Third International Conference on Genetics (1906)
By EDWARD A. BUNYARD, F.R.H.S.
THE study of xenia, or the influence of foreign pollen upon the maternal structure, has aroused much interest and some experiment since the first days of artificial hybridisation, and on few subjects has there been expressed greater difference of opinion. A convenient summary of the earliest observations will be found in Darwin's "Animals and Plants under Domestication," and it was this varying evidence which led me some years ago to commence some experiments, the results of which, with evidence from other sources, I shall ask you to consider.
It is desirable at first to have some exact definition of what we mean by xenia, as the somewhat vague way in which this word has been used by many writers has contributed to the difficulties with which this subject is surrounded. Professor Tschermak suggests that xenia shall be applied to those cases where the pollen shall have caused, apart from the egg-cell and embryo-sac, variation corresponding to the pollen-parent upon the vegetative parts of the mother-plant. This definition seems to be exact, and I suggest that the word "xenia" should be confined to these cases. This will leave on one side all those cases where the embryo or embryo-sac is affected, such cases being due to Mendelian dominance or double fertilisation.
The discovery by Guignard of this latter process has thrown so much light on cases of supposed xenia that I may, perhaps, be permitted to quote from Professor Wilson's book on "The Cell in Development and Inheritance" (p. 221) a short description of this phenomenon:
"The pollen-cell possesses two generative nuclei, and one of these conjugates with the egg-nucleus, thus effecting fertilisation, the other conjugates with one of the polar nuclei thrown off in the course of mitosis. By a division of the fertilised egg-cell arises the embryo, while by division of the compound nucleus resulting from the fusion of the polar nucleus and the second sperm-nucleus are formed the endosperm-cells."
It will thus be seen that the embryo-sac is purely hybrid in its structure, and any effect of foreign pollen that may appear in it should, I suggest, be referred to as the effect of double fertilisation, as the meaning of xenia (guest-gift) prevents one from supposing that it was intended to apply to something which the guest himself brings and takes away with him.
That a large number of cases of supposed xenia must now be referred to double fertilisation can hardly be doubted, and some of these will be mentioned below, but we may select as a typical case the experiments made by Major Trevor Clarke and mentioned by Darwin in which a biennial stock with light brown seeds was pollinated with a variety having violet-black seeds. The resulting seeds showed in many cases the effect of so-called xenia. If, however, we examine the seeds of the ordinary biennial stock, we find that the black seeds have transparent seed-coats and the colour is entirely in the endosperm layer, the result seen, therefore, was undoubtedly due to double fertilisation, and not xenia.
The other effect which has been taken for xenia is that of Mendelian dominance. In Pisum this error was easy to make, and all early records of xenia in this genus are open to this objection.
In the experiments detailed below, which I have made myself, every precaution has been taken to ensure that sources of error shall have been eliminated. The flowers were emasculated before dehiscence of the anthers and carefully bagged, and pollen taken also from bagged flowers.
Many experiments have been made with this subject, and cases of supposed xenia have been frequently recorded.
In 1902 I obtained a race with a red seed-coat, which was mixed in with the commercial yellow variety. This was cultivated, and without exception came true from seed.
This was crossed with the yellow variety, the pigment being held in this case also in the seed-coat, but no xenia was visible. Seeds of the cross were sown the following year, but owing to climatic conditions no seeds were produced.
Webber has made many interesting experiments with maize, and much care was taken to ensure that the varieties chosen reproduced their seed characters truly. In some few cases the colour of the endosperm layer was changed as a result of double fertilisation, but the changes of form figured seem likely to be due to Mendelian dominance.
De Vries, in an interesting paper in the " Revue Générale de Botanique," April 1900, quotes Vilmorin as having observed changes of colour only in his experiments and no changes of form of seed, and on his own part suggests, as did Koernicke, that those changes occur only when an older character meets a younger, an idea similar to that once entertained with regard to Mendelian dominance. I can find no trace of any influence of the pericarp having been noted in maize.
BEANS—RUNNER AND DWARF.
I have tried very many experiments with these varieties of beans, but have had no success at all, the operation of emasculation being extremely difficult to perform, and the light colour of the pollen renders it almost impossible to ascertain if the stigma is pollinated or not. It is, however, notable that in these widely-grown plants, where in all cases the pigment is contained in the pericarp, I can find no recorded cases of xenia.
In the case of peas we come to the stronghold of xenia, as in this genus the evidence is most plentiful.
My own experiments were unfortunately mostly carried out with 'William the First' as a pollen-parent, and as I found out too late the variable character of the seed colour, my results are of no value. I have, however, repeated Darwin's experiment of crossing the blue-podded pea on to an ordinary green-podded variety, but in none of the eight pods which resulted could 1 see any influence of the pollen-parent.
In view of the great influence that premature ripening and other conditions of culture have upon seed colour and the lack of any chemical or microscopical tests, judgment must be suspended as to the occurrence of xenia in peas until a more exact knowledge of the pigments in question is available.
PEACHES AND NECTARINES.
In the course of crossing these fruits I have made some between the white and yellow-fleshed varieties such as 'Pineapple' nectarine (yellow) x 'Elruge' Nectarine (white), 'Pitmaston Orange' (yellow) x 'Elruge' (white), 'Sea Eagle' peach (white) x 'Barrington' peach (yellow).
In none of the above was any influence visible to the unaided eye, but the seedlings which are growing now may give some signs of their hybrid origin.
In these fruits xenia had been often recorded, and in view of the fact that there is no doubt that all effects seen are true influences of the pericarp, they offer a subject for experiment of peculiar interest. The case of the 'St. Valery' apple, mentioned by Darwin, which produced no pollen, but on being pollinated by other varieties gave fruits which resembled those of the male parent, will be well remembered. The case having thus been stated, it was but natural that corroborative evidence commenced to flow. My experiments, with one presumable exception, have so far failed to augment this stream.
The varieties chosen differed much in size and colour. For differences of size and form the following were used:— 'Stone's' (large) x 'Old Nonpareil' (small russet), 'Rosemary Russet' (medium) x 'Stone's,' 'Rosemary Russet' x 'The Queen' (large, flat), 'Sturmer Pippin' (small) x 'The Queen.'
Differences of colour:— 'Gloria Mundi' (green) x 'Rosemary Russet,' 'Gloria Mundi' x 'Hoary Morning' (distinct stripes), 'Stone's' (green) x 'Hoary Morning,' 'Stone's' x 'Old Nonpareil.'
In all the above cases no change was seen, though the crossing of a russet skin with a smooth green would seem to offer an easy and exact method of detecting xenia.
The only case in which any direct influence was seen was in the case of crossing a flower of 'Sandringham Apple' (large, faint stripes) with the pollen of 'Bismarck' (bright, non-striped red).
This fruit ripened and fell off three weeks before other uncrossed fruit on the tree, and was in shape and colour quite out of character, resembling a fine fruit of 'Cox's Orange.' This case was reported in the JOURN. R.H.S. vol. xxiv. part 4, p. 1899.
This is the only case of presumable xenia I have ever seen. The resulting seedlings are now growing.
SUMMARY OF RESULTS.
So far as my own experience goes, it seems that the occurrence of xenia as an influence of the pericarp is of rare occurrence, and, in my opinion, considerably more experiment is needed to establish the phenomena on a firm basis of positive fact. The methods of the recognition of colour are at present too inexact, and some chemical or microscopical test is needed. The experiments with apples and peaches and nectarines are open, of course, to the objection that the pollen of these fruits as hybrids would be mixed; but even then the resulting fruits might be expected to show some variation, though not corresponding to the pollen-parent. I cannot, perhaps, sum up the position better than in the words used by Bacon in his preface to the "Novum Organum" in reference to science in general, when he says "that it is not so much an opinion to be held as a work to be done."