ON XENIA. 297 
ON XENIA. 
By Epwarp A. Bunyarp, 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 asa 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 
