EXPERIMENTAL EMBRYOLOGY 389 



All the work so far has been of an exploratory nature only, but 

 some of the attempts made in this direction deserve special mention. 

 In 1936 La Rue reported having grown embryos of Taraxacum, 

 Chrysanthemum, Lactuca, Coreopsis, Lycopersicum, Nicotiana, Bryo- 

 phyllum, Zea, and Vallota which were only 0.5 mm. in length. His 

 experiments, as well as those of some of his predecessors, made it 

 clear that inorganic media alone are not adequate for the culture of 

 young embryos, since they also require sugar and other heat-stable 

 factors present in such substances as yeast extract or fibrin digest. 

 In some of his culture media, La Rue substituted indoleacetic acid 

 for yeast extract. 4 



Since embryos are nourished inside the seed by the endosperm, 

 Van Overbeek et al. (1942) thought of using coconut milk as one of 

 the ingredients of the culture medium. By adding this they suc- 

 ceeded in growing embryos of Datura stramonium which were still 

 in the heart-shaped stage and measured only 0.15 mm. in length 

 (the mature embryo is approximately 6 mm. long). Further study 

 revealed, however, that, in addition to the "embryo factor" neces- 

 sary for the growth of the embryo, coconut milk also contains one 

 deleterious substance which inhibits root growth and another which 

 causes a callus-like growth but no differentiation. 



Blakeslee and Satina (1944) found that powdered malt extract 

 shows embryo factor activity if sterilized by nitration instead of 

 by heat. At the same time Van Overbeek, Siu, and Haagen-Smit 

 (1944) discovered that extracts of Datura ovules, yeast, wheat germ, 

 and almond meal also possess this quality. 6 Finally, they claim to 

 have obtained a purified embryo factor preparation which, on the 

 basis of dry weight, showed an activity 170 times that of coconut 

 milk. 



Unfortunately neither coconut milk nor the purified "embryo 

 factor" preparations have given any uniform results in the hands of 

 other workers. Working with selfed and hybrid embryos involving 

 four species of Datura, Sanders (1948, 1950) has explained that 

 differences exist not only in the nutritive requirements of the differ- 

 ent species and their hybrids but also in those of embryos of the 



4 La Rue was also able to culture small bits of embryonic tissues, only 0.5 mm. 

 in length, and raise complete plants from them in Lactuca canadensis, Taraxacum 

 officinale, Chrysanthemum leucanthemum, and Lycopersicum esculentum. 



5 Kent and Brink (1947) report the presence of "embryo factor" in casein hy- 

 drolysate and in water extracts of date, banana, and tomato. 



