Part 13

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.