THE ORIGIN OF SPECIES IN NATURE, AND SUGGESTIONS FOR
EXPERIMENTS TO INDUCE VARIETIES TO ARISE UNDER CULTIVATION.
By the Rev. Prof. G. HENSLOW, M.A., F.R.H.S., V.M.H.
[Lecture delivered at the Gardens of the Royal Horticultural Society, Chiswick, July 13, 1898.]
INTRODUCTION.—WHAT IS A "SPECIES"?—It is a term used by botanists to indicate a certain amount of differences between one and another kind of the same genus. Hence a species is known by a collection of morphological characters, presumed to be constant, and taken from any or all parts of a plant. A variety only differs from a species in having a less amount of differences. There is no recognised standard as to the amount or number of differences which separate varieties from species; hence systematic botanists have greatly differed in their use of these terms. The point to be remembered is that neither one nor the other is a fixed entity in nature; but a so-called variety or species can change its form under altered circumstances; and the direct cause of changes of form or of variations of structure in plants is a change of environment.
|*Hooker's Student's Flora, p. 346.|
A good example of "forms" being assumed by a common plant under different soils, &c. may be seen in the knot-grass (Polygonum aviculare, L.). Thus Sir J. D. Hooker describes the varieties:—"P. aviculare, L., proper; P. littorale, Link, Littoral; the passage to P. maritimum, L., sea-shores; var. agrestinum, Jord., the common robust field form; arenastrum, Boreau, a sand-loving prostrate one; microspermum, Jord., a small fruited one; and rurivagum, Jord., a wayside one; subspecies, P. Roberti, Loisel, sandy shores."*
In Nature, new varieties, Sir J. D. Hooker observes, are mostly found on the confines of the geographical area of the species: and when plants are grown for experiment in widely different regions, they are generally found to lose their special features, and to take on those of the plants among which they now live; so that lowland forms assume alpine or arctic features when grown at high altitudes and latitudes. Plants of arid districts gradually lose their spiny and poverty-stricken appearances when grown in a rich and moist soil.
|*Gard. Chron., 1896, p. 586. Figs. 93, 94.|
Hence it is obviously more likely that one would induce plants to vary by transferring them to as different external conditions as possible. Mr. Elwes informed me that the many bulbous plants he brought from the East change so in all their parts in his garden, that they can scarcely be recognised after three or four years; as, e.g., Tulipa Kolpakowskyana.* Of course, great differences exist in the natural capacity of plants to change; some are very refractory, others supply numerous cultivated varieties ; but every experience tends to show that all plants can vary if a sufficiently active environment be provided to call out their latent powers of response.
ILLUSTRATIONS OF RAPID CHANGES IN STRUCTURE.
From Dry to Moist Conditions.—One of the most marked and comparatively sudden alterations of structure that take place on a change of environment is seen in that of inhabitants of dry, poor soils with a dry atmosphere, when they are removed to a moist one. Thus a common feature of not a few plants of the former condition is to be spinescent; whether the spines be branches, as in the Rest-harrow, or leaves, as in the Barberry. Experiments have shown that if the Rest-harrow (Ononis spinosa) be grown, either from seed or from cuttings, in a moist soil and atmosphere, the spines soon cease to be formed, and the plants assume more or less the character of the wild and spineless form, O. inermis or O. repens. Similarly the leaf-spines of the Barberry will develop out into true leaves under similar conditions; while hairiness, a characteristic of drought, disappears.
Analogous results have occurred when wild plants bearing spines have been cultivated in, of course, a good soil: when they become non-spinescent, as Pears and Plums and some Roses.
From an Aquatic to a Land Soil.—Many plants are amphibious, i.e., though usually aquatic and wholly or partly submerged, they can grow on land equally well by adapting the minute structures of their roots, stems, and leaves to either medium, air or water. If they be transferred from one to the other, then all the submerged foliage dies, but new foliage is immediately developed suitable to the changed medium.
A common cultivated example is seen in Richardia Aethiopica, usually grown in pots, but it is an aquatic plant in its native habitat.
There is no apparent reason why water-lilies should not be grown in a garden border; if the experiment be made, either by sowing the seed or by gradually adapting the thick rhizome to put out suitable roots, by supplying it with a wet soil at first. The experiment is worth trying, either with water-lilies or any other aquatic plants which might be thought suitable for the garden.
As water has the effect of producing degeneration of the tissues in aquatic plants as compared with land plants, they often grow stronger when on land than when in their normal condition under water.
Changes between an Erect and Prostrate Habit.—The erect habit of growth is common with plants growing thickly together, but if they are isolated on an exposed surface they will often assume a prostrate habit. This is due to the ground being warmer than the air above it. We may describe this tendency by the term "thermotropism," i.e. "a turning heatwards." This response to an inequality of temperatures will account for the plants acquiring a prostrate habit.
Thus Malva sylvestris, the common Mallow, when growing in shady places with other herbs will be erect, but on the roadsides it becomes perfectly prostrate. It is so prevalent in this condition on the limestone of Malta, that it has been named M. Nicaeensis, but it is simply a prostrate and more hairy form of the common Mallow. A very familiar example is seen in the lesser Bindweed, Convolvulus arvensis, for this plant is a true stem climber when growing among other erect plants, but assumes a thoroughly prostrate habit when growing on banks by roadsides, &c.
Similarly a prostrate form is characteristic of high Alpine plants, and when lowland plants are grown in those regions they, too, then become prostrate in habit.
Fleshy Types.—Many seaside plants are remarkable for the fleshy character of their leaves and stems, as Plantago maritima, Samphire, &c. This is due to the presence of salt, and it can be imitated by watering inland plants with a saline solution. Hence it is customary to give salt to some maritime plants which are cultivated, as Asparagus and Sea Kale; while all the cabbage tribe would doubtless be assisted by it, if it were thought necessary, as they are natives of sea-cliffs; but they retain their fleshy character by heredity.
Several plants growing at Bad Nauheim, 200 miles from the nearest sea-coast, have acquired a fleshy texture in consequence of the abundance of saline waters there.
Variegation.—This phenomenon is produced by several causes; though it is not quite clear why some plants with a variegated foliage may grow in the same soil with others not variegated.
The general absence of colour has been called "Chlorosis," and the disease appears to be the result of the absence or deficiency of certain ingredients in the soil requisite for a vigorous growth and a fully green colour.
Thus, a variegated strawberry remained constant so long as it grew in a dry soil; but when it was transferred to a cold or moist one, its variegation quickly disappeared. A variegated laurel grew well in a not very deep soil for three years, but when the roots could penetrate into the sub-soil composed of chalk, the leaves became green again.
Professor A. Church has investigated the subject, and finds that Chlorosis may be divided into four groups—(1) Etiolation, due to insufficient light; (2) Albinism, when there is a relative excess of potash and deficiency of lime; (3) Icterus, due to a deficiency of iron; (4) Wheat-yellow, on account of a deficiency of potash.
Mr. Penhallon found that Peach-yellow was due to a deficiency of magnesia.
Following these discoveries, it seems obvious that experiments might be made in which soils deficient in the above-mentioned ingredients might be used, to see if variegation could not be induced. As, however, all cases of variegation are abnormal and unhealthy conditions, it is probable that a permanency would be difficult to secure, unless the soil remained of the same character to produce it. Still, as M. Carrière observes of plants, "everything tends to become hereditary," therefore variegations may in time become so fixed in the constitution, that they might not disappear even in a good soil. Thus coleus, hollies, pelargonia, &o., appear now to be pretty well fixed in the various colorations of their foliage.
Dwarfing.—There are many more dwarf annuals than perennials in cultivation. This is only because the latter are not usually raised from seed.
On the appearance of a dwarf, it is necessary to isolate it; so that it be not crossed with taller ones. Then one must keep selecting seed from the shortest of the seedlings, till the "nanism" be fixed. This fixing varies from one to six years; but it is not known why there should be this variation in time.
The methods of producing dwarfs are possibly several. The following have been suggested. Bearing in mind that the object is to "arrest vegetative growth," anything that will do this may produce dwarfing, not only in an individual, but in its progeny.
By autumn sowing (Aug.-Sept.): When it is too late for a plant to flower, it produces a more compact vegetation. If it be sown in spring, successive prickings out and transplanting, so that each plant grows freely, will result in strong thick-set plants. "This process will favour the development of the lower ramifications at the expense of the main stem; we thus create an individual, comparatively dwarf. If now we collect seed from plants thus grown, and if we give the came treatment to them as to their parents, we shall obtain year after year 'plants which we shall have made to develop a certain tendency to nanism.' That is, after some years, they will be more apt to produce dwarf varieties." Verlot adds that the greater number of cultivated dwarfs were of varieties sown in autumn; or if in spring, they have been subjected to successive transplanting. Of the first he mentions eleven varieties, such as Calceolaria plantaginea, Senecio cruentus (garden cineraria), Oenothera Drummondii, Scabiosa atropurpurea, Iberis umbellata, &c.
Of those sown in spring-time, he mentions Impatiens balsamina, Callistephus sinensis, Tagetes patula, T. erecta, and T. signata.
With regard to the procuring of dwarfs by fecundation of flowers, ordinary crossing has usually an opposite tendency in making the offspring more vigorous; but Mr. McNab found that the best dwarf varieties of Rhododendron were obtained by using pollen taken from the anther of the shorter stamens.
As numerous irregular flowers have stamens of different lengths, this experience opens out a new field for experiments with Labiatae, Scrophularineae, Leguminosae, &c.
M. Verlot observes that when plants are cut back early to make dwarf plants of them, though it may usually only affect the individual plant, yet he thinks that if it be habitually submitted to this treatment, the seeds are subsequently more likely to give rise to dwarf plants.
As an illustration, the writer has lately had occasion to notice a tennis lawn, part of which was returfed from a field last winter. The whole has been left to grow uncut. The result is that while the new turf rapidly grew tall, as it would have done in the field if left for hay, the old lawn-turf and other flowering plants mixed with the grass have remained more or less in a much dwarfer condition.
Similarly, Mr. Veitch found that cuttings from the miniature trees made by the Japanese and struck in a border refuse to grow. If, however, they be grafted on other and vigorous plants of the same kind, they then grow out vigorously. It would be interesting to see if seeds of such tiny trees produce dwarfs also.
It would seem, therefore, probable that whatever causes tend to check growth, if persisted in long enough, may in time have an hereditary effect.
It would be therefore advisable to try experiments besides the repeated pricking out alluded to:— (1) Reducing the roots; (2) reducing the foliage; (3) cutting off the terminal shoots; (4) selecting small seeds; (5) crossing with pollen from the smaller stamens, wherever there is an inequality; (6) using pollen from the smallest flowers on the plant; (7) poor soil.
Double Flowers.—These result from various alterations in the structure of flowers, coupled with an increase in the number of petals. The question is, what are the causes which induce the production of double flowers? M Verlot observes: "A rich soil, a culture inducing a luxuriant vegetation, are those under the influence of which we see duplication generally to arise in our gardens."
On the other hand, Mr. Barren observed that: "Double flowers growing on a sandy soil at Sutton keep truer to doubling than on a wet, heavier soil at Chiswick." Mr. Wolley Dod corroborates this fact, for he says that his own cold and wet soil tends to make his double daffodils to become single.
Mr. Darwin, some fifty-five years ago, noticed and described, in the Gardeners' Chronicle (1848, p. 628), some double flowered Gentiana Amarella, "which grew on a very hard, dry, bare, chalk bank." Similarly he found on an adjoining field of "wretchedly sterile clay great numbers of Ranunculus repens, producing half-double flowers." He then asks the question, "Is it, then, too bold a theory to suppose that all double flowers are first rendered, by some change in their natural condition, to a certain degree sterile?" When a double-flowering plant has this affection well fixed in its constitution, then it would seem that it is benefited by a rich soil; "petalody" having set in, it may affect every part of the flower—stamens first, then pistil or calyx, and finally the petals may be multiplied indefinitely, so that a flower of the double stock may contain more than fifty petals.
That the petalody can be "in the blood," so to say, is seen from the fact that, as no seed can be raised from a "perfectly" double stock, they can be procured from the "single" flowers. For by suppressing the anthers of flowers before they shed their pollen, the seeds (M. Verlot observes) developed in the ovaries of these flowers produce double-flowering plants with great facility—viz. 60 to 70 per cent. If the anthers be not removed, then the percentage drops to 20 to 30 of double-flowering offspring, the number of seed being reduced to five or six in a pod, which produced double-flowering plants, instead of from forty to fifty.
As another influence, that of age may be mentioned. Thus, seed of Matthiola annua, sown immediately after being gathered, produced few double-flowering plants; while seed three to four years old produced many. Wallflowers gave similar results.
Yet another fact may be mentioned which bears out the same contention. It is found that old, strong root-stocks of Dahlias produce strong growing plants, but they do not "double" well. Heavy foliage and rich colouring are, as a rule, adverse to doubling.
The conclusion to be drawn from the above facts is that it is not a rich soil which first induces doubling, but a poor one; but let the doubling be once thoroughly set up in the plant's constitution and it then seems that a rich soil will probably enhance it.
As soon as the slightest indication of petalody of the stamens has appeared, by one or more of them having a minute petal-like appendage; then that particular flower in which the change has occurred must be fertilised with its own pollen, all other pollen being rigidly excluded. The progeny will, in all probability, prove to be semi- or quite double. Such was the experience with Mr. Heal, who raised the Balsamae-flora section of the East Indian Greenhouse Rhododendrons in this manner.
I will conclude by quoting a passage from Bacon's "Naturall History," Century vi. § 513. "It is a curiosity also to make Flowers double, Which is effected by Often Removing them into New Earth; As on the contrary Part, Double Flowers, by neglecting, and not Remouing, proue Single. And the Way to doe it speedily, is to sow or set Seeds, or Slips of Flowers; and as soone as they come vp, to remoue them into New Ground, that is good."