Morphogenesis 219 



The forces which, in the egg cell of a vascular 

 Cryptogam, for example, result in the first three 

 mitoses segregating cells which normally develop 

 into stem growing point, cotyledon, root, and sus- 

 pensor, respectively, are sufficiently clean-cut to 

 offer material capable of clear and definite analy- 

 sis, once a technique for such studies is available. 

 The tissue culture technique should make possible 

 the development of a true science of plant em- 

 bryology. 



One method of approach to problems of organi- 

 zation which has proved especially effective in 

 animal embryology is, as we have seen, that of 

 transplantation, developed to a high degree by 

 Harrison and by Spemann (1936, 431, 432). With 

 plant material, the Boysen-Jensen-Paal-Went 

 technique of transplanting Avena coleoptile tips, 

 or agar blocks containing extracts from such tips, 

 is essentially a comparable technique, the coleop- 

 tile or agar block (Went, 1928, 379) acting in the 

 same way as an organizer in the sense of Spemann. 

 If plant members such as coleoptile tips (Avery 

 and La Rue), root tips (White et al), stem tips 

 and buds (White), leaf primordia, etc., can be 

 grown in culture, can be subjected to controlled 

 environmental influences, and can then be trans- 

 planted into otherwise intact host plants, the ques- 

 tion of organ influences at a distance, of correla- 



