Part 12

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[83], 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.

[83] Usually correlated characters, as Mendel knew.

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[84]:

[84] _Animals and Plants_, 2nd ed. 1885, p. 428.

“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[85]. 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.

[85] “_Eine andere Frage ist jedoch, ob der Einfluss des Pollens auf den Keim schon äusserlich an diesen letzteren sichtbar sein kann. Darwin führt mehrere hierher gehörige Fälle an, und wahrscheinlich sind auch die Resultate der von Gärtner über diesen Gegenstand ausgeführten Experimente hier zu erwähnen, wenn es auch nicht ganz deutlich ist, ob der von Gärtner erwähnte directe Einfluss des Pollens sich nur innerhalb der Grenzen des Keimes merklich macht oder nicht._” p. 490.

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[86]. 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.”

[86] Appendix to paper of Goss, _Trans. Hort. Soc._ v. 1822, pub. 1824 (_not_ 1848, as given by Professor Weldon), p. 236.

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[87].

[87] 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[88], 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.

[88] (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[89].

[89] 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_[90].” I may warn those interested in these questions, that in estimating changes due to ripening, _dead_ seeds are not available.

[90] “_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[91] 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.

[91] 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[92]. Phenomena somewhat similar to these will be met with in Laxton’s case of the “maple” seeded peas (see p. 161).

[92] 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[93] 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.

[93] 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.

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.

[94] 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[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!”

[95] Correns found a similar result.