B. Seed-coats and shapes.
1. Seed-coats. Professor Weldon lays some stress on the results obtained by Correns[91] in crossing a pea having green cotyledons and a thin almost colourless coat (grüne späte Erfurter Folger-erbse) with two purple-flowered varieties. The latter are what are known in England as “grey” peas, though the term grey is not generally appropriate.
In these varieties the cotyledon-colour is yellow and the coats are usually highly coloured or orange-brown. In reciprocal crosses Correns found no change from the maternal seed-coat-colour or seed-shape. On sowing these peas he obtained plants bearing peas which, using the terminology of Mendel and others, he speaks of as the “first generation.”
These peas varied in the colour of their seed-coats from an almost colourless form slightly tinged with green like the one parent to the orange-brown of the other parent. The seeds varied in this respect not only from plant to plant, but from pod to pod, and from seed to seed, as Professor Correns has informed me.
The peas with more highly-coloured coats were sown and gave rise to plants with seeds showing the whole range of seed-coat-colours again.
Professor Weldon states that in this case neither the law of dominance nor the law of segregation was observed; and the same is the opinion of Correns, who, as I understand, inclines to regard the colour-distribution as indicating a “mosaic” formation. This is perhaps conceivable; and in that case the statement that there was no dominance would be true, and it would also be true that the unit of segregation, if any, was smaller than the individual plant and may in fact be the individual seed.
A final decision of this question is as yet impossible. Nevertheless from Professor Correns I have learnt one point of importance, namely, that the coats of all these seeds were thick, like that of the coloured and as usual dominant form. There is no “mosaic” of coats like one parent and coats like the other, though there may be a mosaic of colours. In regard to the distribution of colour however the possibility does not seem to me excluded that we are here dealing with changes influenced by conditions. I have grown a “grey” pea and noticed that the seed-coats ripened in my garden differ considerably and not quite uniformly from those received from and probably ripened in France, mine being mostly pale and greyish, instead of reddish-brown. We have elsewhere seen (p. [120]) that pigments of the seed-coat-colour may be very sensitive to conditions, and slight differences of moisture, for example, may in some measure account for the differences in colour. Among my crosses I have a pod of such “grey” peas fertilised by Laxton’s Alpha (green cotyledons, coat transparent). It contained five seeds, of which four were red-brown on one side and grey with purple specks on the other. The fifth was of the grey colour on both sides. I regard this difference not as indicating segregation of character but merely as comparable with the difference between the two sides of a ripe apple, and I have little doubt that Correns’ case may be of the same nature[92]. Phenomena somewhat similar to these will be met with in Laxton’s case of the “maple” seeded peas (see p. [161]).
2. Seed-shapes. Here Professor Weldon has three sets of alleged exceptions to the rule of dominance of round shape over wrinkled. The first are Rimpau’s cases, the second are Tschermak’s cases, the third group are cases of “grey” peas, which we will treat in a separate section (see pp. [153] and [158]).
(a) Rimpau’s cases. Professor Weldon quotes Rimpau as having crossed wrinkled and round peas[93] and found the second hybrid generation dimorphic as usual. The wrinkled peas were selected and sown and gave wrinkled peas and round peas, becoming “true” to the wrinkled character in one case only in the fifth year, while in the second case—that of a Telephone cross—there was a mixture of round and wrinkled similarly resulting from wrinkled seed for two years, but the experiment was not continued.
These at first sight look like genuine exceptions. In reality, however, they are capable of a simple explanation. It must be remembered that Rimpau was working in ignorance of Mendel’s results, was not testing any rule, and was not on the look out for irregularities. Now all who have crossed wrinkled and round peas on even a moderate scale will have met with the fact that there is frequently some wrinkling in the cross-bred seeds. Though round when compared with the true wrinkled, these are often somewhat more wrinkled than the round type, and in irregular degrees. For my own part I fully anticipate that we may find rare cases of complete blending in this respect though I do not as yet know one.