194 



EMBRYOGENESIS IN PLANTS 



Wardlaw, 1954). In Pinus ponderosa, Buchholz and Stiemert (1945) 

 and Buchholz (1946) have demonstrated that there is a close and direct 

 correlation between seed size, endosperm size, embryo size and cotyle- 

 don number. The embryos of large seeds, at the time of differentiation 

 of the shoot apex and the cotyledon primordia, are much larger than 

 those of small seeds, and have many cotyledons. The number of coty- 

 ledons may range from 6 to 16, embryos with these numbers being 



B 



D 



I . I, ' I I i/ f' 1' ii, ' .V I 



m\ 





Fig. 42. Variable cotyledon number in embryos 



A-E, Cedriis libani. A, Ordinary embryo with no fusion of cotyledons. B, C, 

 Earlier and later stages in fusion. D, A large cotyledon that appears to have arisen 

 by fusion. E, Evidence of cotyledon abortion. ¥-G, Pinus banksiana. F, Fusion of 

 two cotyledons. G, Transverse section of a group of cotyledons; s, shoot apex 



(x 16, after Buchholz). 



found in seeds with a volume range from 50-140 cu. mm. {see also 

 Butts and Buchholz, 1940). 



Embryonic Selection. Buchholz has shown that for a period of 

 about five weeks during the early embryogeny little differentiation takes 

 place, the several embryos within the ovule competing for survival. 

 This has been described as the period of embryonic selection. Once 

 predominance has been attained by one embryo, it grows rapidly and 

 differentiates all its organs in 10-12 days; seed ripening is completed 

 in a few weeks with little change in seed size. When the embryo eventu- 

 ally attains to its full size. Fig. 37, it is surrounded by the enlarged female 

 gametophyte which has grown so as virtually to fill the remainder of 

 the seed cavity, the whole being enclosed within the hard stony layer 

 which constitutes the testa or seed coat. 



