INBREEDING AND CROSSBREEDING IN SEED DEVELOPMENT 87 



The concurrent development of the embryo, on the other hand, is relatively 

 slow. The zygote divides to form a two-celled proembryo. Successive divi- 

 sions of the apical cell give rise first to a six-celled proembr>'o and then to the 

 initials of the definitive embryo. 



The pronounced difference in rate of development of the two tissues is il- 

 lustrated by the fact that at 144 hours the modal number of cells in the 

 embryo is only 16, whereas the typical number of nuclei in the endosperm 

 at this time is 128. Rapid and precocious development of the endosperm as 

 seen in alfalfa is characteristic of the angiosperms in general. The much 

 higher level of activity of the endosperm is presumptive evidence tlvit this 

 tissue, rather than the embryo, is especially subject to develo{)mental upsets 

 in the young seed. Data available in the present instance provide direct con- 

 firmation of this interpretation. 



The comparative rates of growth of endosperm and embryo in the selfed 

 and crossed alfalfa series up to 144 hours after pollination are illustrated in 

 Figure 5.1. Not only are the values for the embryo low, but also there is 

 little difference between those for the inbred and crossbred series. The con- 

 clusion appears warranted that the direct effect of inbreeding on the embryo 

 at this stage, if indeed there is a demonstrable effect, is too small to account 

 for the high frequency of seed collapse. In contrast, there is a very sharp 

 decline in rate of nuclear division in the endosperm, following enforced self- 

 fertilization of this naturally cross-fertilized plant. The lower rate is shown 

 from the first division onward. There are about twice as many nuclei present 

 at 144 hours in the crossbred as in the inbred endosperms. 



Due to the partial self-incompatibility in alfalfa, fertilization on the 

 average, is slightly delayed following selfing. A comparison of the rate of 

 growth of the two classes of endosperms independent of time as shown in 

 Figure 5.2, however, establishes the reality of the difference in rate of growth 

 between the inbred and crossbred endosperms. When the seeds are arrayed 

 in terms of cell numbers of the enclosed embryo, it is found that for all nine 

 classes occurring in the material the endosperms are more advanced in the 

 crossbred than in the inbred series. That is to say, the embryos at a given 

 stage of development have associated with them more vigorously growing 

 endosperms following cross-fertilization than after selfing. Moreover, the 

 decrease in size resulting from the inbreeding is so large that one is led im- 

 mediately to suspect that herein lies the primary cause of the frequent seed 

 collapse following selfing. 



Why should impairment in rate of endosperm growth lead to arrested seed 

 development? The answer in the present case is clear. As was pointed out 

 earlier, double fertilization initiates not only endosperm and embryo develop- 

 ment, but also a new cycle of growth in the integuments. The latter compete 

 directly with the endosperm for the nutrients moving into the young seed. 

 If the endosperm is developing subnormally, a disproportionate amount of 



