:254 PLANT GROWTH SUBSTANCES 



coleoptile primordia and the local degeneration of the endosperm in 

 the regions of embryonic differentiation. The author postulates that 

 hormones are liberated from the degenerating tissue, and that they 

 influence nuclear divisions, cell enlargement, and tissue differentiation. 

 Among other studies of this general problem have been those of 

 Brink and Cooper (2). Their approach has been to observe a variety of 

 cases of arrested seed development, and to attempt from an analysis of 

 these to arrive at an understanding of the important aspects of normal 

 development. They conclude that in the angiosperms the endosperm 

 typically plays an important part in the nutrition of the young embryo. 

 The endosperm, because of its genetic constitution, seems well suited 

 to compete successfully with the integuments which are at the start 

 bigger than the embryo or endosperm, and which are themselves 

 actively enlarging. The aggressive endosperm is able to establish itself 

 as a physiologically dominant tissue in the seed and simultaneously to 

 perform for the young embryo certain nutritional functions of which the 

 embryo is not yet capable. 



The endosperm is not, however, always of prime importance, as shown 

 by Cooper and Brink (3) in their study of the common dandelion. This 

 plant is an autonomous apomict, in which no fertilization occurs, either 

 of the egg or of the central nucleus. In contrast to a related sexual 

 species with the typically precocious endosperm, there appears to be 

 with this material no regular interdependence of the embryo and endo- 

 sperm. Many of the seeds show embryos that are much more advanced 

 than the corresponding endosperms. One regular feature in the common 

 dandelion is, however, the presence in the ovule, at the time the embryo 

 starts developing, of an extensive amount of stored food material. 



These examples are but a few of many studies which have shown that 

 the very young embryo is not independent but must rely for its nutri- 

 tion on the presence of some other tissue, be it endosperm or a functional 

 substitute for it. The actual compositions of the materials used by the 

 embryo have not been determined, but indirect evidence points to the 

 importance both of hormones and of substances of direct nutritional 

 value. 



The control of the growth of older embryos, especially in the stages 

 just prior to maturation, is also a question of interest. During the later 

 portion of embryonic growth, embryos are generally autotrophic with 

 respect to any special growth substances. They are often capable of 



