Part 28

8. Debarbieux by nain de Bretagne (very small; yellow cotyledons; very round) gave three pods, 12 seeds, all yellow cotyledons, of which two pods had eight seeds identical in shape with Debarbieux, while the third had four seeds like Debarbieux but more dimpled. The reciprocal cross gave two seeds exactly like nain de Bretagne.

But it may be objected that the shape of this large grey pea is very peculiar; and that it maintains its type remarkably when fertilised by many distinct varieties though its pollen effects little or no change in them; for, so long as round varieties of sativum are used as mothers, this is true as we have seen. But when once it is understood that in Graue Riesen there is no question of wrinkling, seeing that the variety behaves as a round variety, the shape and especially the size of the seed must be treated as a maternal property.

It is certainly subject to considerable changes according to conditions. Those ripened in my garden are without exception much larger and flatter than Vilmorin’s seeds (now two years old) from which they grew. The colour of the coats is also much duller. These changes are just what is to be expected from the English climate--taken with the fact that my sample of this variety was late sown.

Why the distinction between the shape of Graue Riesen and that of ordinary round peas should be a matter of maternal physiology we do not know. The question is one for the botanical chemist. But there is evidently very considerable regularity, the seeds borne by the cross-breds exhibiting the form of the “grey” pea, which is then a dominant character as much as the seed-coat characters are. And that is what Tschermak’s Graue Riesen crosses actually did, thereby exhibiting dominance in a very clear form. To interject these cases as a mystery without pointing out how easily they can be reconciled with the “law of dominance” may throw an unskilled reader into gratuitous doubt.

Finally, since the wrinkled peas, Laxton’s Alpha and British Queen, pollinated by a large flat mange-tout, witness Nos. 3 and 6 above, became round in both cases where this experiment was made, we here merely see the usual dominance of the non-wrinkled character; though of course if a round-seeded mother be used there can be no departure from the maternal shape, as far as roundness is concerned.

Correns’ observations on the shapes of a “grey” pea crossed with a round shelling pea, also quoted by Professor Weldon as showing no dominance of roundness, are of course of the same nature as those just discussed.

C. Evidence of Knight and Laxton.

In the last two sections we have seen that in using peas of the “grey” class, i.e. with brown, red, or purplish coats, special phenomena are to be looked for, and also that in the case of large “indented” peas, the phenomena of size and shape may show some divergence from that simple form of the phenomenon of dominance seen when ordinary round and wrinkled are crossed. Here the fuller discussion of these phenomena must have been left to await further experiment, were it not that we have other evidence bearing on the same questions.

The first is that of Knight’s well-known experiments, long familiar but until now hopelessly mysterious. I have not space to quote the various interpretations which Knight and others have put upon them, but as the Mendelian principle at once gives a complete account of the whole, this is scarcely necessary, though the matter is full of historical interest.

Crossing a white pea with a very large grey purple-flowered form Knight (21) found that the peas so produced “were not in any sensible degree different from those afforded by other plants of the same [white] variety; owing, I imagine, to the external covering of the seed (as I have found in other plants) being furnished entirely by the female.” All grew very tall, and had colours of male parent. The seeds they produced were dark grey.

Thus avoiding the error of Seton, see p. 144. There is no xenia perhaps because the seed-coat of mother was a transparent coat.

As heterozygotes often do.

Dominance of the purple form.

Dominance of the grey coat as a maternal character.

“I had frequent occasion to observe, in this plant [the hybrid], a stronger tendency to produce purple blossoms, and coloured seeds, than white ones; for when I introduced the farina of a purple blossom into a white one, the whole of the seeds in the succeeding year became coloured [viz. DR × D giving DD and DR]; but, when I endeavoured to discharge this colour, by reversing the process, a part only of them afforded plants with white blossoms; this part sometimes occupying one end of the pod, and being at times irregularly intermixed with those which, when sown, retained their colour” [viz. DR × R giving DR and RR] (draws conclusions, now obviously erroneous).

Sherwood’s view (J. R. Hort. Soc. XXII. p. 252) that this was the origin of the “Wrinkled” pea, seems very dubious.

In this account we have nothing not readily intelligible in the light of Mendel’s hypothesis.

The next evidence is supplied by an exceptionally complete record of a most valuable experiment made by Laxton. The whole story is replete with interest, and as it not only carries us on somewhat beyond the point reached by Mendel, but furnishes an excellent illustration of how his principles may be applied, I give the whole account in Laxton’s words, only altering the paragraphing for clearness, and adding a commentary. The paper appears in Jour. Hort. Soc. N.S. III. 1872, p. 10, and very slightly abbreviated in Jour. of Hort. XVIII. 1870, p. 86. Some points in the same article do not specially relate to this section, but for simplicity I treat the whole together.

It will be well known to all practical horticulturalists that Laxton, originally of Stamford, made and brought out a large number of the best known modern peas. The firm is now in Bedford.

It is not too much to say that two years ago the whole of this story would have been a maze of bewildering confusion. There are still some points in it that we cannot fully comprehend, for the case is one of far more than ordinary complexity, but the general outlines are now clear. In attempting to elucidate the phenomena it will be remembered that there are no statistics (those given being inapplicable), and the several offspring are only imperfectly referred to the several classes of seeds. This being so, our rationale cannot hope to be complete. Laxton states that as the seeds of peas are liable to change colour with keeping, for this and other reasons he sent to the Society a part of the seeds resulting from his experiment before it was brought to a conclusion.

“The seeds exhibited were derived from a single experiment. Amongst these seeds will be observed some of several remarkable colours, including black, violet, purple-streaked and spotted, maple, grey, greenish, white, and almost every intermediate tint, the varied colours being apparently produced on the outer coat or envelope of the cotyledons only.

The peas were selected for their colours, &c., from the third year’s sowing in 1869 of the produce of a cross in 1866 of the early round white-seeded and white-flowered garden variety “Ringleader,” which is about 2-1/2 ft. in height, fertilised by the pollen of the common purple-flowered “maple” pea, which is taller than “Ringleader,” and has slightly indented seeds. I effected impregnation by removing the anthers of the seed-bearer, and applying the pollen at an early stage. This cross produced a pod containing five round white peas, exactly like the ordinary “Ringleader” seeds.

A round white ♀ × grey ♂ giving the usual result, round, “white” (yellow) seeds.

In 1867 I sowed these seeds, and all five produced tall purple-flowered purplish-stemmed plants, and the seeds, with few exceptions, had all maple or brownish-streaked envelopes of various shades; the remainder had entirely violet or deep purple-coloured envelopes: in shape the peas were partly indented; but a few were round. Some of the plants ripened off earlier than the “maple,” which, in comparison with “Ringleader,” is a late variety; and although the pods were in many instances partially abortive, the produce was very large.

Tall heterozygotes, with normal dominance of purple flowers.

Here we see dominance of the pigmented seed-coat as a maternal character over white seed-coat. The colours of the seed-coats are described as essentially two: maple or brown-streaked, and violet, the latter being a small minority. As the sequel shows, the latter are heterozygotes, not breeding true. Now Mendel found, and the fact has been confirmed both by Correns and myself, that crossing a grey pea which is capable of producing purple leads to such production as a form of xenia.

We have here therefore in the purple seeds the union of dissimilar gametes, with production of xenia. But as the brown-streaked seeds are also in part heterozygous, the splitting of a compound allelomorph has probably taken place, though without precise statistics and allotment of offspring among the several seeds the point is uncertain. The colour of seed-coats in “grey” peas and probably “maples” also is, as was stated on p. 150, sensitive to conditions, but the whole difference between “maples” and purple is too much to attribute safely to such irregularity. “Maple” is the word used to describe certain seed-coats which are pigmented with intricate brown mottlings on a paler buff ground. In French they are perdrix.

This is not, as it stands, explicable. It seems from this point and also from what follows that if the account is truly given, some of the plants may have been mosaic with segregation of characters in particular flowers; but see subsequent note.