Mendel's Principles of Heredity: A Defence · William Bateson
A. In regard to cotyledon colour: Preliminary.
I have been at some pains to show how the contradictory results, no doubt sometimes occurring, on which Professor Weldon lays such stress, may be comprehended without any injury to Mendel’s main conclusions. This excursion was made to save trouble with future discoverers of exceptions, though the existence of such facts need scarcely disturb many minds. As regards the dominance of yellow cotyledon-colour over green the whole number of genuine unconformable cases is likely to prove very small indeed, though in regard to the dominance of round shape over wrinkled we may be prepared for more discrepancies. Indeed my own crosses alone are sufficient to show that in using some varieties irregularities are to be expected. Considering also that the shapes of peas depend unquestionably on more than one pair of allelomorphs I fully expect regular blending in some cases.
As however it may be more satisfactory to the reader and to Professor Weldon if I follow him through his “contradictory” evidence I will endeavour to do so. Those who have even a slight practical acquaintance with the phenomena of heredity will sympathize with me in the difficulty I feel in treating this section of his arguments with that gravity he conceives the occasion to demand.
In following the path of the critic it will be necessary for me to trouble the reader with a number of details of a humble order, but the journey will not prove devoid of entertainment.
Now exceptions are always interesting and suggestive things, and sometimes hold a key to great mysteries. Still when a few exceptions are found disobeying rules elsewhere conformed to by large classes of phenomena it is not an unsafe course to consider, with such care as the case permits, whether the exceptions may not be due to exceptional causes, or failing such causes whether there may be any possibility of error. But to Professor Weldon, an exception is an exception--and as such may prove a very serviceable missile; so he gathers them as they were “smooth stones from the brook.”
Before examining the quality of this rather miscellaneous ammunition I would wish to draw the non-botanical reader’s attention to one or two facts of a general nature.
For our present purpose the seed of a pea may be considered as consisting of two parts, the embryo with its cotyledons, enclosed in a seed-coat. It has been known for about a century that this coat or skin is a maternal structure, being part of the mother plant just as much as the pods are, and consequently not belonging to the next generation at all. If then any changes take place in it consequent on fertilisation, they are to be regarded not as in any sense a transmission of character by heredity, but rather as of the nature of an “infection.” If on the other hand it is desired to study the influence of hereditary transmission on seed-coat characters, then the crossed seeds must be sown and the seed-coats of their seeds studied. Such infective changes in maternal tissues have been known from early times, a notable collection of them having been made especially by Darwin; and for these cases Focke suggested the convenient word Xenia. With this familiar fact I would not for a moment suppose Professor Weldon unacquainted, though it was with some surprise that I found in his paper no reference to the phenomenon.
For as it happens, xenia is not at all a rare occurrence with certain varieties of peas; though in them, as I believe is generally the case with this phenomenon, it is highly irregular in its manifestations, being doubtless dependent on slight differences of conditions during ripening.
The coats of peas differ greatly in different varieties, being sometimes thick and white or yellow, sometimes thick and highly pigmented with green or other colours, in both of which cases it may be impossible to judge the cotyledon-colour without peeling off the opaque coat; or the coats may be very thin, colourless and transparent, so that the cotyledon-colour is seen at once. It was such a transparent form that Mendel says he used for his experiments with cotyledon-colour. In order to see xenia a pea with a pigmented seed-coat should be taken as seed-parent, and crossed with a variety having a different cotyledon-colour. There is then a fair chance of seeing this phenomenon, but much still depends on the variety. For example, Fillbasket has green cotyledons and seed-coat green except near the hilar surface. Crossed with Serpette nain blanc (yellow cotyledons and yellow coat) this variety gave three pods with 17 seeds in which the seed-coats were almost full yellow (xenia). Three other pods (25 seeds), similarly produced, showed slight xenia, and one pod with eight seeds showed little or none.
On the other hand Fillbasket fertilised with nain de Bretagne (yellow cotyledons, seed-coats yellow to yellowish green) gave six pods with 39 seeds showing slight xenia, distinct in a few seeds but absent in most.
Examples of xenia produced by the contrary proceeding, namely fertilising a yellow pea with a green, may indubitably occur and I have seen doubtful cases; but as by the nature of the case these are negative phenomena, i.e. the seed-coat remaining greenish and not going through its normal maturation changes, they must always be equivocal, and would require special confirmation before other causes were excluded.
Lastly, the special change (xenia) Mendel saw in “grey” peas, appearance or increase of purple pigment in the thick coats, following crossing, is common but also irregular.
If a transparent coated form be taken as seed-parent there is no appreciable xenia, so far as I know, and such a phenomenon would certainly be paradoxical.
In this connection it is interesting to observe that Giltay, whom Professor Weldon quotes as having obtained purely Mendelian results, got no xenia though searching for it. If the reader goes carefully through Giltay’s numerous cases, he will find, almost without doubt, that none of them were such as produce it. Reading Giant, as Giltay states, has a transparent skin, and the only xenia likely to occur in the other cases would be of the peculiar and uncertain kind seen in using “grey” peas. Professor Weldon notes that Giltay, who evidently worked with extreme care, peeled his seeds before describing them, a course which Professor Weldon, not recognizing the distinction between the varieties with opaque and transparent coats, himself wisely recommends. The coincidence of the peeled seeds giving simple Mendelian results is one which might have alarmed a critic less intrepid than Professor Weldon.
Bearing in mind, then, that the coats of peas may be transparent or opaque; and in the latter case may be variously pigmented, green, grey, reddish, purplish, etc.; that in any of the latter cases there may or may not be xenia; the reader will perceive that to use the statements of an author, whether scientific or lay, to the effect that on crossing varieties he obtained peas of such and such colours without specifying at all whether the coats were transparent or whether the colours he saw were coat- or cotyledon-colours is a proceeding fraught with peculiar and special risks.
(1) Gärtner’s cases. Professor Weldon gives, as exceptions, a series of Gärtner’s observations. Using several varieties, amongst them Pisum sativum macrospermum, a “grey” pea, with coloured flowers and seed-coats, he obtained results partly Mendelian and partly, as now alleged, contradictory. The latter consist of seeds “dirty yellow” and “yellowish green,” whereas it is suggested they should have been simply yellow.
Now students of this department of natural history will know that these same observations of Gärtner’s, whether rightly or wrongly, have been doing duty for more than half a century as stock illustrations of xenia. In this capacity they have served two generations of naturalists. The ground nowadays may be unfamiliar, but others have travelled it before and recorded their impressions. Darwin, for example, has the following passage:
“These statements led Gärtner, who was highly sceptical on the subject, carefully to try a long series of experiments; he selected the most constant varieties, and the results conclusively showed that the colour of the skin of the pea is modified when pollen of a differently coloured variety is used.” (The italics are mine.)
In the true spirit of inquiry Professor Weldon doubtless reflected,
“’Tis not Antiquity nor Author, That makes Truth Truth, altho’ Time’s Daughter”;
but perhaps a word of caution to the reader that another interpretation exists would have been in place. It cannot be without amazement therefore that we find him appropriating these examples as referring to cotyledon-colour, with never a hint that the point is doubtful.
Giltay, without going into details, points out the ambiguity. As Professor Weldon refers to the writings both of Darwin and Giltay, it is still more remarkable that he should regard the phenomenon as clearly one of cotyledon-colour and not coat-colour as Darwin and many other writers have supposed.
Without going further it would be highly improbable that Gärtner is speaking solely or even chiefly of the cotyledons, from the circumstance that these observations are given as evidence of “the influence of foreign pollen on the female organs”; and that Gärtner was perfectly aware of the fact that the coat of the seed was a maternal structure is evident from his statement to that effect on p. 80.
To go into the whole question in detail would require considerable space; but indeed it is unnecessary to labour the point. The reader who examines Gärtner’s account with care, especially the peculiar phenomena obtained in the case of the “grey” pea (macrospermum), with specimens before him, will have no difficulty in recognizing that Gärtner is simply describing the seeds as they looked in their coats, and is not attempting to distinguish cotyledon-characters and coat-characters. If he had peeled them, which in the case of “grey” peas would be absolutely necessary to see cotyledon-colour, he must surely have said so.
Had he done so, he would have found the cotyledons full yellow in every ripe seed; for I venture to assert that anyone who tries, as we have, crosses between a yellow-cotyledoned “grey” pea, such as Gärtner’s was, with any pure green variety will see that there is no question whatever as to absolute dominance of the yellow cotyledon-character here, more striking than in any other case. If exceptions are to be looked for, they will not be found there; and, except in so far as they show simple dominance of yellow, Gärtner’s observations cannot be cited in this connection at all.
(2) Seton’s case. Another exception given by Professor Weldon is much more interesting and instructive. It is the curious case of Seton. Told in the words of the critic it is as follows:--
“Mr Alexander Seton crossed the flowers of Dwarf Imperial, ‘a well-known green variety of the Pea,’ with the pollen of ‘a white free-growing variety.’ Four hybrid seeds were obtained, ‘which did not differ in appearance from the others of the female parent.’ These seeds therefore did not obey the law of dominance, or if the statement be preferred, greenness became dominant in this case. The seeds were sown, and produced plants bearing ‘green’ and ‘white’ seeds side by side in the same pod. An excellent coloured figure of one of these pods is given (loc. cit. Plate 9, Fig. 1), and is the only figure I have found which illustrates segregation of colours in hybrid Peas of the second generation.”
Now if Professor Weldon had applied to this case the same independence of judgment he evinced in dismissing Darwin’s interpretation of Gärtner’s observations, he might have reached a valuable result. Knowing how difficult it is to give all the points in a brief citation, I turned up the original passage, where I find it stated that the mixed seeds of the second generation “were all completely either of one colour or the other, none of them having an intermediate tint, as Mr Seton had expected.” The utility of this observation of the absence of intermediates, is that it goes some way to dispose of the suggestion of xenia as a cause contributing to the result.
Moreover, feeling perfectly clear, from the fact of the absence of intermediates, that the case must be one of simple dominance in spite of first appearances, I suggest the following account with every confidence that it is the true one. There have been several “Imperials,” though Dwarf Imperial, in a form which I can feel sure is Seton’s form, I have not succeeded in seeing; but from Vilmorin’s description that the peas when ripe are “franchement verts” I feel no doubt it was a green pea with a green skin. If it had had a transparent skin this description would be inapplicable. Having then a green skin, which may be assumed with every probability of truth, the seeds, even though the cotyledons were yellow, might, especially if examined fresh, be indistinguishable from those of the maternal type. Next from the fact of the mixture in the second generation we learn that the semi-transparent seed-coat of the paternal form was dominant as a plant-character, and indeed the coloured plate makes this fairly evident. It will be understood that this explanation is as yet suggestive, but from the facts of the second generation, any supposition that there was real irregularity in dominance in this case is out of the question.
(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.
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.
(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.
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.
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).
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.
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.
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.
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!”
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.”
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.
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.
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.
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.
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