INBREEDING AND CROSSBREEDING IN SEED DEVELOPMENT 



91 



arithmetically. It will be noted from Figure 5.3 that the mean embryo cell 

 number in T. officinale, before the endosperm mother cell divides (0 cycle), 

 is about 16. The corresponding value T. kok-saghyz is 1. This is a reflection of 

 the fact that the embryo in the apomictic sj)ecies usually starts growth in 

 advance of the endosperm. Although they start from different levels, the two 

 curves are not greatly dissimilar. The embryo in the common dandelion, on 

 the average, is consistently larger in the young seed than that of T. kok- 

 saghyz, relative to given stages in endosperm develo])ment. 



100 



T. officinale 



T. kok-soghyz 



4 



K.SJ 



12 3 4 5 6 7 



NUMBER OF DIVISION CYCLES IN ENDOSPERM 



Fig. 5.3 — Early growth of embryo of T . kok-saghyz and T. officinale in relation to endo- 

 sperm size. After Cooper and Brink, 1949. 



More instructive than the mean values on which Figure 5.3 are based, is 

 the variability in the frequency distributions concerned. The data are sum- 

 marized in Table 5.2. A logarithmic scale was used in expressing embryo 

 sizes merely as a convenient way of summarizing the widely dispersed values. 

 As mentioned above, growth of the embryo during this period is approxi- 

 mately linear. 



Table 2 reveals that the variability is low in embryo cell number at suc- 

 cessive stages of endosperm development in T. kok-saghyz. This means that 

 embryo and endosperm are closely synchronized in their growth in the sexual 

 species. The variability in embryo size in the apomict, on the other hand, is 

 enormous. For example, in seeds in which the endosperm is still at the mother 

 cell stage (0 cycle), the associated embryos are distributed over all size 

 classes from 1 to 128. The standard deviation for embryo cell number is 



