INBREEDING AND CROSSBREEDING IN SEED DEVELOPMENT 



89 



collapse of the endosperm then ensues. Significantly, breakdown of the 

 endosperm tissue begins in the region opposite the end of the vascular 

 bundle where the inner integument is especially hyperactive. Following col- 

 lapse of the endosperm, the young seed dies. 



SEED DEVELOPMENT WITHOUT FERTILIZATION 



There are a few species of flowering plants in which both endosperm and 

 embryo develop without fertilization. These so-called autonomous apomicts 



1-celled 

 proembri/o 



2-celled 

 proem bryo 



3-celled 

 proembryo 



4-celled 

 proembryo 



5-ceiied 

 proembryo 



6-celied 

 proembryo 



2-celied 

 emb.*5usp. 



3-ceiled 

 emb.-t-5u5p. 



4-celled 

 emb.+sasp. 



1 5elf-fcrtilization 

 cross-fertilization 



10 20 50 40 50 60 70 



Number of endosperm nuclei 



Fig. 5.2 — Number of endosperm nuclei associated with proembryos and embryos at various 

 stages of development following self- and cross-fertilization. After Brink and Cooper, 1940. 



should provide an independent test of the hypothesis that aggressive develop- 

 ment of the endosperm is requisite to seed development, and that the sec- 

 ondary fertilization is a device by which aggressiveness of the tissue is en- 

 hanced. On the basis of the reasoning applied to sexual species, one would 

 e.xpect to find in autonomous apomicts that the embryo is not basically de- 

 pendent on an active endosperm for its nourishment. So far as I am aware, 

 the evidence bearing directly on this question is limited to a single study 

 which Cooper and I carried out on the common dandelion, Taraxacum 

 officinale (Cooper and Brink, 1949). 



The common dandelion is triploid (3x = 24). The regularity and abun- 

 dance of seed production in the plant is well known. A full complement of seed 



