Journal of Horticulture and Cottage Gardener, 32-34 (July 11, 1872)
THE RELATIVE INFLUENCE OF PARENTAGE IN FLOWERING PLANTS
J. Denny, M.D.
One of the chief objects of my paper is to urge the study of a subject full of scientific interest, and of the greatest importance to the practical horticulturist, but which for the want of the accumulation of data derived from accurate experiments, at the present is involved in much obscurity.
If we could by the observation of results acquired through the medium of a series of carefully performed experiments in artificial fertilisation, obtain any reliable evidence indicative of the relative influence the male (or pollen) and the female (or seed) parents bear in the production of their progeny, it would assist us immensely in carrying out our own designs for the improvement in the form and colour of our flowers, and the quality of our fruits and vegetables.
If, for instance, we knew that either parent, and which, was prepotent in conveying to its offspring certain qualities, say of flavour and aroma, or of size and form, or of quality as regards the texture of our fruits; of colour, perfume, form, substance, and the various qualities we may wish to perpetuate or modify in our flowers, we should be able to form some proximate idea, á priori, of the result that would follow our fertilisations.
A knowledge, too, of the ancestry of the varieties we purpose employing would also be desirable, to enable us to make allowances for the modifications likely to ensue from the tendency to reversion towards an ancestral type—a propensity which seems to be inherent in all plants that have been much changed from their original state by artificial breeding.
It would also be a matter of scientific interest, as well as of practical importance, perhaps, to know if the proportionate influence borne by the respective parents in crossing varieties is the same as in crossing species?
Whether, as the admission of fecundation is no test of the plants employed belonging to the same species, we have any well-defined line of demarcation or practical test by which wa can distinguish between species and varieties, so that we may know when to employ correctly the term hybridisation, and when cross-breeding?
Whether there exists any real difference in the powers or quality of the pollen of the long and short stamens from which we may expect to derive any specific effect on the progeny by the exclusive employment of the one or the other, or to succeed more readily in effecting difficult crosses?
Whether certain states of the atmosphere, and, if so, what apparent conditions of it, favour fecundation?
Whether any clue can be obtained, or suggestions offered, to account for the antipathies that are found to exist between apparent varieties, as well as affinities between what are considered by botanists to be distinct species, precluding fertilisation in the former, and rendering it easy in the latter?
These are a few of the most important points that are constantly occurring to the practical horticulturist. To how many of them does our knowledge admit of a satisfactory reply being given?
There are, doubtless, many present whose vast practical experience in the artificial fertilisation of our fruits, flowers, and shrubs would enable them to give most valuable information upon most of these points; and as the purport of this meeting, I take it, is intended to be for the genial discussion and for the interchange of knowledge and ideas, I trust to the generosity of those who are able to assist me in making the subject I have ventured to broach interesting by throwing more general light upon it than my circumscribed experience will afford.
From early youth I have taken much interest in artificial fertilisation, but kept no registered account of my crosses, or their results, until the controversy arose respecting the tricolored Pelargoniums, as to whether their leaf-markings could be reproduced by fertilisation and seed, or whether they were sports only, and owing to a diseased condition of the plant.
To ascertain for my own satisfaction the correct theory upon these points, as well as with the object of obtaining, if possible, some information regarding the relative powers the respective parents exert over their progeny, I commenced a series of experiments upon the scarlet section of the Pelargonium, employing varieties of the most opposite and varied character, and crossing them in every conceivable way.
I conducted these experiments, too, with the utmost possible care and minuteness of detail, both as regards the methods I adopted for preventing self or insect fertilisation, for ensuring the fertilisation being effected by the desired pollen only; and as regards the keeping an exact register of every cross, as well as a record of the results.
By this means I soon arrived at a satisfactory conclusion as regards the points at issue respecting the transmission of variegation of the foliage by fertilisation, from the fact of its being manifested to a greater or less degree in as large a proportion as from 50 to 60 per cent. of the offspring, where the green Zonal had been fertilised by the pollen of the variegated; I also obtained some valuable information indicative of the powers the respective parents exert upon various other points in connection with the transmission and modification of the foliage and habit of the plant, as well as of the colour and form of the flower.
From the information thus derived, I am of opinion that by careful and persistent fertilisation, under the guidance of the observation of results, it is possible to produce almost any modification in the character and habit of our plants, and variety of colour and form in our flowers we might desire; for I am satisfied that by these means we possess a much greater power of moulding our flowers in accordance with preconceived design than is generally supposed; and, moreover, I think it possible that ultimately some insight may be obtained into the working of the laws that govern procreation in the vegetable kingdom, and that produce variation in our fruits and flowers.
The result of my experience derived from these experiments, as regards the relative influence of the parents, certainly tends in the reverse direction to my previous ideas, which were derived from books, from which I gleaned that the form of the flower and constitution and habit of the plant were inherited from its mother, whilst the colour of the flower only was supposed to be conveyed by the father. The recorded results of my crossings indicate an immense preponderance of influence over the progeny on the part of the father, in all respects—in colour and in form, in the quality, in size, and substance of the flower, as well as in the production of variegation of the foliage, and in the habit and constitution of the plant also, provided the plants employed are of equal strength.
I wish to be distinct upon this point of relative strength of the parents, because it seems to me that upon the equality or the preponderance of strength on either side very much hinges, as regards the results we obtain from our crossings, for power of constitution exerts most unmistakeable influence, and where it preponderates on the part of the seed parent it will modify the otherwise prepotent influence of the pollen parent. This modifying influence manifests itself most as regards the habit and foliage of the plant, and next as regards the form and substance of the flower, and lastly as regards the production of a blend in the colour of the flower.
To instance what I mean (I am alluding to the Pelargonium), if the pollen of a flower of brilliant and decided colour, but of bad form and substance, belonging to a plant of weakly constitution, be applied to the stigma of a finely formed thick-petalled flower of a plant possessing a vigorous constitution, some few of their progeny will be influenced towards improvement in the form and substance of the flowers and habit of plant, with, perhaps, some blend in the colour, showing that the preponderance of vigour in the seed parent had exerted a certain amount of influence; but even under these circumstances much the greater proportion of the progeny would either resemble the father in all respects, or show reversion towards former progenitors, or an original type.
I will quote a case or two in point from my note-book. During the summer of 1869 I raised about 140 seedlings from crossings between Lord Derby and Leonidas. In about half of these Lord Derby was the pollen, and Leonidas the seed parent; and half resulted from crosses effected the reverse way. The flower of Lord Derby possessed the fine qualities, both as regards form of petal and smoothness of texture, but was wanting in depth and brilliancy of colour, and in substance also; and the plant was deficient in vigour of constitution as compared with Leonidas.
The flower of Leonidas was much inferior as regards form and quality, but of greater substance and brilliancy of colour, as well as larger than Lord Derby; and the plant possessed a vigorous constitution.
These seedlings flowered during the spring and summer of 1870 of that portion in which Lord Derby was used as pollen parent, and Leonidas as seed parent, about one-third resembled in all respects their father, a few produced flowers very considerably in advance of Lord Derby in size, substance, and in colour of the flower, and with a superior constitution and habit of plant, showing the influence of the mother in combination with the father's. (I would instance Sir Charles Napier as an example, and which resulted from this cross). Of the remaining two-thirds, a few very nearly resembled in flower Leonidas except being paler in colour and having a somewhat increased breadth of petal, resulting from the father's influence (for instance, Iago); but a large proportion were inferior, showing reversion towards an ancestral type.
Of that portion in which Leonidas was used as pollen, and Lord Derby as seed parent, nearly half resembled in all respects their father, and the rest were much inferior; not one showed that any appreciable amount of influence had been exerted by the mother towards improvement. It will be observed that in this cross the pollen parent possessed both the inferior flower and the most powerful constitution also. As regards the habit of these seedlings, they were all more robust than their mother's.
The same season I raised about sixty seedlings from a cross between Celestial and Lord Derby. Celestial, which was used as pollen parent, possessed a brilliant magenta-coloured flower, but of very bad form and substance, and possessed a weakly constitution; from this batch of seedlings a few produced flowers of a colour very similar to their father's, but somewhat less brilliant, and with a great improvement as regards the form, quality, size, and substance of the flower, accompanied, too, with a fair habit and constitution of plant, showing a marked influence on the part of the mother, which in this cross was decidedly the stronger of the two parents. Ianthe resulted from this cross. The remainder of this batch were mostly of very bad form and quality of flower, and weakly constitutions; but there were some very brilliant and novel colours, interesting examples of colour blending, amongst them were carmine, rose-crimson, pinks, and vivid scarlets—some in all respects resembled Celestial.
My large seedling Nosegay Wellington was the result of a cross between Le Grand (Nosegay) and Leonidas, Le Grand being used as pollen parent. Here the plants were about equally vigorous. "Wellington resembles in the character of its flower its father, but with an increased breadth of petal derived from its mother; the colour of the flower is nearly that of the father's also, but it is somewhat a blend, the purple hue of Le Grand and the deep scarlet of Leonidas having produced a very dark crimson scarlet, almost maroon. The foliage, too, of Wellington is most distinctly of the Nosegay type; its habit still more vigorous than either parent.
In breeding for variegates, and using the variegates (which, as a rule, are wanting in vigour) as pollen parents, and the robust green Zonals as seed parents, about half the number of their progeny showed variegation, and possessed weakly constitutions, the remainder being green Zonals; upon the order of procedure being reversed, by which the pollen parent became the parent of very much the greater vigour, the mother's influence was almost nil.
I believe that it is owing to the existence of a difference in the vigour of the respective parents that the production of novelties and varieties in our flowers (and probably in our fruits too) mainly depends, and that were it not for a preponderance of power on the mother's side, the progeny would almost invariably resemble the father; and hence the immutability of our flowers and vegetables, which are annually reproduced from seed, the result of self-fertilisation.
But I consider another source of the production of novelties and variation exists in the tendency in all flowers (and fruits) that have been artificially bred up to a state far in advance of their original condition, to revert towards former progenitors (especially under the influence of self-fertilisation), by which means new combinations of ancestral properties are formed, and hence new varieties.
Even under artificial fertilisation I find in the Pelargonium this tendency to reversion to exert very considerable modifying influences. Especially have I observed it as regards the colour of the flower; for instance, the magenta shades that have been produced upon the scarlet Pelargoniums have resulted from the crossing of pinks upon scarlets; and very many of my seedlings, the offspring resulting from the crossing of two magenta-coloured flowers, have produced pink ones as well as scarlets, showing reversion to both the colours of their immediate ancestors.
It is a point worthy of observation whether the colour of a flower, or a change in the character of a plant that has been recently obtained, are conveyed to their offspring in the same proportion as to numbers, and with the same certainty as those of long standing. I think not.
I must also mention a remarkable instance of reversion as regards foliage that has occurred in two of a number of seedlings raised this spring from Violet Hill Nosegay as seed parent, crossed by Ianthe, with the object of obtaining variety in the spring of 1864, with a view to its breaking into variegation, but which it did not do, but was selected, and subsequently sent I out, for its flower, and on account of its dwarf habit of growth.
My notes would furnish innumerable examples in support of the theories I have founded upon them, did time admit of my going further into detail. I would observe that I have purposely quoted the results of crossings which produced varieties that have been sent out by Mr. W. Paul, that they might, if desired, be referred to, and compared with their parents.
A close analogy seems to me to exist between the vegetable and the animal kingdoms as regards the ill effects produced by breeding in-and-in, and the good resulting from crossing opposites, for I find it to be necessary for the maintenance of improvement in the flower and the constitution of my seedlings, to introduce fresh varieties to breed from annually; and I find that crossing two flowers of the finest qualities does not produce such satisfactory results as where one of much inferior quality is employed. Of course it will be inferred from my previous observations that I use the superior-quality flower as pollen parent.
I am of opinion that the decadence in many of our old florists' flowers is owing to their having been bred in-and-in, and from the repeated crossing of flowers of a precisely similar strain and qualities, with the object (and probably supposed only means) of reproducing flowers possessing certain peculiarities in markings, or form, in accordance with the rigid rules prescribed for these flowers.
As regards the condition of the atmosphere that favours the effecting of difficult crosses, it would be no easy matter to note with any degree of certainty the precise period of each successful attempt, nor the precise condition of the atmosphere at the time; we read of special crosses having been effected under certain conditions of it, but I have never seen it specified what these conditions were. My experience indicates that bright clear weather, and the hours of sunshine, are conducive to fecundation.
I have alluded to the antipathies and affinities we find to exist, without any explicable cause; for instance, I have found it impossible to fertilise three or four varieties of the scarlet Pelargonium—viz., the Duke of Cornwall, Dr. Muret, Beauté de Suresnes, and all that section of the doubles which sprang from Beauté de Suresnes, which to all appearance are mere varieties of the Zonal section, save with one another; and, showing the existence of affinity between what are supposed to be distinct species, I have fertilised without much difficulty a variety (peltatum elegans) of the Ivy-leaved section by the pollen of the Zonal.
I have also alluded to the possible difference in the respective influence of the parents in true hybridisation. Upon this point I have not sufficient evidence to form a fair opinion; but certainly in the seedlings I have raised between the Ivy-leaved and the Zonal sections, their foliage (with the exception of some distinctive evidence of their being hybrids) resembles almost entirely that of their mother, which you will observe is the reverse of my experience of the results produced between varieties.
Much has been written and said upon the difference in the quality and powers of the pollen of the short stamens; and if the supposed difference really does exist, it is a matter of considerable practical importance, and one worthy of further scientific investigation; but my experiments have hitherto failed to satisfy me of their possessing any difference.
In an admirable article upon hybridisation, written by Isaac Anderson-Henry, Esq. (and which at different periods has appeared in nearly all the horticultural journals), he says, "that, owing to the granules of the short stamens being smaller than those of the long ones, they can the more easily descend the tubules leading from the stigma to the ovaries, and consequently facilitate the crossing of a large-flowered variety, or species, upon a smaller one."
I have not been able to detect this difference in size, although I have many times placed the granules of the long and short stamens side by side under a powerful microscope; nor, I believe, is it the opinion of physiologists of the present day that they do descend these tubules at all—in fact it has been shown that they send down filaments through them to the ovules.
The arrangement of the anthers upon filaments of different lengths looks to me like a provision to ensure all parts of the body and legs of the insect coming into contact with the pollen as it passes down the flower to obtain the nectar, thereby rendering the fertilisation of the next flower it visits the more certain.
The visible effects of impregnation are frequently manifested with a rapidity almost equalling that of an electrical phenomenon. I have observed the petals of the Pelargonium which, before impregnation, were quite firm, to fall within a few seconds of the application of the pollen to the stigma—a result due, I conclude, either to the immediate diversion of nourishment from the then superfluous part of the flower to the organs of generation, or to the existence in the vegetable kingdom of a power analogous to the nervous in the animal, but of which we are as yet in total ignorance.
Lastly, I would remark that, to enable reliable conclusions to be drawn upon any of these points, we require an accumulation of data derived from the careful observation of very many unbiased workers, whose results have been obtained from experiments conducted with scientific precision upon all our flowers and fruits.
Such an accumulation of recorded facts, if they could be obtained, would prove a source of the greatest interest to the philosopher, by their tendency to throw light upon the working of Nature’s laws, and could not but afford most valuable information for the guidance of the practical horticulturist; and moreover, by freeing horticulture from all empiricism, place it in its true and legitimate position among the modern sciences.
[With regard to the influence of the size of pollen grains upon hybrid-formation, the following seems to be a case in point. It is quoted from the Report of the Massachusetts Horticultural Society for 1872, p. 184:—"Mr. Wylie found that generally the pollen grains of grapes were of oblong form, while those of the Scuppernong, the great wild grape of the South, were smaller and more spherical. He found that he could not fertilise the Scuppernong with pollen from other species, but he did succeed in impregnating the foreign grapes with pollen from the Scuppernong. His inference was that the pollen-grains being smaller in the Scuppernong than in other varieties, the canal through which they have to pass to reach the ovule in that species is also smaller, and thus he explained the results of his experiments. The smaller grains could pass through the large tube, but the larger grains could not pass through the small tube." There is the same misapprehension as to the part played by the pollen-grains as is alluded to above, but this does not affect the fact stated.—EDS.]