June, 191/] Xenia Following Fertilization 277 



fertilization in the angiosperms the pollen tube discharges its 

 contents of the two male cells into the embryo sac. One of 

 these cells reaches the egg cell and by fusing with it completes 

 fertilization. Rapid divisions take place in the fertilized egg 

 and the embryo is formed. In order for the endosperm to be 

 formed, the second male nucleus must enter into a fusion also 

 and with the definitive nucleus which has arisen as the result 

 of the fusion of the polar nuclei. Hence the name triple fusion 

 apth^ describes this union. 



Xenia and Xeniophyte Tissue. 



The resulting endosperm is therefore a unique tissue in the 

 Plant Kingdom. Since it is a fusion product, as is the embryo, 

 it must contain more than the haploid (x, or gametophytic) , 

 number of chromosomes. On the other hand, although formed 

 similarly to the sporophyte, it contains more than the 2x or 

 diploid number. Since the definitive nucleus is made up of 

 two polar nuclei, each containing a full set of chromosomes and 

 another set is brought in by the male cell, the endosperm must 

 have at least 3x, or a triploid number of chromosomes. How- 

 ever, the number is not necessarily limited to three, for in some 

 cases adjacent cells start fusing with the polars before the male 

 cell joins in the fusion. Because of its unparalleled origin, the 

 endosperm is different from the sporophyte and yet, unlike 

 the so-called "endosperm" of gymnosperms, is not female 

 gametophyte. So Trelease (10) has named this tissue, 

 which he characterizes as "neglected," the xeniophyte, for 

 it is here and here only that xenia will be expressed if it is to 

 appear. 



The formation of the xeniophyte is so closely associated 

 with the formation of the sporophyte, that one would expect 

 to find them closely related to one another in the plant's life 

 processes. This is actually the condition. They are side by 

 side in the seed and in sprouting the growing embryo parasiti- 

 cally consumes the reserve store of food materials of w^hich the 

 xeniophyte is composed. As far as the heredity is concerned, 

 this use which the plant is able to make of the stored food 

 materials of the endosperm disposes of the nx chromosome 

 number of the xeniophyte tissue, and together with that any 

 irregularities in the expression of the contained chromosomes. 



