Part 22

Professor Weldon refers to no experiments of his own and presumably has made none. Had he done so he would have learnt many things about dominance in peas, whether of the yellow cotyledon-colour or of the round form, that might have pointed him to caution.

In the year 1900 Messrs Vilmorin-Andrieux & Co. were kind enough to send to the Cambridge Botanic Garden on my behalf a set of samples of the varieties of Pisum and Phaseolus, an exhibit of which had greatly interested me at the Paris Exhibition of that year. In the past summer I grew a number of these and made some preliminary cross-fertilizations among them (about 80 being available for these deductions) with a view to a future study of certain problems, Mendelian and others. In this work I had the benefit of the assistance of Miss Killby of Newnham College. Her cultivations and crosses were made independently of my own, but our results are almost identical. The experience showed me, what a naturalist would expect and practical men know already, that a great deal turns on the variety used; that some varieties are very sensitive to conditions while others maintain their type sturdily; that in using certain varieties Mendel’s experience as to dominance is regularly fulfilled, while in the case of other varieties irregularities and even some contradictions occur. That the dominance of yellow cotyledon-colour over green, and the dominance of the smooth form over the wrinkled, is a general truth for Pisum sativum appears at once; that it is a universal truth I cannot believe any competent naturalist would imagine, still less assert. Mendel certainly never did. When he speaks of the “law” or “laws” that he has established for Pisum he is referring to his own discovery of the purity of the germ-cells, that of the statistical distribution of characters among them, and the statistical grouping of the different germ-cells in fertilization, and not to the “Law of Dominance” which he never drafted and does not propound.

The issue will be clearer if I here state briefly what, as far as my experience goes, are the facts in regard to the characters cotyledon-colour and seed-shapes in peas. I have not opportunity for more than a passing consideration of the seed-coats of pure forms; that is a maternal character, a fact I am not sure Professor Weldon fully appreciates. Though that may be incredible, it is evident from many passages that he has not, in quoting authorities, considered the consequences of this circumstance.

The whole question as to seed-coat colour is most complex. Conditions of growth and ripening have a great effect on it. Mr Arthur Sutton has shown me samples of Ne Plus Ultra grown in England and abroad. This pea has yellow cotyledons with seed-coats either yellow or “blue.” The foreign sample contained a much greater proportion of the former. He told me that generally speaking this is the case with samples ripened in a hot, dry climate.

Unquestionable Xenia appears occasionally, and will be spoken of later. Moreover to experiment with such a plant-character an extra generation has to be sown and cultivated. Consequently the evidence is meagre.

The normal characters: colour of cotyledons and seed-coats.

Culinary peas (P. sativum, omitting purple sorts) can primarily be classified on colour into two groups, yellow and green. In the green certain pigmentary matters persist in the ripe seed which disappear or are decomposed in the yellow as the seed ripens. But it may be observed that the “green” class itself is treated as of two divisions, green and blue. In the seedsmen’s lists the classification is made on the external appearance of the seed, without regard to whether the colour is due to the seed-coat, the cotyledons, or both. As a rule perhaps yellow coats contain yellow cotyledons, and green coats green cotyledons, though yellow cotyledons in green coats are common, e.g. Gradus, of which the cotyledons are yellow while the seed-coats are about as often green as yellow (or “white,” as it is called technically). Those called “blue” consist mostly of seeds which have green cotyledons seen through transparent skins, or yellow cotyledons combined with green skins. The skins may be roughly classified into thin and transparent, or thick and generally at some stage pigmented. In numerous varieties the colour of the cotyledon is wholly yellow, or wholly green. Next there are many varieties which are constant in habit and other properties but have seeds belonging to these two colour categories in various proportions. How far these proportions are known to be constant I cannot ascertain.

Of such varieties showing mixture of cotyledon-colours nearly all can be described as dimorphic in colour. For example in Sutton’s Nonpareil Marrowfat the cotyledons are almost always either yellow or green, with some piebalds, and the colours of the seed-coats are scarcely less distinctly dimorphic. In some varieties which exist in both colours intermediates are so common that one cannot assert any regular dimorphism.

Knowing my interest in this subject Professor Weldon was so good as to forward to me a series of his peas arranged to form a scale of colours and shapes, as represented in his Plate I. I have no doubt that the use of such colour-scales will much facilitate future study of these problems.

There are some varieties which have cotyledons green and intermediate shading to greenish yellow, like Stratagem quoted by Professor Weldon. Others have yellow and intermediate shading to yellowish green, such as McLean’s Best of all. I am quite disposed to think there may be truly monomorphic varieties with cotyledons permanently of intermediate colour only, but so far I have not seen one. The variety with greatest irregularity (apart from regular dimorphism) in cotyledon-colour I have seen is a sample of “mange-tout à rames, à grain vert,” but it was a good deal injured by weevils (Bruchus), which always cause irregularity or change of colour.

I notice that Vilmorin in the well-known Plantes Potagères, 1883, classifies the intermediate-coloured peas with the green.

Similarly though tall and dwarf are Mendelian characters, peas occur of all heights and are usually classified as tall, half-dwarfs, and dwarfs.

Lastly in some varieties there are many piebalds or mosaics.

From what has been said it will be evident that the description of a pea in an old book as having been green, blue, white, and so forth, unless the cotyledon-colour is distinguished from seed-coat colour, needs careful consideration before inferences are drawn from it.

Shape.

In regard to shape, if we keep to ordinary shelling peas, the facts are somewhat similar, but as shape is probably more sensitive to conditions than cotyledon-colour (not than seed-coat colour) there are irregularities to be perhaps ascribed to this cause. Broadly, however, there are two main divisions, round and wrinkled. It is unquestioned that between these two types every intermediate occurs. Here again a vast number of varieties can be at once classified into round and wrinkled (the classification commonly used), others are intermediate normally. Here also I suspect some fairly clear sub-divisions might be made in the wrinkled group and in the round group too, but I would not assert this as a fact.

I cannot ascertain from botanists what is the nature of the difference between round and wrinkled peas, though no doubt it will be easily discovered. In maize the round seeds contain much unconverted starch, while in the wrinkled or sugar-maize this seems to be converted in great measure as the seed ripens; with the result that, on drying, the walls collapse. In such seeds we may perhaps suppose that the process of conversion, which in round seeds takes place on germination, is begun earlier, and perhaps the variation essentially consists in the premature appearance of the converting ferment. It would be most rash to suggest that such a process may be operating in the pea, for the phenomenon may have many causes; but however that may be, there is evidently a difference of such a nature that when the water dries out of the seed on ripening, its walls collapse; and this collapse may occur in varying degrees.

Wrinkling must of course be distinguished further from the squaring due to the peas pressing against each other in the pod.

In connexion with these considerations I may mention that Vilmorin makes the interesting statement that most peas retain their vitality three years, dying as a rule rapidly after that time is passed, though occasionally seeds seven or eight years old are alive; but that wrinkled peas germinate as a rule less well than round, and do not retain their vitality so long as the round. Vilmorin-Andrieux, Plantes Potagères, 1883, p. 423. Similar statements regarding the behaviour of wrinkled peas in India are made by Firminger, Gardening for India, 3rd ed. 1874, p. 146.

In respect of shape the seeds of a variety otherwise stable are as a rule fairly uniform, the co-existence of both shapes and of intermediates between them in the same variety is not infrequent. As Professor Weldon has said, Telephone is a good example of an extreme case of mixture of both colours and shapes. William I. is another. It may be mentioned that regular dimorphism in respect of shape is not so common as dimorphism in respect of colour. Of great numbers of varieties seen at Messrs Suttons’ I saw none so distinctly dimorphic in shape as William I. which nevertheless contains all grades commonly.

So far I have spoken of the shapes of ordinary English culinary peas. But if we extend our observations to the shapes of large-seeded peas, which occur for the most part among the sugar-peas (mange-touts), of the “grey” peas with coloured flowers, etc., there are fresh complications to be considered.

Professor Weldon does not wholly avoid these (as Mendel did in regard to shape) and we will follow him through his difficulties hereafter. For the present let me say that the classes round and wrinkled are not readily applicable to those other varieties and are not so applied either by Mendel or other practical writers on these subjects. To use the terms indicated in the Introduction, seed-shape depends on more than one pair of allelomorphs--possibly on several.

Stability and Variability.

Generally speaking peas which when seen in bulk are monomorphic in colour and shape, will give fairly true and uniform offspring (but such strict monomorphism is rather exceptional). Instances to the contrary occur, and in my own brief experience I have seen some. In a row of Fill-basket grown from selected seed there were two plants of different habit, seed-shape, etc. Each bore pods with seeds few though large and round. Again Blue Peter (blue and round) and Laxton’s Alpha (blue and wrinkled), grown in my garden and left to nature uncovered, have each given a considerable proportion of seeds with yellow cotyledons, about 20% in the case of Laxton’s Alpha. The distribution of these on the plants I cannot state. The plants bearing them in each case sprang from green-cotyledoned seeds taken from samples containing presumably unselected green seeds only. A part of this exceptional result may be due to crossing, but heterogeneity of conditions especially in or after ripening is a more likely cause, hypotheses I hope to investigate next season. Hitherto I had supposed the crossing, if any, to be done by Bruchus or Thrips, but Tschermak also suspects Megachile, the leaf-cutter bee, which abounds in my garden.

Cotyledon-colour is not nearly so sensitive to ordinary changes in conditions as coat-colour, provided the coat be uninjured. But even in monomorphic green varieties, a seed which for any cause has burst on ripening, has the exposed parts of its cotyledons yellow. The same may be the case in seeds of green varieties injured by Bruchus or birds. These facts make one hesitate before denying the effects of conditions on the cotyledon-colour even of uninjured seeds, and the variation described above may have been simply weathering. The seeds were gathered very late and many were burst in Laxton’s Alpha. I do not yet know they are alive.

Whatever the cause, these irregularities may undoubtedly occur; and if they be proved to be largely independent of crossing and conditions, this will in nowise vitiate the truth of the Mendelian principle. For in that case it may simply be variability. Such true variation, or sporting, in the pea is referred to by many observers. Upon this subject I have received most valuable facts from Mr Arthur Sutton, who has very kindly interested himself in these inquiries. He tells me that several highly bred varieties, selected with every possible care, commonly throw a small but constant proportion of poor and almost vetch-like plants, with short pods and small round seeds, which are hoed out by experienced men each year before ripening. Other high-class varieties always, wherever grown, and when far from other sorts, produce a small percentage of some one or more definite “sports.” Of these peculiar sports he has sent me a collection of twelve, taken from as many standard varieties, each “sport” being represented by eight seeds, which though quite distinct from the type agree with each other in almost all cases.

In two cases, he tells me, these seed-sports sown separately have been found to give plants identical with the standard type and must therefore be regarded as sports in seed characters only; in other cases change of plant-type is associated with the change of seed-type.

In most standard varieties these definite sports are not very common, but in a few they are common enough to require continual removal by selection.