R. A. BRINK 



University of Wisconsin 



Chapter 5 



Inbreeding and Crossbreeding 

 in Seed Development 



It is now generally recognized that the effects on growth of inbreeding and 

 crossbreeding are intimately interwoven in the whole complex fabric of 

 development and reproduction. Not only are the effects widespread and 

 often of major consequence in the economy of the organism, but sometimes 

 they are manifested in devious ways. Such is the case in the seed of flowering 

 plants. 



The success or failure of seed development turns primarily, not on the 

 embryo which embodies the line of descent, but upon an accessory organ of 

 reproduction, the endosperm. The novel origin and sensitivity of this latter 

 tissue to changes in genetic composition render early seed development one 

 of the critical stages in the life cycle of flowering plants. My colleague, D. C. 

 Cooper, and I have been exploring these relations during the past decade. An 

 attempt will be made here to review some of the evidence upon which our 

 point of view rests, and to call attention to some of the broader implications 

 of the main facts. 



As a means of bringing the important aspects of the problem in flowering 

 plants into focus, seed development in the angiosperms and gymnosperms 

 will be compared. Essential features of the general hypothesis by which we 

 have been guided will then be set fo"rth. The central role of the endosperm in 

 formation of the angiosperm seed and the responsiveness of this tissue to 

 variations in genetic composition will be illustrated by a consideration of the 

 immediate effects of self- and cross-fertilization in alfalfa. It will then be 

 shown that the means by which the embryo in the common dandelion, an 

 autonomous apomict, is nourished is of a type which would be expected 

 according to the hypothesis proposed. 



* Paper from the Deparlment of Genetics, College of Agriculture, University of Wiscon- 

 sin, No. 432. 



81 



