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.
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[94], 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.
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[95]. 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!”
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.