33^ PLANT GROWTH SUBSTANCES 



of fruit growth and prompt abscission. When the embryo was destroyed 

 between stages two and three the fruit failed to reach full size and 

 ripened quickly. Destruction of the embryo in stage three was followed 

 by increased fruit growth and somewhat earlier ripening. From these 

 and other studies (88) it is quite evident that the embryo has a definite 

 bearing on fruit formation. Naturally occurring hormones undoubtedly 

 play a role in this relationship. Most probably they regulate the metabo- 

 lism and nutrition not only of the embryo but also of the fruit. Much 

 evidence that hormones are important factors in the development of 

 young fruit comes from the successful use of various synthetic growth 

 regulators in production of parthenocarpy (32). Moreover, it has been 

 found in several instances that seeds of developing fruit are rich in 

 hormone content (22,36). 



The situation in the apple as regards the relative period and rate of 

 development of the embryo, endosperm, nucellus, and integuments is 

 similar to that of a drupe fruit (24,80,76). But since the bulk of the apple 

 is made up of accessory tissues (torus), it does not show cyclic stages of 

 growth (89). A fairly good correlation, however, exists between seed 

 (embryo) number and size of apples during early stages of their develop- 

 ment (39). Later on, before ripening, this relationship seems to vary 

 considerably depending primarily on the size of the crop and the avail- 

 able food supply (74). Seeds do, of course, have a local effect on growth 

 of the pericarp (86) and torus (74). 



By means of artificial culture of embryos of various ages, isolated from 

 their natural environments and associated tissues, information has ac- 

 cumulated as to their nutrient requirements (74,67,69). In most instances 

 it has been found that in addition to the necessary inorganic substances 

 and certain organic nutrilites, an embryo factor is indispensable. This 

 has been discovered in yeast (44, 92, 43), tomato juice (43), malt 

 extracts, and a number of other plant-tissue extracts and substances. 

 Older embryos, being autotrophic, do not seem to require this factor. 

 Van Overbeek (67,69) has successfully used a liquid endosperm, coco- 

 nut milk, for the culture of young Datura and other embryos. At 

 least two important factors probably are present in coconut milk: a 

 thermolabile one, which causes embryos to grow rapidly, and a heat 

 stable factor that inhibits root development of the embryo. 



Embryo culture is of considerable aid in obtaining plants from certain 

 self- and cross-sterile matings and in the production of seedlings from 



