86 



R. A. BRINK 



of that of its neighbors in the same ovary. Furthermore, the quickly succeed- 

 ing secondary effects of fertilization, such as enlargement of the surrounding 

 fruit, are at a minimum. Studies on the reproductive physiology of the flower- 

 ing plants are rendered difficult by the multiplicity of changes which are 

 eventually set in motion in the tissues of the seed, the fruit, and the maternal 

 plant following fertilization. The sequence and interrelations of the events 

 immediately subsequent to syngamy are simpler to analyze than those which 

 occur later, in view of the fact that each very young seed may be considered 

 to behave independently of the others. 



The data in Table 5.1 show that, for each of the seven plants tested, the 



TABLE 5.1 



FREQUENCY OF FERTILE OVULES COLL.APSING IN SEVEN .ALFALFA 

 PLANTS FOLLOWING SELF- AND CROSS-FERTILIZATION. DATA BASED 

 ON COLLECTIONS AT 72, 96, 120, AND 144 HOURS .\FTER POLLINATION 

 (AFTER COOPER AND BRINK, 1940) 



frequency of seeds collapsing is much higher in the selfed than in the crossed 

 series. The proportions vary in different individuals from about 3 to 1 to 

 over 11 to 1. On the average, approximately five times as many seeds con- 

 taining inbred endosperms and embryos collapse within the first six days 

 after pollination as in the crossbred group. Since other factors were not 

 varied, the decrease in survival in the selfed series must be attributed to 

 the inbreeding. 



The evidence, both general and particular, points to the endosperm 

 rather than the embryo as the seat of the inbreeding depression effect. The 

 endosperm in alfalfa is free nucleate up to about 144 hours after pollination, 

 although it develops as a cellular tissue thereafter. Successive waves of 

 mitotic divisions traverse the tissue, the number of nuclei being doubled in 

 each cycle. Thus growth during this period proceeds at an exponential rate. 



