82 R. A. BRINK 



An illustration will next be given of endosperm failure as an isolating 

 mechanism. Finally, the significance of the present results for the problem 

 of artificially rearing embryos whose development in the seed is blocked by 

 endosperm disfunction will be pointed out. 



Complete literature citations are not given. These may be found in the 

 summary paper (Brink and Cooper, 1947) in which much additional evidence 

 bearing on the present thesis also is presented. 



The endosperm is a special structure intercalated between the female 

 parent and the embryo, serving to mediate the relations between the two. 

 The tissue originates from the central cell of the female gametophyte, follow- 

 ing a fertilization distinct from that giving rise to the embryo. The secondary 

 fertilization is unusual in that two identical haploid nuclei of maternal origin 

 are united with one contributed by the pollen. The endosperm thus becomes 

 3x in chromosome number in contrast with the 2x condition of the embryo 

 and the mother plant, respectively. Endosperm and embryo carry the same 

 kinds of genes, but the genie balance may be unlike in the two tissues by 

 virtue of the double contribution to the endosperm from the maternal 

 parent. A further element of genetic heterogeneity in the seed arises from the 

 fact that nucellus and integuments, which are maternal structures, may 

 differ in genotype from the endosperm and embryo which they enclose, 

 since they belong to the previous generation. 



These facts, of course, have long been known. Certain of their implica- 

 tions, however, are only now becoming apparent. Particularly is this true of 

 the secondary fertilization on which our attention will be focussed. 



A word should be said at this point concerning the manner in which the 

 endosperm should be visualized. Many are familiar with the tissue only in 

 the mature seeds of species in which the endosperm persists as a storage 

 organ. This condition, well known in the cereals, for example, is exceptional 

 among flowering plants, and represents a secondary adaptation of signifi- 

 cance mainly for the future seedling. In most species the endosperm either 

 does not persist in the fully developed seed or occurs therein as a residue 

 only. On the other hand, the endosperm is regularly a prominent organ in 

 the juvenile seed. It is especially active directly following fertilization, during 

 what may be termed the lag phase of embryo growth. This period is seldom 

 longer than a few days, and varies according to the species. In spite of its 

 typically ephemeral character, the endosperm plays a critical role in (1) 

 transforming the mature ovule into a young seed and (2) nourishing the 

 embryo during its initial period of growth. We are here concerned with the 

 endosperm in these two relationships only. 



THE SEED IN GYMNOSPERMS AND ANGIOSPERMS 



It is helpful in understanding the significance of the secondary fertilization 

 to compare the circumstances of seed development in the angiosperms with 



