Part 26

Since the above passage was written I find the “Imperials” described in “Report of Chiswick Trials,” Proc. R. Hort. Soc. 1860, I. p. 340, as “skin thick”; and on p. 360 “skin thick, blue”; which finally disposes of this “exception.”

(3) Tschermak’s exceptions. These are a much more acceptable lot than those we have been considering. Tschermak was thoroughly alive to the seed-coat question and consequently any exception stated as an unqualified fact on his authority must be accepted. The nature of these cases we shall see. Among the many varieties he used, some being not monomorphic, it would have been surprising if he had not found true irregularities in dominance.

(3 a) Buchsbaum case. This variety, growing in the open, gave once a pod in which every seed but one was green. In stating this case Professor Weldon refers to Buchsbaum as “a yellow-seeded variety.” Tschermak, however, describes it as having “gelbes, öfters gelblich-grünes Speichergewebe” (cotyledons); and again says the cotyledon-colour is “allerdings gerade bei Buchsbaum zur Spontanvariation nach gelb-grün neigend!” The (!) is Tschermak’s. Therefore Professor Weldon can hardly claim Buchsbaum as “yellow-seeded” without qualification.

(36), p. 502 and (37), p. 663.

Buchsbaum in fact is in all probability a blend-form and certainly not a true, stable yellow. One of the green seeds mentioned above grew and gave 15 yellows and three greens, and the result showed pretty clearly, as Tschermak says, that there had been an accidental cross with a tall green.

On another occasion Telephone ♀ (another impure green) × Buchsbaum gave four yellow smooth and two green wrinkled, but one [? both: the grammar is obscure] of the greens did not germinate.

Professor Weldon should have alluded to this. Dead seeds have no bearing on these questions, seeing that their characters may be pathological. The same seeds are later described as “wie Telephone selbst,” so, apart from the possibility of death, they may also have been self-fertilised.

(3 b) Telephone cases. Telephone, crossed with at least one yellow variety (Auvergne) gave all or some green or greenish. These I have no doubt are good cases of “defective dominance” of yellow. But it must be noted that Telephone is an impure green. Nominally a green, it is as Professor Weldon has satisfied himself, very irregular in colour, having many intermediates shading to pure yellow and many piebalds. It is the variety from which alone Professor Weldon made his colour-scale. I desire therefore to call special attention to the fact that Telephone, though not a pure green, Tschermak’s sample being as he says “gelblichweiss grün,” a yellowish-white-green in cotyledon-colour, is the variety which has so far contributed the clearest evidence of the green colour dominating in its crosses with a yellow; and that Buchsbaum is probably a similar case. To this point we shall return. It may not be superfluous to mention also that one cross between Fillbasket (a thorough green) and Telephone gave three yellowish green seeds (Tschermak, (36), p. 501).

(3 c) Couturier cases. This fully yellow variety in crosses with two fully green sorts gave seeds either yellow or greenish yellow. In one case Fillbasket ♀ fertilised by Couturier gave mixed seeds, green and yellow. For any evidence to the contrary, the green in this case may have been self-fertilised. Nevertheless, taking the evidence together, I think it is most likely that Couturier is a genuine case of imperfect dominance of yellow. If so, it is the only true “exception” in crosses between stable forms.

* * * * *

We have now narrowed down Professor Weldon’s exceptions to dominance of cotyledon-colour to two varieties, one yellow (Couturier), and one yellow “tending to green” (Buchsbaum), which show imperfect dominance of yellow; and one variety, Telephone, an impure and irregular green, which shows occasional but uncertain dominance of green.

What may be the meaning of the phenomenon shown by the unstable or mosaic varieties we cannot tell; but I venture to suggest that when we more fully appreciate the nature and genesis of the gametes, it will be found that the peculiarities of heredity seen in these cases have more in common with those of “false hybridism” (see p. 34) than with any true failure of dominance.

Before, however, feeling quite satisfied in regard even to this residuum of exceptions, one would wish to learn the subsequent fate of these aberrant seeds and how their offspring differed from that of their sisters. One only of them can I yet trace, viz. the green seed from Telephone ♀ × Buchsbaum ♂, which proved a veritable “green dominant.” As for the remainder, Tschermak promises in his first paper to watch them. But in his second paper the only passage I can find relating to them declares that perhaps some of the questionable cases he mentioned in his first paper “are attributable to similar isolated anomalies in dominance; some proved themselves by subsequent cultivation to be cases of accidental self-fertilisation; others failed to germinate.” I may warn those interested in these questions, that in estimating changes due to ripening, dead seeds are not available.

“Vielleicht sind einige der l.c. 507 bis 508 erwähnten fraglichen Fälle auf ähnliche vereinzelte Anomalien der Merkmalswerthigkeit zu beziehen; einige erwiesen sich allerdings beim Anbau als Producte ungewollter Selbstbefruchtung, andere keimten nicht.”

B. Seed-coats and shapes.

1. Seed-coats. Professor Weldon lays some stress on the results obtained by Correns 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.

Regarding this case I have to thank Professor Correns for a good deal of information which he kindly sent me in response to my inquiry. I am thus able to supplement the published account in some particulars.

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. Phenomena somewhat similar to these will be met with in Laxton’s case of the “maple” seeded peas (see p. 161).

Mr Hurst, of Burbage, tells me that in varieties having coats green or white, e.g. American Wonder, the white coats are mostly from early, the green from later pods, the tints depending on conditions and exposure.

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 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.

In the first case Knight’s Marrow with Victoria, both ways; in the second Victoria with Telephone, both ways.

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.

Rimpau gives a photograph of eight peas (Fig. 146) which he says represent the wrinkled form derived from this cross. It is evident that these are not from one pod but a miscellaneous selection. On close inspection it will be seen that while the remainder are shown with their cotyledon-surfaces upwards, the two peas at the lower end of the row are represented with their hilar-surfaces upwards. Remembering this it will be recognized that these two lower peas are in fact not fully wrinkled peas but almost certainly round “hybrids,” and the depression is merely that which is often seen in round peas (such as Fillbasket), squared by mutual pressure. Such peas, when sown, might of course give some round.