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[64].
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[65]. 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[66]. 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.
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[67]; and this collapse may occur in varying degrees.
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.