Part 27

As Tschermak writes ((37), p. 658), experience has shown him that cross-bred seeds with character transitional between “round” and “wrinkled” behave as hybrids, and have both wrinkled and round offspring, and he now reckons them accordingly with the round dominants.

Note further the fact that Rimpau found the wrinkled form came true in the fifth year, while the round gave at first more, later fewer, wrinkleds, not coming true till the ninth year. This makes it quite clear that there was dominance of the round form, but that the heterozygotes were not so sharply distinguishable from the two pure forms as to be separated at once by a person not on the look-out for the distinctions. Nevertheless there was sufficient difference to lead to a practical distinction of the cross-breds both from the pure dominants and from the pure recessives.

The Telephone case may have been of the same nature; though, as we have seen above, this pea is peculiar in its colour-heredity and may quite well have followed a different rule in shape also. As stated before, the wrinkled offspring were not cultivated after the third year, but the round seeds are said to have still given some wrinkleds in the eighth year after the cross, as would be expected in a simple Mendelian case.

(b) Tschermak’s cases. The cases Professor Weldon quotes from Tschermak all relate to crosses with Telephone again, and this fact taken with the certainty that the colour-heredity of Telephone is abnormal makes it fairly clear that there is here something of a really exceptional character. What the real nature of the exception is, and how far it is to be taken as contradicting the “law of dominance,” is quite another matter.

3. Other phenomena, especially regarding seed-shapes, in the case of “grey” peas. Modern evidence. Professor Weldon quotes from Tschermak the interesting facts about the “grey” pea, Graue Riesen, but does not attempt to elucidate them. He is not on very safe ground in adducing these phenomena as conflicting with the “law of dominance.” Let us see whither we are led if we consider these cases. On p. 124 I mentioned that the classes round and wrinkled do not properly hold if we try to extend them to large-seeded sorts, and that these cases require separate consideration. In many of such peas, which usually belong either to the classes of sugar-peas (mange-touts) or “grey” peas (with coloured flowers), the seeds would be rather described as irregularly indented, lumpy or stony, than by any use of the terms round or wrinkled. One sugar-pea (Debarbieux) which I have used has large flattish, smooth, yellow seeds with white skins, and this also in its crossings follows the rules about to be described for the large-seeded “grey” peas.

Gärtner’s macrospermum was evidently one of these, though from the further account (p. 498) it was probably more wrinkled. There are of course mange-touts which have perfectly round seeds. Mendel himself showed that the mange-tout character, the soft constricted pod, was transferable. There are also mange-touts with fully wrinkled seeds and “grey” peas with small seeds (see Vilmorin-Andrieux, Plantes Potagères, 1883).

In the large “grey” peas the most conspicuous feature is the seed-coat, which is grey, brownish, or of a bright reddish colour. Such seed-coats are often speckled with purple, and on boiling these seed-coats turn dark brown. They are in fact the very peas used by Mendel in making up his third pair of characters. Regarding them Professor Weldon, stating they may be considered separately, writes as follows:--

“Tschermak has crossed Graue Riesen with five races of P. sativum, and he finds that the form of the first hybrid seeds follows the female parent, so that if races of P. sativum with round smooth seeds be crossed with Graue Riesen (which has flattened, feebly wrinkled seeds) the hybrids will be round and smooth or flattened and wrinkled, as the P. sativum or the Graue Riesen is used as female parent. There is here a more complex phenomenon than at first sight appears; because if the flowers of the first hybrid generation are self-fertilised, the resulting seeds of the second generation invariably resemble those of the Graue Riesen in shape, although in colour they follow Mendel’s law of segregation!”

Correns found a similar result.

From this account who would not infer that we have here some mystery which does not accord with the Mendelian principles? As a matter of fact the case is dominance in a perfectly obvious if distinct form.

Graue Riesen, a large grey sugar-pea, the pois sans parchemin géant of the French seedsmen, has full-yellow cotyledons and a highly coloured seed-coat of varying tints. In shape the seed is somewhat flattened with irregular slight indentations, lightly wrinkled if the term be preferred. Tschermak speaks of it in his first paper as “Same flach, zusammengedrückt”--a flat, compressed seed; in his second paper as “flache, oft schwach gerunzelte Cotyledonen-form,” or cotyledon-shape, flat, often feebly wrinkled, as Professor Weldon translates.

First-crosses made from this variety, each with a different form of P. sativum, are stated on the authority of Tschermak’s five cases, to follow exclusively the maternal seed-shape. From “schwach gerunzelte,” “feebly wrinkled,” Professor Weldon easily passes to “wrinkled,” and tells us that according as a round sativum or the Graue Riesen is used as mother, the first-cross seeds “will be round and smooth or flattened and wrinkled.”

As a matter of fact, however, the seeds of Graue Riesen though slightly wrinkled do not belong to the “wrinkled” class; but if the classification “wrinkled” and “round” is to be extended to such peas at all, they belong to the round. Mendel is careful to state that his round class are “either spherical or roundish, the depressions on the surface, when there are any, always slight”; while the “wrinkled” class are “irregularly angular, deeply wrinkled.”

“Entweder kugelrund oder rundlich, die Einsenkungen, wenn welche an der Oberfläche vorkommen, immer nur seicht, oder sie sind unregelmässig kantig, tief runzlig (P. quadratum).”

On this description alone it would be very likely that Graue Riesen should fall into the round class, and as such it behaves in its crosses, being dominant over wrinkled (see Nos. 3 and 6, below). I can see that in this case Professor Weldon has been partly misled by expressions of Tschermak’s, but the facts of the second generation should have aroused suspicion. Neither author notices that as all five varieties crossed by Tschermak with Graue Riesen were round, the possibilities are not exhausted. Had Tschermak tried a really wrinkled sativum with Graue Riesen he would have seen this obvious explanation.

As some of my own few observations of first-crosses bear on this point I may quote them, imperfect though they are.

I grew the purple-flowered sugar-pea “Pois sans parchemin géant à très large cosse,” a soft-podded “mange-tout” pea, flowers and seed-coats coloured, from Vilmorin’s, probably identical with Graue Riesen.

1. One flower of this variety fertilised with Pois très nain de Bretagne (very small seed; yellow cotyledons; very round) gave seven seeds indistinguishable (in their coats) from those of the mother, save for a doubtful increase in purple pigmentation of coats.

2. Fertilised by Laxton’s Alpha (green; wrinkled; coats transparent), two flowers gave 11 seeds exactly as above, the purple being in this case clearly increased.

In the following the purple sugar-pea was father.

3. Laxton’s Alpha (green; wrinkled; coats transparent) fertilised by the purple sugar-pea gave one pod of four seeds with yellow cotyledons and round form.

4. Fillbasket (green; smooth but squared; coats green) fertilised by the purple sugar-pea gave one pod with six seeds, yellow cotyledons; Fillbasket size and shape; but the normally green coat yellowed near the hilum by xenia.

The colour is the peculiarly deep yellow of the “grey” mange-tout.

5. Express (“blue”-green cotyledons and transparent skins; round) fertilised with purple sugar-pea gave one pod with four seeds, yellow cotyledons, shape round, much as in Fillbasket.

6. British Queen (yellow cotyledons, wrinkled, white coats) ♀ × purple sugar-pea gave two pods with seven seeds, cotyledons yellow, coats tinged greenish (xenia?), all round.

So much for the “Purple” sugar-pea.

I got similar results with Mange-tout Debarbieux. This is a soft-podded Mange-tout or sugar-pea, with white flowers, large, flattish, smooth seeds, scarcely dimpled; yellow cotyledons.

7. Debarbieux fertilised by Serpette nain blanc (yellow cotyledons; wrinkled; white skin; dwarf) gave one pod with six seeds, size and shape of Debarbieux, with slight dimpling.