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].
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.
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]:
“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.)