84 R. A. BRINK 



largement of the cells in the integuments. Thus, with the exception of the 

 nucellus which is broken down and absorbed by the rapidly expanding endo- 

 sperm, all the elements of the young seed which were previously quiescent, 

 suddenly spring into active growth following syngamy. 



Consideration of these differences between the seeds of gymnosperms and 

 angiosperms led us some ten years ago to explore the hypothesis that the 

 secondary fertilization in angiosperms is essentially a means of enhancing 

 the competitive power of the endosperm relative to the maternal portions of 

 the seed — by conferring upon the endosperm the advantages of hybridity. 

 The nutritive requirements of the young seed suddenly are raised from a low 

 to a high level since fertilization starts a new cycle of growth in the massive 

 integuments. The nutrient supply, on the other hand, quickly falls to the 

 plane which can be maintained by movement of foods into the seed from 

 other parts of the plant as a result of exhaustion of the limited ovule reserves. 



It seemed reasonable to assume that, within the seed, the incoming nutri- 

 ents would tend to be partitioned between the different tissues according to 

 the respective amounts of growth occurring in them. On this basis, the ex- 

 tensively developed integuments would consume the major portion. The 

 diminutive endosperm and embryo would receive but a small fraction of the 

 total. Under these conditions, failure of the young seed through starvation of 

 the embryo could arise, unless the endosperm — as the nutritive agent of the 

 embryo — were endowed with special properties which offset its initially small 

 size. It seemed essential that the endosperm, by one means or another, be 

 enabled to quickly acquire a position of physiological dominance in the 

 juvenile seed in order to insure continued development. 



Two genetic characteristics of the endosperm suggest themselves as being 

 important in this connection. The first is the triploid condition of the nuclei. 

 Little is known of the physiological effects of ploidy in general, and virtually 

 nothing of its meaning in special situations of this kind. One suspects, how- 

 ever, that the endosperm gains some advantage from its extra chromosome 

 garniture, as such, in mediating the relations between the diploid maternal 

 parent and the young diploid embryo. It is also probably significant that, 

 whereas the embryo inherits equally from the two parents, two-thirds of the 

 endosperm's genie complement is derived from the plant upon which it is 

 nutritionally dependent and one-third of the complement from the male 

 parent. 



Heterozygosis is the second characteristic of the endosperm which might 

 enhance the inherent physiological efficiency of this tissue. The possibility of 

 heterozygosity arises, of course, from the biparental origin of the endosperm 

 mother nucleus. The condition is realized in matings between genetically 

 different plants. Haploidy of the endosperm, as occurs in the gymnosperms, 

 appears to be genetically insufficient for seed development in the flowering 

 plants. Early post-fertilization circumstances, particularly the dependence 



