The Gardeners' Chronicle p. 42 (Jan 13, 1877)
THE FERTILISATION OF PLANTS
|*Cross and Self Fertilisation of Plants By C. Darwin.|
MR. DARWIN'S new book,* like its predecessors, is a perfect mine of facts and generalisations; and it has been no easy task to digest it. I, however, purpose endeavouring to give as concisely as possible some of the chief facts and conclusions which are of more especial value to horticulturists.
In the introductory chapter Mr. Darwin describes the plans pursued, which were as follows. Confining his observations almost entirely to cultivated and conspicuous flowers, he protected by a net the parent plants. Several flowers were fertilised with their own pollen, the seedlings resulting being called ''self-fertilised plants.'' Other flowers on the same plant, the stamens not being removed, so that Nature might be closely followed, were fertilised with pollen from a distinct plant of the same stock, growing, however, under the same conditions. Their seedlings he calls "intercrossed plants.'' I shall limit the word "crossed" to plants derived by a cross with an entirely new stock. As the two kinds came up, whenever one of each kind appeared simultaneously, they were planted one on each side of a pot. Proceeding thus until from six to twenty or more pairs were established, the remaining seedlings were planted thickly in a large pot or in the open ground, often thus having to struggle for existence, with results to be mentioned hereafter. Conditions of soil, light, moisture, &c., were made as exactly alike as possible.
To determine the results, the relative heights of the fully-grown plants were compared, and a long table (A) in chapter vii. gives them for fifty-four species of thirty orders the height of the intercrossed being fixed at 100—the mean of all the self-fertilised is 87. Mr. Darwin, however, omits certain cases which are more favourable to the latter kind; thus, while inserting all ten generations of Ipomoea, he omits all mention of the wonderful self-fertilising and tall variety he named "Hero." Again, of Mimulus, while inserting the first to the third generations, he omits the fourth to the seventh inclusive, when the self-fertilised exceeded the intercrossed in height for while the average difference for the first three generations is shown by the proportion of 100:65, in the subsequent years the ratios were as 100:110, 100:126, and even as 100:137 neither has he inserted the second year's growth of Eschscholtzia, viz., 100:100 nor that of Lobelia fulgens, viz., 100:167—the self-fertilised being in these vastly superior to the intercrossed! Had our author added these—and I cannot see any good reason for their omission—the resulting mean ratio would have been as 100:118! Height, however, as Mr. Darwin confesses, is not always a satisfactory nor even trustworthy test; for it shows only one element of vigour; and if a plant be unnaturally ''drawn," as occurred with Dianthus, not even that. The weight of the entire plant, had it been possible to ascertain it in all cases, would have been more desirable. Some few plants were weighed, and a table (II) is added, and the difference is often striking. Thus the height of intercrosssed plants of Brassica oleracea is to that of the self-fertilised as 100:95, that is practically the same; but their respective weights were as 100:37 giving a difference between the percentages of fifty-eight! That difference is exceptionally great; for if we take the eleven plants that were weighed, a similar difference between the weights and heights of the mean is twenty-six. Unfortunately weighing could not be always done and, as might be expected, the heights in some cases gave adverse results, i.e., the self-fertilised grew exceptionally taller than the intercrossed. The reverse, however, was the general rule. Hence it is concluded that intercrossed plants grown under the same conditions have, at least fir the first few generations, a decided advantage unless the plant, like Pisum sativum, has been cultivated for many years and propagated solely by self-fertilisation, for in such a case intercrossing does no good. This advantage is shown in the greater vigour of the intercrossed, especially under competition, though it does not always show itself from the very first, the self-fertilised seedlings being often equal to if not taller than the others; but soon after the preliminary stage is past, then the intercrossed begin to surpass their rivals, and ultimately evince greater luxuriance, darker foliage, greater fertility, and brighter tints in the flowers. But after several generations of close interbreeding, the latter become more uniform in colour. This fact is associated with another—viz., that although intercrossing is beneficial at first, subsequently it does no good at all; and then closely interbred and related plants become equal to, if not inferior to, plants of the same kind which have been repeatedly self-fertilised. Mr. Darwin more than once attributes the slight differences between certain self-fertilised and intercrossed plants to the deterioration of the latter through constant inbreeding; but he does not observe that actual self-fertilisation is a still closer form of inbreeding; hence such results tell in favour of the "lasting'' powers of self-fertilisation—that they have not deteriorated, at all events, in the same ratio as the intercrossed. This, again, is analogous to the fact that intercrossing distinct flowers on the same plant often does no good, or else gives results inferior to self-fertilisation; excepting in the case of some nearly self-sterile plants. 'thus, Mr. Darwin speaks of Ipomoea:— ''That the self-fertilised plants grew a little taller, were heavier and generally flowered before those derived from a cross between two flowers on the same plant." A few cases, such as Digitalis, Eschscholtzia, Corydalis cava and Oncidium are mentioned, in which a slight advantage accrued to the offspring of flowers fertilised with pollen from others on the same plant. It is an observable fact that these four plants are nearly self-sterile, so that it would seem that the flowers on the same plant had become more and more highly individualised in some way, though it would be dangerous to generalise from so few cases, especially as it was not the case with some other self-sterile plants. An interesting case, not recorded by Mr. Darwin, is the Victoria regia, which is said to have yielded 25 seeds only when naturally self-fertilised; 60 when artificially self-fertilised; 100, when by pollen from a different flower on the same plant, and as many as 300 seeds when intercrossed with pollen from a separate plant.
The third series of experiments brought out the most important result of all—that when either intercrossed or self-fertilised plants were crossed with pollen from plants raised from an entirely new stock, the ancestors of which had grown at quite different localities, then the resulting "crossed plants" vastly superseded the self-fertilised, and very greatly, through to a less extent, the intercrossed plants of nearly related origin. Every element of vigour was usually enhanced, and colours became much more varied and pronounced. These experiments thus strongly corroborate the custom of seed growers and propagators, of introducing fresh seed from distant localities. Here, then, we may see how Nature brings to her aid various means to effect this. Insects do the first office in intercrossing the closely related plants growing in the same district; and the strong persistency in Nature would imply that all the individuals of s species growing in the same locality were not only closely related, but were seldom crossed with pollen brought from a great distance. This is the first step in the progress of improvement. As the species spread into other areas, birds, especially, perhaps, "of passage," will carry about seeds from different stocks, and having introduced them into new areas, insects will then fertilise the old long-residents by means of the newly bird-imported seedlings; while the fact that insects and birds often feed more or less exclusively on one and the same kind of plant respectively, will favour such crossing. Illustrations will be hereafter given of the benefits which result from crossing distinct stocks.
It may be interesting to the reader to know that Dean Herbert long ago anticipated these results of Mr. Darwin, for he says (Amaryllideae, p. 371):—"I am inclined to think that I have derived advantage from impregnating the flowers from which I wished to obtain seed with pollen from another individual of the same variety, or, at leant, from another flower rather than its own . . . and especially from an individual grown in a different soil or aspect."
Though self-fertilisation does not usually produce such marked results as intercrossing, nor the latter such benefit as crossing with a distinct stock, yet it must not be supposed that, therefore, plants cannot fertilise themselves. The known number self-sterile plants is very small, i.e., plants in which the flower's own pollen is inefficient on its own stigma; and even these would probably become self-fertile under certain climatal conditions, as some are known to do, especially on the reduction of temperature. Moreover, in several instances amongst Mr. Darwin's experiments the self-fertilised were more prolific and more vigorous than the intercrossed and in some genera highly self-fertile forms appeared and transmitted their self-fertilising properties to their descendants: so that, as was the case with Hero in Ipomoea, they did not even profit from a cross with a distinct stock! Then again many plants possess "cleistogene" flowers as the Violet; i.e., minute nearly apetalous and closed flower-buds, which produce an abundance of good seed; while on the other hand conspicuous flowers adapted for intercrossing often set no seed at all in the absence of insects. Lastly, a considerable number of our wild genera with very inconspicuous self-fertilising flowers, such as Chickweed, Knotweed, Fumitory, &c., are abundant in individuals, prolific, and very rapid seeders, vigorous in growth for their size, and when transplanted into other regions of the globe, as Polygonum aviculare into New Zealand, grow to much larger dimensions, and, what is more remarkable, completely invert the rule deduced from Mr. Darwin's observations on cultivated plants—for they oust the native population, and are wonderfully vigorous in the struggle for life! On the other hand Mr. Darwin's experiments on cultivated plants clearly show the great superiority of intercrossed, and especially crossed, plants when under competition with self-fertilised individuals. Mr. Darwin, though often speaking of the "evils" of self-fertilisation (a term which I hope to show requires a relative interpretation only), is obliged to confess "that self-fertilisation is in some measure more beneficial than 'crossing,' unless the cross bring with it some decided and preponderant advantage; and this is only to be acquired by different constitutions in the parents."
Some horticultural importance lies in the fact that the colouring of self-fertilised plants always tends to become extremely uniform in successive generations. This tendency already appeared in the later generations of closely related intercrossed plants subjected to the same environing conditions but it is even more marked in the case of self-fertilised plants; and an obvious fact issues out of this—that if horticulturists wish to retain and render permanent any particular ''strain," they should carefully select it, and then endeavour to fix it by securing self-fertilisation as much as possible, accompanied by entire freedom from competition.
The first chapter of Mr. Darwin's work being introductory, is followed by five chapters in which are recorded the results of his experiments on a large series of plants, the seventh chapter containing tables and a summary of these results. I propose in the next communication to select some of time more interesting cases. The subsequent contributions to the Gardeners' Chronicle will treat of the various matters dealt with by Mr. Darwin in the remaining five chapters of the book.
THE first series of experiments was made on the so-called Convolvulus major (Ipomoea purpurea), which, having conspicuous corollas, is greatly intercrossed by humble-bees, but is also highly self-fertile, so that the number of seeds produced by intercrossing and self-fertilisation differed very slightly. Mr. Darwin experimented on this plant for ten years, planting seedlings of exactly the same growth on opposite sides of pots, and the general results were that the excess in height per cent, of the former over the latter varied from 14 to 46; or, taking the intercrossed plants as 100, the mean height of the self-fertilised was 77. The intercrossed plants showed greater vigour and weight. A chief difference was in the number of capsules Produced, although the seeds per capsule differed but slightly, so that the actual fertility of the intercrossed as compared with that of the self-fertilised was as (from) 100:35 to 100:64.
Mr. Darwin next fertilised flowers of the ninth intercrossed generation with pollen from a new stock, while other flowers were again intercrossed. The results were greatly in favour of the new cross in height, as 100:78, i.e., almost the same ratio as that of the mean of the intercrossed for ten years to the self-fertilised, or as 100:77. In weight the ratio was 100:51. An exceptional plant appeared in the sixth generation, which Mr. Darwin called Hero, as being remarkable for its strong self-fertilising powers, and for showing no benefit when its children and grandchildren were not merely intercrossed, but crossed even by a new stock!
Mimulus luteus afforded quite analogous results. For three years the intercrossed exceeded the self-fertilised in height as 100:65. In the fourth generation a new variety appeared, which grew taller, had whiter and larger flowers, and transmitted these characters with great fidelity, so that all the plants in the later self-fertilised generations belonged to it. These frequently exceeded the intercrossed in height, even in the ratio of 137:100, and in fertility as 147:100. As with Hero so with this pale variety of Mimulus, intercrossing did no good. But it differed from Hero when crossed by a new stock, for the heights of the eighth self-fertilised generation thus crossed were to that of the ninth self-fertilised generation as 100:52, and in fertility as 100:3.
"Better evidence," says Mr. Darwin, "could hardly be desired of the potent influence of a cross with a fresh stock on plants which had been self-fertilised for eight generations, and had been cultivated all the time under nearly uniform conditions."
Here, then, is a most important fact for horticulturists, and it is associated with another of equal value, that greater variety of colours are produced in proportion to the crossing; while on the other band perpetual self-fertilisation seems invariably to reduce them to a uniform tint. Mr. Darwin first noticed this in the seventh generation of Ipomoea, of which all the flowers were of a uniform remarkably rich dark purple tint, which remained constant up to the tenth and last generation raised. Likewise had the repeatedly intercrossed flowers a much more uniform colour than those originally raised from purchased seed Similarly with Mimulus, the original plants varied greatly in the colour of their flowers, so that hardly two individuals were quite alike, the corolla being of all shades of yellow, with the most diversified blotches of purple, crimson, orange and coppery brown, probably the result of much intercrossing. On the other hand, the form which appeared having great self-fertility was not only tall, but its flowers were large, nearly white, and blotched with crimson, and retained a surprising uniformity throughout later generations.
Brassica oleracea.—This, like other cruciferous plants, is adapted for crossing with its two shorter stamens, and self-fertilisation by its four longer ones; and, if varieties are grown together, it is consequently very difficult, as Mr. Darwin proved, to raise pure kinds. Height in this case proved quite fallacious, but weight showed the vast superiority of intercrossing by the ratio 100:37, while the fertility was as 100:5. Finally, crossing with s new stork increased the weight of the cross to the ratio of 100:22. Mr. Darwin tested the statement that a cut-leaved curled and variegated white-green Cabbage would not cross with a similar crimson-green Cabbage. This was an error, but the former was partially sterile, which may possibly account for the statement. The latter crossed by the former afforded curious results. A few reverted to a pure green, and became very vigorous, but many more of the self-fertilised seedlings of crimson-green thus reverted, and grew taller by 2 inches than the crossed seedlings. hence reversion to a more natural condition acted more powerfully on their growth than the influence of crossing with a semi-sterile variety.
It would have been interesting for gardeners had Mr. Darwin experimented on the effects of crossing upon ''root" plants. He only remarks that the effects of Kohl Rabi were particularly plain in the enlarged stems of the mongrel seedlings of varieties of Cabbages grown together.
Pisum sativum is fully self-fertile in England, and is rarely intercrossed, as no British insects are usually strong enough to effect it, hence varieties grown together remain pure. Knight's, produced by artificial crossing, lasted over sixty years, and were always self-fertilised. Owing to the varieties being self-fertilised for many years, s cross brought no benefit at all, or rather had a deteriorating effect for the heights were as 100:115; but Mr. Knight proved that a short variety crossed by a tall one gave rise to offspring twice its height. Mr. Laxton also proved that crossed varieties gave rise to prolific offspring. From this species we learn that when a plant has after many generations become, as it were, habituated to self-fertilisation, then intercrossing does no good. This we have seen is paralleled by Hero in Ipomoea, and probably by Lathyrus odoratus, though Mr. Darwin did not test this species; though he crossed different varieties varying in colour, and obtained a new subvariety, and in the next generation the offspring varied still more—proving the good effect of crossing varieties if colour-variations are required. On the other hand a spontaneous variety may appear, and then this may prove true to its kind.
Phaseolus multiflorus, or Scarlet Runner, is very sterile in the absence of fertilising insects, but self-fertile if the petals be only mechanically moved. The cause of the great variability in the colouring of the seed-skins is disputed, but seems most likely to be due to intercrossing. The crossed and self-fertilised plants differed but little, if anything, in height and fertility. P. vulgaris, though closely allied to the preceding, differs from it in being highly self-fertile, yet varieties cross freely if planted together.
Pelargonium zonale.—This genus is strongly proterandrous and almost self-sterile, but in pale-coloured varieties the pistil matures early and the plants then become "great seeders." The physiological importance of this fact will be alluded to hereafter. In Mr. Darwin's first experiments he crossed plants derived by cuttings from the same stock, and no benefit accrued—a result similar to one obtained by crossing plants of Origanum propagated by stolons. A cross with the pollen from a different plant gave offspring, compared in height with one from a flower fertilised by pollen from another flower on the same plant, in proportion of 100:74. It possessed also greater vigour. It would have been a desideratum to know what, if any, effect was produced by crossing on the "zones:" for Mr. Grieve found the pollen of Zonal Pelargonium to affect Geranium pratense, both in the colour of the flowers as well as in the variegation of leaves (Gardeners' Chronicle, July 8, 1876), but nothing is said about it.
Dianthus Caryophyllus.—This plant is also strongly proterandrous, and great care must be taken to prevent varieties crossing if pure strains are required. The number of seeds from crossed and self-fertilised parent plants (as was not unfrequently the case at first) differed but little, the ratio being as 100:92. That of the heights of the second generation were about as 100:86 when both were grown in the open ground, but when crowded in pots, the result of struggling for existence, which self-fertilised plants usually manifest, was very apparent in the relative heights being as 100:58. In the third generation, however, the heights, though grown in pots, were as 100:99. They were, however, "drawn," being ''light and thin," but when tested by weight the latter showed the ratio of 100:49—a fact which well illustrated the danger of estimating solely by heights. Another curious fact happened in the third generation. The self-fertilised became much more fertile than the intercrossed, the number of seeds being as 125:100, This was probably due to the sexual organs maturing more nearly together, and so being less dichogamous. This is similar to Pelargonium and Primula, which may also become self-fertilising from a like cause. A fresh cross from a new stock was now introduced. The first effect appeared to indicate a decrease of fertility, but the offspring, compared with the self-fertilised, were in weight as 100:33, and their relative heights as 100:81. Lastly, the pale pink or rose colour of the fourth self-fertilised generation were ''as uniform in tint as those of a wild species." The flowers of the fourth intercrossed generation were likewise nearly uniform; but the newly-crossed plants varied extremely in colour.
Petunia violacea.—A dingy purple variety was cultivated for five generations. The ratio of the average heights of the intercrossed to the self-fertilised was 100:71; but in the third generation the difference was reversed, for the heights were then as 100:131. Mr. Darwin's explanation (p. 275) is, that the seeds from which the self-fertilised plants of the third generation were raised, w ere not well ripened, which fact, he says, gave them an advantage (?), and refers to a similar case in Iberis, but in p. 192 he attributes the inferiority of the very same crossed plants of this third generation of Petunia to their not having been sufficiently ripened, and thus producing weakly plants compared with the self-fertilised, as (he again says) also occurred with Iberis; but on p. 103 we read:— I record in my notes that the self-fertilised seeds [of Iberis] from which the self-fertilised plants were raised [and which were 5 or 6 inches taller than the crossed], were not so well ripened as the crossed.'' Hence, there are both errors of statement as well as adverse reasonings from the data! Mr. Darwin also accounts for the greater growth of the eighth generation of Ipomoea from their having been raised from unhealthy parents (pp. 33, 39). Surely, if it be beneficial to have unripened seeds and unhealthy parents, at least as far as height is concerned—and Mr. Darwin has selected that as his standard of superiority—natural selection ought to have prevented seeds from maturing, and encouraged the visits of aphides! But I think we may find another explanation. On p. 274 Mr. Darwin adds a very significant sentence:—"It is a remarkable fact that in one pot in which the plants of both lots grew extremely crowded, the intercrossed were thrice as tall as the self-fertilised." This implies that in the other pots they were not so crowded. Now, there is abundant evidence throughout the book that intercrossed and perhaps especially crossed plants, i.e., with new stocks, show, as a rule, a very great superiority over self-fertilised plants when struggling in competition; but, when not crowded, or when grown in open ground, then the difference between their heights and weights is greatly reduced, and may be actually reversed. Indeed Mr. Darwin had already stated this fact in his Animals and Plants under Domestication, vol. ii., p. 128, in alluding to his method of culture:—"It is of importance that the two lots of seed should be sown or planted on opposite sides of the same pot, so that the seedlings may struggle against each other, for if sown separately in ample and good soil, there is often but little difference in their growth." I will return to this fact hereafter. Hence I would attribute the vigour shown by the self-fertilised plants of the third generation of Petunia to less crowding. And here I would venture to remark that Mr. Darwin's plan of sowing both intercrossed and self-fertilised seedlings in the same pot—though with the express object of imitating Nature, and thereby revealing the superiority of the former—would have been benefited had he also grown similar pairs in separate pots. Then, there is reason to know that the relative heights would have been much nearer uniformity. As it is, the absolute vigour of neither can be estimated, though the relative vigour is clearly in favour of the intercrossed. Mr. Darwin, indeed, himself elsewhere recognises the great importance of freedom of growth for self-fertilisation to be successful; for in speaking of an experiment with an equal-styled "perfectly self-fertilising red-flowered variety of Primula veris," he says:—"Judging from the previous generations, the extreme unproductiveness of the self-fertilised plants in this experiment was wholly due to their having been subjected to unfavourable conditions, and to severe competition with crossed plants; for had they grown separately in good soil, it is almost certain that they would have produced a large number of capsules."
Canna Warscewiczii affords another case resembling Pisum sativum, for this species is highly self-fertile, and ''as plants are cultivated in pots, and are not exposed to competition, they have been long subjected to uniform conditions;" and Mr. Darwin adds, "we have no right to expect much or any good from intercrossing plants thus descended and thus treated." And no good did follow, except that the intercrossed plant, yielded rather more seeds, viz., as 100:85. But the self-fertilised plants mostly flowered before the intercrossed.
There are two more plants, Eschscholtzia californica and Reseda odorata, I will remark upon, for I think Mr. Darwin has drawn erroneous conclusions from them.
If the reader will carefully peruse the first paragraph of chap. xii., pp. 436-7, I think he would be best to suppose that Reseda and Eschscholtzia were good examples to illustrate the "injurious effects of self-fertilisation, for as [some individuals of] these are sterile with their own pollen, they have been long naturally crossed, and the artificial crosses in my experiments," says Mr. Darwin, ''cannot have increased the vigour of the offspring beyond that of their progenitors. Therefore the difference between the self-fertilised and crossed plants raised by me cannot be attributed to the superiority of the crossed, but to the inferiority of the self-fertilized seedlings, due to the injurious effects of self-fertilisation."
The last sentence, which I have italicised, is very misleading.
In the introductory remarks on Eschscholtzia (p. 109), the second sentence directly contradicts the above, for it runs as follows:—"A cross greatly increases the productiveness of the flowers of the parent plant." This is repeated on p. 275, in the words, "A cross does some good."
Mr. Darwin received seed from Brazil, where Eschscholtzia is ''absolutely self-sterile," and, therefore, unlike English-grown plants, which are more or less self-fertile. It proved, however, to be not so sterile in this country; hence he was able to raise self-fertilised plants; the relative fertility of the intercrossed to the self-fertilised being as 100:15—where the "15" represents so much absolute gain, and that is considerable for a single generation; but in the next generation the relative fertility appears now as 100:86.7! Turning next to the comparative results between the heights of the first generations—i.e., grandchildren of those grown in Brazil—the heights are as 100:101, and in the second generation as 100:116! Mr. Darwin observes on this:—"As the grand-parents absolutely required cross-fertilisation, I expected that self-fertilisation would have proved very injurious to these seedlings. . . . But the result showed that my anticipation was erroneous."
He next crossed the Brazilian plant with an English stock, and it would be inferred by analogy that great benefit should accrue; but the self-fertilised actually beat the crossed in height as 109:100, and in weight as 118:100; and Mr. Darwin further remarks that the self-fertilised were also apparently superior in hardiness. The only point, then, where the crossed were superior to the self-fertilised was in fertility; but when we remember that Eschscholtzia is absolutely self-sterile in Brazil, it is more surprising to find the ratio so near equality in two generations.
Surely, then, it is unfortunate that Mr. Darwin should have specially alluded to this plant as proving the "injuriousness" of self-fertilisation, for his own data not only fail to support his conclusion, but maintain exactly the reverse, for the acquirement of self-fertilisation is, per se, so much actual gain to the plant of a new and, ready means of propagation; secondly, having acquired that, such plants, by gaining new constitutional elements by transportation to this country, became more vigorous than the normally intercrossed, or even than plants crossed with a new stock.
Eschscholtzia is not a unique case, for Reseda, likewise mentioned as supporting the idea of the injuriousness of self-fertilisation, also tells against such being the case.
Reseda lutea is sometimes absolutely self-sterile. Some plants, however, produced a few spontaneously self-fertilised capsules; propagating from these latter in pots, the results gave the mean ratio of heights as 100:85, and in open ground as 100:82. And Mr. Darwin adds:—"It is a singular fact that the tallest plant in the two rows was one of the self-fertilised." The ratios of their weights was still more striking—viz., of those in pots 100:21, but in open ground as 100:40.
Reseda odorata.—This species shows degrees of fertility from absolute self-sterility to full self-fertility. Arranging Mr. Darwin's results as follows:—I. Intercrossed plants from the highly self-fertile parents, compared with self-fertilised. a (in pots), heights were as 100:82, weights as 100:67; b (in open ground), I height as 100:105 (weights not given). II. Intercrossed plants raised from a semi-self-sterile parent, and compared with self-fertilised plants, raised from the same parent. Mr. Darwin here makes a remark similar to that upon Eschscholtzia:—"I expected that the seedlings from this semi-self-sterile plant would have profited in a higher degree from a cross than did the seedlings from the fully self-fertile plant, but my anticipation was quite wrong." a, height as 100:92, weight as 100:99; b, height as 100:90 (weights not given).
Now, comparing these two sets of results, a remarkable fact is seen, that in she first generation the plants from fully self-fertile and self-fertilised parents grown in pots (a) do not give such high results as the self-fertilised plants raised from semi-sterile parents under the same circumstances. But while the former show a vast improvement when grown in the open ground (I., b), the latter show none!
Now it is a general rule, as already stated, that self-fertilised plants cannot stand competition at all equal to intercrossed plants; but when planted in the open ground the contrast is greatly lessened, i.e., the ratio becomes more nearly one of equality, in other words the self-fertilised recover vigour in a higher ratio than the intercrossed are improved by freedom of growth. So here, from I. (b) it is to be inferred that the self-fertilised have a greater ''elasticity" of growth, so to say, than the intercrossed. This was likewise the case with the weights of Reseda lutea, II. (a) would seem to show that the vigour of the offspring from parents semi-sterile is relatively greater than that of the offspring from fully fertile parents when compared with the intercrossed plants of each kind; and had not Eschscholtzia showed a similar fact these results of Reseda could scarcely have been considered trustworthy.
Hence it is very unfortunate for Mr. Darwin's argument to have especially selected Reseda and Eschscholtzia, for they certainly do not prove any "injurious " effects to arise from self-fertilisation, but just the reverse!
The fact appears to me to be this:—The word ''injurious" is a purely relative term. If a plant is so highly differentiated that it has not only become adapted to insect agency but also to be self-sterile, then, of course, to put its own pollen upon the stigma of a flower, rather than that of another plant, may be said to be, relatively to the latter process, at least useless if not injurious. But when we see a plant thus usually self-sterile becoming, under changed circumstances, self-fertile, this power of self-fertilisation is actually so much positive gain, as Mr. Darwin admits "the obvious fact, that the production of seed is the chief end of the act of fertilisation, and that this end can be gained by hermaphrodite plants with incomparably greater certainty by self-fertilisation." Now it is clear that many plants have become highly differentiated by adaptation to insects, and in becoming so, the pollen has physiologically changed correlatively to such a degree as to be useless on the flower's own stigma. This therefore is so much absolute loss to the plant, as it now has to depend, with less certainty, upon the chance visit of insects; but when the plant can recover its lost power, and especially when it can acquire fresh and new constitutional elements, then, as we see in Reseda and Eschscholtzia, the first result is the absolute re-gain of self-fertilisation, while this may be accompanied by an equality with, or even a superiority in vigour to, the intercrossed.
In a later communication I purpose continuing the subject of self-fertilisation.
MR. DARWIN has been good enough to notice one of my criticisms and to explain it as due to a misprint of "self-fertilised" for "crossed," so that the sentence on p. 275 now runs as follows:—"The most probable explanation [of the ratio 100:131] is that the seeds from which the crossed plants of the third generation were raised were not well ripened [i.e., this will account for the self-fertilised beating the crossed], for I observed an analogous case with Iberis. Self-fertilised seedlings of this latter plant, which were known to have been produced from seeds not well matured, grew from the first much more quickly than the crossed plants, which were raised from better matured seeds; so that having thus once got a great start they were enabled ever afterwards to retain their advantage. . . and when in full flower were to 6 inches higher than the crossed" (p. 103). So that if I were hypercritical in detecting an error due to the misprint, my inference is not altered, viz., that Mr. Darwin attributes the inferiority of the crossed Petunias to unripened seeds, and the superiority of the self-fertilised Iberia to the same cause! So that the case of Iberis, though in harmony with the original misprint, is no longer analogous, but exactly the reverse. I echo Mr. Darwin's hope that any reader who is interested in the subject will not take my interpretation of his statements without consulting his hook.
Alter having devoted seven chapters to the details of his experiments, and to tables and summaries of the results, Mr. Darwin, in the eighth and ninth, deals with the comparative advantages and the reverse of crossing and of self-fertilisation respectively. Then follow two chapters on "the means of fertilisation and the habits of insects in relation to the fertilisation of flowers," and the last (twelfth) is devoted to general results.
Mr. Darwin commences the eighth chapter by observing that, "as the seedlings were planted on opposite aides of the same put, they had to compete with one another, and the greater height, weight, and fertility of the [inter] crossed plants may be attributed to their possessing greater innate constitutional vigour." He, however, notices that whilst very young—i.e., before competition began—they were, with some exceptions, of equal height but afterwards the intercrossed exceeded the self-fertilised. He next observes (p. 286), "that the [inter] crossed plants have an inherent superiority independently of competition, was sometimes well shown when both lots were planted separately in good soil in the open ground." But the word "sometimes" I have italicised is rather misleading, for on p. 288 the force of this sentence is reversed, where he says of plants thus grown—"The result was in several cases (but not so invariably as might have been expected), that the [inter] crossed plants did not exceed in height the self-fertilised in nearly so great a degree as when grown in pairs in the pots."
|*Animals and Plants under Domestication, vol. ii., p. 128.|
The sentence in brackets is an important admission in favour of self-fertilised plants when they can grow freely and agrees with the tables, which clearly show such to be the case (vide, e.g., A., Vandellia, Eschscholtzia, Reseda, Petunia, 5th gen.; Beta, B., Reseda lutea; C., Nicotiana). The rule, therefore, appears to be that when self-fertilised plants are compared with intercrossed in open ground, the vigour of the former often evinces itself either in an equal degree or in a higher ratio than the intercrossed. Mr. Darwin's later experiences as detailed in this book thus corroborate his statement given in 1868: "If [the seeds are] sown in ample and good soil, there is often but little difference in their growth."* Nevertheless as plants for the most part are certainly compelled to compete with one another in Nature, the superiority of intercrossed and crossed plants is specially shown in that capacity, Mr. Darwin next observes that "the innate power of the [inter] crossed plants to resist unfavourable conditions far better than did the self-fertilised plants, was shown on two occasions in a curious manner . . . when both sets were grown under extremely unfavourable conditions."
He here refers to seedlings of the plants of Petunia and Iberis, referred to in the last paper, of which when sown in sand and burnt earth, i.e., without any organic matter, the intercrossed surpassed the self-fertilised in height until they all died; a fact obviously of no great trustworthiness for deducing a general result, especially as the seeds were in an abnormal condition. Mr. Darwin refers to some analogous case with the first generation of Nicotiana. Unfortunately he does not appear to have recorded it, for his other observations on that plant are all in favour of the self-fertilised, not only in the first year during which he says "the self-fertilised were greatly superior to the [inter] crossed," but also mostly in the two following generations!
|*Mr. A. Bennett has also observed the same fact. Nature 1869, p. 11.|
The stronger constitution of the intercrossed was also seen in withstanding sudden changes of temperature—i.e., from the house to the open ground—as well as in resisting cold and intemperate weather. He illustrates this fact with Ipomoea, Mimulus, Viola, and Sarothamnus and Nicotiana; Eschscholtzia formed an exception. There appears, on the other hand, however, to be some correlation between a reduction of temperature and self-fertilisation. Several observations have led me to draw the conclusion that flowers, perhaps usually intercrossed, become self-fertilised not only, as has been by others inferred, in the absence of insects which is apparently another and distinct cause, as, e.g., in Pisum sativum and Lathyrus odoratus in England; but when the autumn draws on, and habitually in winter for such of our wild flowers as blossom in that season:* several cases are mentioned incidentally by Mr. Darwin, which corroborate my observations. Thus he speaks of Reseda odorata, that while some plants were quite sterile in one year, three other plants in the next "became loaded with capsules, especially during the early part of the summer; and this fact indicates that temperature produces some effect." What that effect is I think I have discovered, and will explain hereafter.
Another fact in favour of intercrossing is that the intercrossed are less liable to premature death, independently of any apparent external cause. The self-fertilised plants often germinated sooner, but the seedlings oftener died than their competitors. They also arrived at maturity quicker, and began to wither sooner than the intercrossed; just as these latter did before the offspring of a cross with a fresh stock.
Lastly, the intercrossed often threw up a larger number of flower-stems, the result probably of greater vigour, and showed a tendency to flower before the self-fertilised. Mr. Darwin gives lists of species which flowered first. Thus forty-four intercrossed flowered first, either in a majority of the pots or in all. In nine cases a self-fertilised plant flowered first; in five the two lots flowered together. In the case of Cyclamen the intercrossed actually flowered some weeks before the self-fertilised.
"On the whole," concludes Mr. Darwin, "there can be no doubt that the [inter] crossed plants exhibit a tendency to flower before the self-fertilised, almost, though not quite so strongly marked as to grow to a greater height, to weigh more, and to he more fertile."
MR. DARWIN devotes a portion of chapter 8 to "the transmission of the good effects of a cross," and shows that when a previously intercrossed, a crossed, and a self-fertilised plant are allowed to be fertilised naturally by insects, the good effects of the "cross" is seen in the greater fertility and vigour of the offspring of the first two than in that of the self-fertilised; and that this benefit may be continued, not only to the grandchildren, but for many years, as appeared from Mr. A. Knight's varieties of Peas, which were originally raised by crossing, and which kept true for more than sixty years, during which time they retained their superiority, though invariably self-fertilised. This fact, as Mr. Darwin observes, is due to the "force of inheritance being very strong in plants." On the other hand, continued interbreeding between very closely related plants brings ultimately no benefit at all as far as fertility is concerned; and, with reference to colour, such interbreeding is closely analogous to self-fertilisation; that it tends to produce great uniformity of tints, but still much less so than the self-fertilised plants, for Mr. Darwin observed that the seventh to the tenth generations of Ipomoea were absolutely uniform in tint, "like those of a constant species living in a state of Nature." That last sentence is not followed up, but is suggestive of queries. Why are wild flowers so uniform? They must be abundantly intercrossed, and as in Nature we often get a great variety of soils and climatal conditions within reasonable distances, we should think there would be abundant resources for crossing distinct stocks, yet there are many species identical, not only in the same country, but in opposite hemispheres of the globe! However this "retention of specific type" may be explained, the practical benefit of Mr. Darwin's observations to horticulturists is plain, for, as he says:—"We learn from them that new and slight shades of colour may be quickly and firmly fixed, independently of any selection, if the conditions are kept as nearly uniform as is possible, and no intercrossing [much less crossing with a new stock] be permitted."
Mr. Darwin gives a table (D.) of "the relative fertility of crossed, intercrossed, and self-fertilised parentage, the fertility being estimated by the number of seeds per capsule, number of capsules, or else the weight of seeds per capsule, the first being the most valuable method. Eliminating, therefore, from table D., all cases judged by capsules, as well as that by weight of seeds, and also deducting five cases of plants crossed by a new stock, there are left eleven instances for comparison between the intercrossed and self-fertilised; and if 100 be standard for the intercrossed, that of the self-fertilised is 91.5—a result which, considering the data, is very nearly a ratio of equality, and had he introduced into table D. "Hero" and the "white variety of Mimulus," the ratio would have been probably, if not in favour of the self-fertilised, at least one of equality (see p. 50), for when Ipomoea, under a net, gives a ratio of 100:99, and another instance, uncovered, gives a ratio of 100:89, we see at once that there must be conditions which may vary the proportions to a considerable extent one way or the other. Indeed, Mr. Darwin himself says:—"It should be observed that the results [of this table] cannot be considered as fully trustworthy, for the fertility of a plant is a most variable element, depending on its age, health, nature of the soil, amount of water given, and temperature to which it is exposed." Next, selecting from table D. the five cases of plants crossed by a new stock, the highest and lowest ratios are 100:30, and as 100:75, their mean being as 100:49.4, which shows a much more decided degree of fertility for crossed plants than that of intercrossed over self-fertilised.
The mean ratio deduced for the number of capsules produced by the plants of intercrossed and self-fertilised. parentage would be very misleading, for the ratios vary from 100:3.5 to absolute equality ! though the intercrossed plants doubtless as a rule yield more than the self-fertilised. As they are more vigorous plants, this is what one would anticipate; but the seeds per capsule may be the same: thus, of Nolana prostrata thirty intercrossed flowers produced twenty-seven capsules, each with five seeds; thirty-two self-fertilised flowers yielded six capsules; each, however, had also five seeds; hence their fertility is the same, but the proportion of capsules as 100:21.
Table E. gives the innate fertility of four plants crossed with a fresh stock, calculated as 100, and compared with those of intercrossed and self-fertilised respectively, as follows:—Mimulus as 100:4 and :3; Eschscholtzia as 100:45 and :40; Dianthus as 100:45 and :33; and Petunia as 100:54 and :46.
Table F. gives the relative fertility of the flowers of thirty of the "parent plants" when intercrossed and when self-fertilised. Of these six are equal, and four of the self-fertilised are most fertile. The ratios of fertility for the "cleistogene" flowers of the Vandellia and of Ononis minutissima are omitted from this table. They would have lessened the difference in the mean proportion, as they are so highly self-fertile. Mr. Darwin puts "100:67?" as the ratio for the conspicuous flowers of Vandellia, but on p. 90 he says he thinks that 100:100 would be more correct. Subtracting this case with the above, we have twenty left, which give an average ratio of 100:64; but, if we substitute 100:100 for the conspicuous flower of Vandellia, 100:106 for the cleistogene flowers of the same plant, and 100:111 for those of Ononis (deduced from the number of seeds given in p. 167), we get a total mean ratio of 100:96, which is, of course, practically nil; but this conclusion does not destroy the fact, that the whole number of intercrossed plants given in this table are more fertile than the self-fertilised in the proportion of two to one. Mr. Darwin observes that "this fertility ranges from zero to a fertility equalling (or exceeding) that of the [inter] crossed plants. Of this fact no explanation can be offered." Moreover in table G., which is analogous to F., only treating of descendants instead of the parent plants, the infertility of the self-fertilised plants does not decrease, indeed in Dianthus it steadily increased; so that Mr. Darwin observes, "There is, therefore, no evidence at present that the fertility of plants goes on diminishing in successive self-fertilised generations."
As the chapter (9) is here devoted to self-sterility and self-fertility it will be better to defer further remarks until another issue, when my communication will deal with self-fertilisation especially.
I PURPOSE devoting this and the remaining contributions to self-fertilisation. Before, however, making out my case, I would wish to guard against being misunderstood; for my criticisms have, I fear, led readers to imagine I would underrate Mr. Darwin's laborious investigations and their important results. This would be far from my intention. The chief of these results may be briefly summed up thus:—1. Crossing with a distinct stock usually gives enormous advantages, which are continued into subsequent, often many generations. Every element of vigour and prosperity is enhanced. That is clearly established. 2. That more moderate advantages accrue from an intercross with another plant al the same stock, and that the relative advantages of such, over self-fertilisation, gradually disappear in successive generations. 3. The crossing of different flowers on the same plant generally does little or no good, such benefit being mostly (?) in the case of self-sterile plants. 4. Self-fertilisation. This last, Mr. Darwin says, is "injurious," and produces "evil results." Now, it is here where I am compelled to differ from his conclusions, and my previous criticism; have been mainly directed to this point. Allowing for individual exceptions, I maintain that, as a broad general principle, self-fertilisation in the vegetable kingdom is not "injurious" in any ordinary sense of the term. I may be wrong, but that is my conviction, and the present communication will supply my reasons for so thinking.
On p. 26, while recommending his readers to study certain cases in particular, in which "the crossed plants are superior to the self-fertilised in a marked degree," Mr. Darwin adds:—"As instances of self-fertilised plants being equal or superior to the crossed, the experiments on Bartonia, Canna, and the common Pea ought to be read; but in the last case, and probably in that of Canna [and he might have added Bartonia, for the plants were unhealthy], the want of superiority in the crossed plants can be explained;" the explanation being, that as Canna and the Pea have been long cultivated by self-fertilisation, intercrossing did little or no good. But there are other instances, such as the case of "Hero," Mimulus, Reseda, Eschscholtzia, &c., to which this cause does not apply, and which were much superior to their intercrossed rivals.
I will now quote three passages of importance. "We should always keep in mind the obvious fact, that the production of seed is the chief end of the act of fertilisation; and that this end can be gained by hermaphrodite plants with incomparably greater certainty by self-fertilisation than by the union of sexual elements belonging to two distinct flowers or plants" (p. 3). In speaking of the superiority of self-fertilised seedlings of Ipomoea to those raised from flowers fertilised by pollen taken from other flowers on the same plant, he says:—"This is a remarkable fact, which seems to indicate that self-fertilisation is in some measure more advantageous than crossing, unless the cross bring with it, as is generally the case, some decided and preponderant advantage" (p. 61).
Lastly, Mr. Darwin observes:—"The most important conclusion to which I have arrived, is that the mere act of crossing by itself does no good. The good depends on the individuals which are crossed differing slightly in constitution" (p. 27).
Considering them in detail, I shall endeavour to show how self-fertilised plants can and do fulfil the conditions here spoken of by Mr. Darwin.
The following are some of the chief facts connected with the self-fertilisation of plants:—
1 to 4. Plants may be physiologically or else morphologically self-sterile, or both combined; but under certain circumstances many, and probably all such plants acquire or regain the power of self-fertilisation. Mr. Darwin records several cases, as, for example, the Brazilian Eschscholtzias, of which he says, "their self-fertility had evidently increased greatly by being reared for two generations in England." It was similar with Abutilon Darwinii. Some plants of Reseda odorata, as already stated, are self-fertile, while other plants are self-sterile Verbascum phoeniceum fluctuates in the same way; and while V. Thapsus is self-fertile, V. nigrum is self-fertile. Lobelia fulgens was self-sterile in Germany, but Mr. Darwin found it self-fertile. This, like the Brazilian Eschscholtzia, appeared to become self-fertile in consequence of the lower temperature of England; for the latter became again self-sterile in Germany. Orchids may be both morphologically and physiologically self-sterile, while even in some cases the pollen acts as a poison on the stigma of the same flower.
Mr. Darwin regards such self-sterility merely as an "incident," and not due to natural selection to prevent self-fertilisation.
Plants adapted for insect agency may be, and generally are, however, self-fertile. This occurs with many Leguminosae; Pisum sativum is habitually self-fertilised, so is Phaseolus vulgaris, but P. multiflorus has not yet regained the power, and is still morphologically self-sterile. Again, dichogamous flowers, such as pale varieties of Pelargonium, Primula and Dianthus, may gradually become self-fertilising by maturing their stamens and pistil together.
In speaking of the "immediate cause" of self-sterility Mr. Darwin attributes it to the environing "conditions." These, I think, should be called the "proximate" causes, the immediate or ultimate cause being, as I conceive, the preponderating influence of the corolla and nectariferous whorls, which thus by undue growth destroy the equilibrium between the essential organs. Whatever, therefore, may be the cause at work which checks the growth, or development, or energy, in whatever way shown, by the corolla, glands, and nectaries, and perhaps in part the stamens, that same energy is now diverted into the pistil, which thus matures simultaneously with the stamens, and, therefore, earlier than it would otherwise have done. What led me to draw this conclusion was an examination of the order of "emergence," as well as the subsequent growth of the floral organs of a large number of plants; such order being frequently as follows with flowers having conspicuous corollas:—sepals, stamens (if in two whorls, that opposite the sepals arises first, then that opposite the petals), pistil, and lastly the corolla. The latter grows subsequently very rapidly, and then, of course, exceeds the other parts very greatly. On the other hand, with inconspicuous self-fertilising flowers, the pistil often emerges simultaneously with or before the stamens. The former condition I take to be the immediate cause of proterandry, and the latter that of self-fertility; or if the pistil become too much stimulated the equilibrium is again destroyed, and proterogyny is the result. But if energy be diverted from the corolla, as seen in the pale Pelargoniums, then it passes into the next whorl which appears in the order of succession, i.e., the pistil; this latter now regaining its power of growth arrives at maturity simultaneously with the stamens, and self-fertilisation is the result.
Under heading No. 4. I have given six causes, and possibly there may be others which may thus bring about self-fertilisation.
Mr. Darwin alludes to withering of corollas al Viola tricolor, and I can corroborate it, for I found flower-buds of Tradescantia erecta in September last with the corollas shrivelled and matted round the ovary, thus keeping the style bent down upon it and the stigma in contact with the anthers; a number of capsules ripened, and the seeds contained perfectly formed embryos. Mr. Darwin alludes to Buttercups fertilising themselves by closing at night, and perhaps we may generalise upon that; at all events it is suggestive that flowers which close in diffused sunlight (as Anagallis arvensis, Mesembryanthemum, Daisies, &c), in the evening, or at night, may do so for this very purpose.
The above observations led me to deduce the expectation that if the corolla were cut out, the proterandrous or otherwise sterile flowers might become self-fertilising; and I was most agreeably surprised to find cases recorded by Mr. Darwin which exactly fulfilled my prediction, though the operation was performed for a different purpose. "Kurr," writes Mr. Darwin, "removed the whole corolla from a considerable number of flowers, and these yielded seeds. Flowers which are self-fertile would naturally produce seeds under these circumstances, but I am greatly surprised that Delphinium Consolida, as well as another species of Delphinium and Viola tricolor, should have produced a fair supply of seeds when thus treated (note, p. 420). Now, such a result exactly corroborates or proves my deduction, but I hope to test it fully this year, and would be extremely glad if any one would assist me in doing so by thus removing the corollas together with some of the stamens, as well as alone, especially before development, and carefully protecting those flowers thus mutilated from the approach of insects. Of course it need only be done in the case of proterandrous flowers.
Another cause of self-fertilisation is the lowering of the temperature. Allusion has already been made to this in the case of the Brazilian Eschscholtzia, which, though self-sterile in South America, became self-fertile in England. Mr. A. Bennet also called attention to the fact that some of our wild flowers, when they blossom in winter, do not open their buds, and are self-fertilising in consequence, and I can corroborate his observations by many other instances. The rationale being, as I believe, that the reduction of temperature checks the growth of the corolla and the secretion of the glands.
Again, certain white and pale coloured varieties are great seeders; and this, again, I suspect, is another cause of the same kind, for whiteness represents loss of vigour. Mr. B. T. Lowne informs me that he found the garden Balsam became white if the soil was deprived of ammonia, but if it be artificially supplied the colour returned—an important fact, to which I would call the attention of horticulturists.
If such be the true explanation of proterandry and self-fertilisation, the question is resolved into one of degrees of compensation, and, so far at least, has nothing to do with " injuriousness" whatever. In fact, it is an absolute loss (and therefore an "injury") to a plant to be self-sterile, for its facility of propagating is therefore largely, if not entirely, checked.
Though I agree with Mr. Darwin that "the inefficiency of a plant's own pollen [physiologically] is in most cases an incidental result [of differentiation], and has not been specially acquired for the sake of preventing self-fertilisation," I cannot agree with him in drawing a different conclusion for morphologically self-sterile plants, for he adds: "On the other hand, there can hardly be a doubt that dichogamy . . . that the hetero-styled condition of certain plants, and that many mechanical structures, have all been acquired so as to check self-fertilisation and to favour cross-fertilisation;" and yet he shows that when plants lose their dichogamy they regain great self-fertilising powers, which he then considers to be an advantage to the plant. Elsewhere, too, Mr. Darwin says "it is difficult to avoid the suspicion that self-fertilisation is in some respects advantageous."
My explanation is that it is simply an unavoidable result of the loss of equilibrium between the pistil and corolline and nectariferous whorls. As long as insects visit a flower they are continually keeping up a sort of irritation at that region; the whole weight of the insect is often thrown upon the corolla. If it be a terminal flower, the insect alights on any petal or petals, and nothing induces the flower to become irregular; but if the flower be axillary, it alights on the anterior side and so (I assume theoretically, until it can be demonstrated or disproved) causes the corolla to be bilateral, by determining a flow of nutriment to that part; though what innate forces may regulate the special peculiarities of structure in different flowers is at present quite incomprehensible.
This constant irritation and the continual drain upon the secretive organs must stimulate them to develope more and more, just as a man's arm increases by work, or the mammae may be made to secrete for prolonged periods; so that, in my view, it is not that insects have gone to the flowers because they were first conspicuous, but have actually themselves determined their conspicuousness. The process may have been a slow one, only affected in many generations; the result has been that nourishment has been delayed from the pistil, and the flowers have become in the majority of instances dichogamous. On the other hand, in the absence of insects, there is no such increase of energy in the outer whorls at the expense of the gynoecium, so that the balance is restored and self-fertilisation resumed.
5. ALTHOUGH in many cases it would appear that intercrossed plants are absolutely more fertile than the self-fertilised, yet Mr. Darwin raised during his experiments some highly self-fertile varieties, which "yielded more seed and produced offspring growing taller than their self-fertilised parents, or than the intercrossed plants of the corresponding generation." Such were "Hero" and the white variety of Mimulus, and a pale variety of Dianthus, while equal-styled Primulas proved more fertile than ordinary plants of the same species legitimately fertilised by pollen from a distinct individual. The ratio of the intercrossed opponent to "Hero" was only 100:100.6, but in the eighth generation two other "Heroes" appeared, whose ratios were as 100:111 and 100:150. Their offspring, however, were not preserved.
6. Mutual adjustments of the essential organs, as well as special constructions, are often to be met with, which thus secure self-fertilisation. Mr. Meehan thus describes Melampryum americanum:—"The curved apex of the pistil is clasped by the stamens and held in contact with the pollen, just as in a cleistogamous Violet." The same occurs with small-flowered Epilobiums, while the styles of Malva rotundifolia curl backwards, as H. Müller has described, and it is the same in other small-flowered species, in order to intertwine amongst the anthers. In Salvia clandestina the elongated stigmas curl back in a similar manner, and lie between the anther-cells, which burst, not downwards, as in other species, but sideways, facing the stigma. The position of the filaments in a position arching over the pistil is very common, as in small-flowered Ranunculi, Potentillae, &c. Such positions appear to be often the normal ones while in the bud, and have thus been retained after the flower has expanded. In flowers with a large number of stamens, Buttercups, &c., the outer stamens burst first, but if the pistil be not crossed, then the inner which dehisce later, perform the function.
7. Inconspicuous flowers are very numerous, and, as H. Müller observes, "must be self-fertilising, or they would become extinct;" and it is observable that they form the majority of our weeds, are excessively vigorous and amongst the commonest of plants. Their origin appears to be by degradation from conspicuous conditions, as many of them have irregular flowers, such as Fumaria officinalis and small-flowered Clovers, which could only have arisen originally by insect agency. I do not agree, therefore, with Prof. Dyer, who regards cleistogamous flowers as "probably survivals of the original type" (Nature, Feb. 15, p. 331), for in all cases I consider them as degraded forms of their more conspicuous congeners, but reverting to self-fertilisation. I cannot, therefore, accept Mr. Darwin's conclusion that some plants "have actually had their flowers purposely rendered inconspicuous;" I take it to be simply and purely a result consequent on the absence of insects.
9. With regard to the (acts of intercrossing and of self-fertilisation we must be careful not to confound the means with the end. Crossing does per se no good unless it bring new constitutional elements. This Mr. Darwin clearly proves. But I do not think he seems to see that self-fertilisation is not injurious except in the sense that a plant cannot introduce into itself fresh vigour. But when self-fertilising plants are introduced into other countries, then they may become excessively vigorous, as the British weeds have done in New Zealand.
|*From a list of British plants in New Zealand, by Kirk, in Transactions of New Zealand Institute, 1868, vol. i., p. 157.|
And it is a significant fact that while, with scarcely an exception, such plants are self-fertilising weeds, their conspicuous allies are wanting. Thus Malva rotundifolia has established itself in New Zealand, Society Islands, Sandwich Islands, Abyssinia and Japan, but M. sylvestris and moschata are conspicuous by their absence, excepting the former, which is in Japan, proving that there is no à priori reason why it should not have gone elsewhere. Euphorbia Peplus and helioscopia are present in New Zealand, both of which Mr. Bennett showed were self-fertilizing, but no other species is known. Rosa rubiginosa is the most conspicuous flower, but oddly enough this is the only British Rose which has established itself in America.* Once more, five of the small flowered species of British Stellaria are scattered about the world, one of which, the highly self-fertile S. media or Chickweed, is found in New Zealand, Tasmania, Kerguelen Island, Auckland and Campbell Islands, South Africa, South America, South Australia, Tropical Asia, Hong-kong, Japan, Madeira; but the larger flowered proterandrous S. Holostea is conspicuous by its total absence from any of these far-distant localities. The inference would appear to be that the necessary insects required to cross conspicuous proterandrous species not being there, such plants have died out, if formerly introduced; and the self-fertilising weeds have thus proved themselves "the best fitted to survive in the struggle for life," in accordance with Mr. Darwin's remark on p. 407. "If any entomophilous species ceased to be visited by insects it would probably perish, unless it were rendered anemophilous" [or self-fertilising].
The next four headings (10 to 13) refer to the relative vigour displayed by self-fertilized plants, and as each has been already alluded to in previous papers little need be now added ; I would, however, allude to the case of Lobelia fulgens. Of this plant three pots containing twelve self-fertilised individuals beat their intercrossed opponents in the mean ratio of 116:100, while in five pots all the intercrossed beat their opponents in the mean ratio of 100:73.5—the total mean ratio for intercrossed compared with the self-fertilised being as 100:91, that is, very nearly equality. Now this case, together with that of Hero and the white Mimulus, would seem to show that when a large number of plants are cultivated or grown for many years, self-fertilised plants may arise quite equal, or even superior to the intercrossed; and as many of Mr. Darwin's experiments were made on single or very few generations, and with even less than ten plants, the above lessens very considerably the relative value of such cases.
14. There is a section in chapter viii., p. 303, headed, "On the Transmission of the Good Effects from a Cross, and of the Evil Effects from Self-fertilisation." The test was in the heights of the plants raised by the intercrossing both the previously intercrossed and also self-fertilised plants. Of the fifty-four species cultivated Mr. Darwin selects three only: the first, Nemophila, which unduly favoured self-fertilisation, must be struck out, as Mr. Darwin says of it, " This experiment was quite worthless." The second is Viola tricolor: the previous generation gave the ratio of the heights of the intercrossed to the self-fertilised as 100:42, but of the descendants from both derived by crossing and producing "an abundance of very fine capsules," it was as 100:82. That is to say, the self-fertilised had improved by the cross, but that the intercrossed had inherited its advantage. The third and last example given is Lathyrus odoratus. The first two generations gave the usual ratios as 100:80 and 100:88 respectively, and the ratio of the heights of their offspring (now derived in both cases from an intercross) as 100:90. Hence, the same remark applies to this as to Viola. Mr. Darwin adds:—"These two lots of seeds were like. wise tried by being sown under very unfavourable conditions in poor exhausted soil, and the plants whose grandparents and great-grandparents had been crossed showed in an unmistakeable manner [my italics] their superior constitutional vigour." Turning back to page 159, it appears that some seeds were sown in the same pot with a Brugmansia, others in poor soil in a shady place in the shubbery. The ratio of the heights of the first lot was 100:88, of the others 100:98—that is, practically the same; for Mr. Darwin elsewhere considers 96 to 104 as equivalent to 100. I do not see, therefore, that the results quite justify the above description. The points which are clearly proved are, first, that constitutional superiority gained by intercrossing is transmitted to the offspring; and, secondly, that nothing here tends to prove the descendants of the self-fertilised to be worse off than their progenitors. Indeed, when we read that Mr. Knight's varieties of Peas, originating from a cross, were subsequently propagated in abundance as a marketable product for sixty years, and that solely by self-fertilisation, it is difficult to see what is meant by "the evil effects;" we may rather ask, was not the "cross" somewhat beholden to the power of self-fertilisation to be able to be kept up so long? Then the question suggests itself, Was the dying out of these varieties or of Mr. Laxton's due to degeneracy, or mainly to fresh varieties competing with them in the market and superseding them?
If we regard the undoubted benefits derived by crossing as a positive good, then it would seem fairer to say that self-fertilisation gives purely negative results.
If two people marry who are consumptive, we might in justice speak of the evil effects or injuriousness of the union, as revealed in their consumptive children. But of the great number of habitually self-fertilising plants, which by their vigour and abundance get called troublesome weeds, as Senecio vulgaris, Cardamine hirsuta, and Stellaria media, &c., nothing can possibly be said which justifies those terms. Hence the heading above quoted should have been "On the Transmission of the Good Effects from a Cross" alone, or with the addition, "and of the correlative negative results of self-fertilisation."
The general inference appears to be this, that self-fertilisation is per se not injurious in the ordinary sense of the term, as implying, for example, weakness of constitution, but only in that a plant cannot introduce by that agency fresh constitutional elements; such being the case, the average vigour of such plants remains stationary. If, however, it can acquire such, in nature by migration to a different locality, artificially by made soil, then the self-fertilized may completely outstrip the intercrossed and beat them in every way. The sole, but doubtless a great, advantage of crossing, lies in such being an important means of introducing fresh constitutional peculiarities. On p. 438 Mr. Darwin observes, "That certain plants, for instance Viola tricolor, Digitalis purpurea, Sarothamnus scoparius, Cyclamen persicum, &c., which have been naturally cross-fertilised for many or all previous generations, should suffer to an extreme degree from a single act of self-fertilisation is a most surprising fact." The inference, however, I would draw is, that these plants probably represent a condition approximating absolute self-sterility, and one generation is not enough for them to recover their full self-fertility; absolute self-sterility having been reached by some species, the number of which "is not at present large" (p. 341). Hence I would group both kinds under the one common cause of sexual differentiation, due to insect agency. Viola seems to show—if I may express it metaphorically—that Nature found she had gone a little too far, and ran the risk of having no offspring at all, and so adopted cleistogamous flowers as well.
This leads me to contest another of Mr. Darwin's conclusions. On p. 455, in the passage beginning "It is an extraordinary fact" [to end of paragraph], he regards the more or less self-sterility of many species as due "to the sexual elements not having been sufficiently differentiated;" but in Animals and Plants under Domestication, vol. ii., p. 140, he gives, what seems to me, the correct explanation, that "the sexual elements of the same flower have become differentiated in relation to each other, almost like those of two distinct species," and he further adds, in direct opposition to his present work:—"We may conclude that it has been naturally acquired for the sake of effectually preventing self-fertilisation." This he now rejects (see p. 345).
When we consider what the plants are which are thus absolutely or more or less self-sterile, we find them scattered about and in no way of—in other respects—low type, as correlation would, à priori, lead us to expect; e.g., Eschscholtzia, Corydalis, Reseda, Lobelia, Verbascum, Passiflora and Orchids, as Oncidium, &c. Now these have either allied species partially or quite self-fertile, or may often become so on changed conditions, that instead of their sexes being not sufficiently, I should prefer Mr. Darwin's former explanation that they were too highly differentiated, and that when they become self-fertile, as Eschscholtzia in England, it is a recovery of, or reversion to, self-fertilisation. Such is my impression: readers can now form their own conclusions as to which they may think is the more probable.