340 PLANT GROWTH SUBSTANCES 



and possibly even the so-called "X-bodies" that flank the egg before 

 and immediately after fertilization, may have a similar function to 

 endosperm but more specifically in nutrition of the egg cell? 



Doubtless in the endosperm-embryo relationship hormones have an 

 important function. Voss (90) observed that isolated embryos of corn 

 require for their development a naturally occurring hormone, which 

 could not be replaced by indoleacetic acid. During germination a hor- 

 mone from endosperm evidently was translocated into the embryo. 

 This has been verified more recently by Guttenberg and Lehle-Joerges 

 (33) who detected in all parts of the embryo a special hormone that 

 originated in the endosperm. 



Fruit Thinning Again 



Now to turn once more to fruit thinning by means of a growth 

 regulator. In this connection we must consider the state of development 

 of the seed at the time the spray is applied and the possible effect of 

 NAA on the embryo and endosperm. According to Tukey (85) and 

 Harrold (36) microscopic embryos and normal endosperm are present 

 in the peach about the approximate time the growth regulator is applied 

 for thinning purposes. Five- to ten-celled embryos and a many-celled 

 endosperm are present in the embryo sac of the apple up to two weeks 

 after fertilization. 



In both stone and pome fruit (peach and apple), with which we are 

 concerned in this discussion, endosperm development after fertilization 

 and triple fusion precedes the division of the zygote nucleus (36,11). 

 It is most important to note here that Bryant (11) observed that, 

 depending on the kind of pollen used in fertihzation, the endosperm 

 nuclei seem to vary considerably in number during this early stage of 

 seed development. Growth of the endosperm usually lagged under self- 

 pollination and when a less efficient pollen was used. This reminds one 

 of the differences in endosperm production, as described by Cooper 

 and Brink (17) in distant crosses oiNicotiatia, Petunia, and other species, 

 wherein, however, the endosperm nuclei divided most rapidly after 

 self-fertilization and more slowly in hybridization, resulting in collapse 

 of the young seeds. This may have a considerable bearing on the dif- 

 ferential abscission of fruit as a result of spraying with NAA. 



That the apple embryo and endosperm when microscopic may be 

 sensitive is indicated by additional circumstantial evidence. As a result 



