INBREEDING AND CROSSBREEDING IN SEED DEVELOPMENT 93 



development. These extreme examples {)oint unmistakably to the conclusion 

 that in the apomictic dandelion the endosperm, as the master tissue in the 

 young seed, has been disestablished. Embryo growth must be sustained by 

 other means. 



The substitute arrangement for nourishing the embryo in T. officinale was 

 disclosed by a histological study of the ovules of this species and T. kok- 

 saghyz. Basically the structure of the ovule is the same in both. As the female 

 gametophyte expands, the nucellus disintegrates so that the gametophyte 

 comes to lie in direct contact with the endothelium which comprises the in- 

 nermost layer of cells of the massive integument. The endothelium [)ersists 

 and appears to function in the transfer of nutrients during the course of seed 

 development. In T. kok-saghyz the inner layers of integumentary cells ad- 

 jacent to the endothelium lose their contents during formation of the game- 

 tophyte, and contain shrunken and misshapen nuclei when the ovule is 

 mature. The cells of the integument immediately surrounding this depleted 

 region are densely cytoplasmic and possess well-defined nuclei. The outer- 

 most parenchymatous cells of the integument are highly vacuolate. The 

 single vascular bundle makes an arc about the greatest circumference of the 

 ovule in both species. Only limited amounts of stainable reserve food ma- 

 terials occur anywhere in the T. kok-saghyz ovule. 



The T. officinale ovule differs conspicuously from that of T. kok-saghyz in 

 possessing an abundance of reserve food. The cells of the integument just 

 outside the endothelium enlarge as the ovule matures and become gorged 

 with a homogeneous material which appears to be proteinaceous in composi- 

 tion. This substance also extends between the cells at the outer edge of the 

 storage region proper. 



This extensive prestorage of protein-rich food material in the integument 

 provides an explanation of the fact that embryo development in the apomict 

 may proceed normally in spite of very limited endosperm growth. The con- 

 ditions render superfluous an aggressively functioning endosperm. The 

 embryo draws directly on a food supply already at hand. From the physio- 

 logical point of view, the nutritive mechanism in the apomict is analogous to 

 that in the gymnosperms. In both these classes of plants certain of the 

 processes essential to seed development, which follow double fertilization in 

 sexual species of flowering plants, are pushed back into the ovule. The 

 secondary fertilization, which through its efifect on vigor of endosperm growth 

 may be looked upon as a means of offsetting the tardy provision of nourish- 

 ment for the embryo, thus can be dispensed with. 



SEED DEVELOPMENT GRADE AND EMBRYO 

 GROWTH POTENTIALITIES 



The conclusion that growth of the angiosperm seed is basically controlled 

 by the endosperm has an interesting corollary. That is, that the grade of seed 

 development attained after a given mating is not a definitive index of the 



