256 PLANT GROWTH SUBSTAN'CES 



more than .3 mm. in length did not require coconut milk for continued 

 growth in litro, while smaller embryos failed to develop even if the 

 medium were supplemented with coconut milk. This study indicates 

 that the requirements for embryo growth var\' among different plant 

 species. 



Curtis (4) found that the growth of embryos of many orchid species 

 was better and more regular when peptone or nucleic acid were present 

 in the medium. Nucleic acid from yeast was also shown by Kent and 

 Brink (7) to be favorable for the growth of barley embryos which were 

 too voung to develop in its absence. The components of the substance 

 which are active in this case have not been determined, but they are 

 known to be heat stable. 



Sanders and Burkholder (9). working with Datura embryos, concen- 

 trated their attention on the effects of various amino acids, and they 

 showed that casein hydrolysate or a mixture of 20 amino acids markedly 

 improved the growth and differentiation of the two species studied. The 

 addition of individual amino acids and mixtures of a few generally 

 resulted in much poorer growth than that occurring on the more com- 

 plete mixture. Some of the individual amino acids or small groups of 

 them caused modifications in the proportions of the embryos, increasing 

 or decreasing the cotyledon size and increasing the number of root 

 primordia. and sometimes premature root growth resulted. The amino 

 acids in the medium thus appeared to influence both the growth and 

 differentiation oi Datura embryos. Spoerl (10), in his studies on orchid 

 embryos, tested the effects of 19 amino acids as nitrogen sources, and 

 found that most of them inhibited growth. Arginine. however, sup- 

 ported good growth of embryos from unripe seeds, while aspartic acid 

 proved to be a satisfactory nitrogen source for older embryos. 



In a studv of the effect of casein hydrolysate on the growth of im- 

 mature Hordeitm embryos, Ziebur and co-workers (13) found that they 

 could not duplicate the effect of this product with a mixture of merely 

 amino acids. Casein hydrolysate has a clear-cut. two-fold effect on 

 barlev embryos: it prevents germination, and it supports embr^^onic 

 growth. It was found that both parts of this effect could also be pro- 

 duced bv supplementing the medium with amino acids, inorganic phos- 

 phate, and sodium chloride, all of which are components of commercial 

 casein hydrolysate. Table i compares the effects of casein hydrolysate 

 and of its three components, used singly and all together. The embryos 



