Chapter IV: Professor Weldon’s Collection of “other Evidence Concerning (1)

DOMINANCE IN PEAS.”

_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[82].

[82] In some transparent coats there is pigment, but so little as a
rule that xenia would be scarcely noticeable.

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[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.

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[96].”

[96] “_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[97]; _Fillbasket_ size and shape; but the normally green
coat yellowed near _the hilum_ by xenia.

[97] 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.

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[98]; 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.

[98] 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[99].” All grew very tall[100], and had colours of male parent[101]. The seeds they produced were dark grey[102].

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

[100] As heterozygotes often do.

[101] Dominance of the purple form.

[102] 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[103]).

[103] 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[104]. 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.

[104] 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[105].

[105] 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[106], 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[107]: in shape the peas were partly indented; but a few were round[108]. 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[109].

[106] Tall heterozygotes, with normal dominance of purple flowers.

[107] 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_.

[108] 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.

[109] As, commonly, in heterozygotes when fertile.

In 1868 I sowed the peas of the preceding year’s growth, and selected various plants for earliness, productiveness, &c. Some of the plants had light-coloured stems and leaves; these all showed white flowers, and produced round white seeds[110]. Others had purple flowers, showed the purple on the stems and at the axils of the stipules, and produced seeds with maple, grey, purple-streaked, or mottled, and a few only, again, with violet-coloured envelopes. Some of the seeds were round, some partially indented[111]. The pods on each plant, in the majority of instances, contained peas of like characters; but in a few cases the peas in the same pod varied slightly, and in some instances a pod or two on the same plant contained seeds all distinct from the remainder[112]. The white-flowered plants were generally dwarfish, of about the height of “Ringleader”; but the coloured-flowered sorts varied altogether as to height, period of ripening, and colour and shape of seed[113]. Those seeds with violet-coloured envelopes produced nearly all maple- or parti-coloured seeds, and only here and there one with a violet-coloured envelope; that colour, again, appeared only incidentally, and in a like degree in the produce of the maple-coloured seeds[114].

[110] Recessive in flower-colour, seed-coat colour, and in seed-shape
as a maternal character: pure recessives as the sequel proved.

[111] These are then a mixture of pure dominants and cross-bred
dominants, and are now inextricably confused. This time the round
seeds may have been all on particular plants--showing recessive
seed-shape as a maternal character. It seems just possible that
this fact suggested the idea of “round” seeds on the _coloured_
plants in the last generation. Till that result is confirmed it
should be regarded as very doubtful on the evidence. But we cannot
at the present time be sure how much difference there was between
these round seeds and the _normal_ maples in point of shape; and
on the whole it seems most probable that the roundness was a mere
fluctuation, such as commonly occurs among the peas with large
indented seeds.

[112] Is this really evidence of segregation of characters, the
flower being the unit? In any case the possibility makes the
experiment well worth repeating, especially as Correns has seen a
phenomenon conceivably similar.

[113] Being a mixture of heterozygotes (probably involving several
pairs of allelomorphs) and homozygotes.

[114] This looks as if the violet colour was merely due to
irregularity of xenia.

In 1869 the seeds of various selections of the previous year were again sown separately; and the white-seeded peas again produced only plants with white flowers and round white seeds[115]. Some of the coloured seeds, which I had expected would produce purple-flowered plants, produced plants with white flowers and round white seeds only[116]; the majority, however, brought plants with purple flowers and with seeds principally marked with purple or grey, the maple- or brown-streaked being in the minority[117]. On some of the purple-flowered plants were again a few pods with peas differing entirely from the remainder on the same plant. In some pods the seeds were all white, in others all black, and in a few, again, all violet[118]; but those plants which bore maple-coloured seeds seemed the most constant and fixed in character of the purple-flowered seedlings[119], and the purplish and grey peas, being of intermediate characters, appeared to vary most[120]. The violet-coloured seeds again produced almost invariably purplish, grey, or maple peas, the clear violet colour only now and then appearing, either wholly in one pod or on a single pea or two in a pod. All the seeds of the purple-flowered plants were again either round or only partially indented; and the plants varied as to height and earliness. In no case, however, does there seem to have been an intermediate-coloured flower; for although in some flowers I thought I found the purple of a lighter shade, I believe this was owing to light, temperature, or other circumstances, and applied equally to the parent maple. I have never noticed a single tinted white flower nor an indented white seed in either of the three years’ produce. The whole produce of the third sowing consisted of seeds of the colours and in the approximate quantities in order as follows,--viz.: 1st, white, about half; 2nd, purplish, grey, and violet (intermediate colours), about three-eighths; and, 3rd, maple, about one-eighth.

[115] Pure recessives.

[116] Pure recessives in coats showing maternal dominant character.

[117] Now recognized as pure homozygotes.

[118] This seems almost certainly segregation by flower-units, and is
as yet inexplicable on any other hypothesis. Especially paradoxical
is the presence of “white” seeds on these plants. The impression is
scarcely resistible that some remarkable phenomenon of segregation
was really seen here.

[119] Being now homozygotes.

[120] Being heterozygotes exclusively.

From the above I gather that the white-flowered white-seeded pea is (if I may use the term) an original variety well fixed and distinct entirely from the maple, that the two do not thoroughly intermingle (for whenever the white flower crops out, the plant and its parts all appear to follow exactly the characters of the white pea), and that the maple is a cross-bred variety which has become somewhat permanent and would seem to include amongst its ancestors one or more bearing seeds either altogether or partly violet- or purple-coloured; for although this colour does not appear on the seed of the “maple,” it is very potent in the variety, and appears in many parts of the plant and its offspring from cross-fertilised flowers, sometimes on the external surface or at the sutures of the pods of the latter, at others on the seeds and stems, and very frequently on the seeds; and whenever it shows itself on any part of the plant, the flowers are invariably purple. My deductions have been confirmed by intercrosses effected between the various white-, blue-, some singularly bright green-seeded peas which I have selected, and the maple- and purple-podded and the purple-flowered sugar peas, and by reversing those crosses.

I have also deduced from my experiments, in accordance with the conclusions of the late Mr Knight and others, that the colours of the envelopes of the seeds of peas immediately resulting from a cross are never changed[121]. I find, however, that the colour and probably the substance of the cotyledons are sometimes, but not always, changed by the cross fertilisation of two different varieties; and I do not agree with Mr Knight that the form and size of the seeds produced are unaltered[122]; for I have on more than one occasion observed that the cotyledons in the seeds directly resulting from a cross of a blue wrinkled pea fertilised by the pollen of a white round variety have been of a greenish-white colour[123], and the seeds nearly round[124] and larger or smaller according as there may have been a difference in the size of the seeds of the two varieties[125].

[121] The nature of this mistake is now clear; for as stated above
xenia is only likely to occur when the maternal seed-coat is
pigmented. The violet coats in this experiment are themselves cases
of xenia.

[122] Knight, it was seen, crossed round ♀ × indented ♂ and
consequently got no change of form.

[123] Cotyledons seen through coat.

[124] Ordinary dominance of round.

[125] This is an extraordinary statement to be given as a general
truth. There are sometimes indications of this kind, but certainly
the facts are not usually as here stated.

I have also noticed that a cross between a round white and a blue wrinkled pea will in the third and fourth generations (second and third years’ produce) at times bring forth blue round, blue wrinkled, white round and white wrinkled peas in the same pods, that the white round seeds, when again sown, will produce only white round seeds, that the white wrinkled seeds will, up to the fourth or fifth generation, produce both blue and white wrinkled and round peas, that the blue round peas will produce blue wrinkled and round peas, but that the blue wrinkled peas will bear only blue wrinkled seeds[126]. This would seem to indicate that the white round and the blue wrinkled peas are distinct varieties derived from ancestors respectively possessing one only of those marked qualities; and, in my opinion, the white round peas trace their origin to a dwarfish pea having white flowers and round white seeds, and the blue wrinkled varieties to a tall variety, having also white flowers but blue wrinkled seeds. It is also noticeable, that from a single cross between two different peas many hundreds of varieties, not only like one or both parents and intermediate, but apparently differing from either, may be produced in the course of three or four years (the shortest time which I have ascertained it takes to attain the climax of variation in the produce of cross-fertilised peas, and until which time it would seem useless to expect a fixed seedling variety to be produced[127]), although a reversion to the characters of either parent, or of any one of the ancestors, may take place at an earlier period.

[126] If we were obliged to suppose that this is a matured conclusion
based on detailed observation it would of course constitute the most
serious “exception” yet recorded. But it is clear that the five
statements are not mutually consistent. We have dominance of round
white in first cross.