210 Plant Tissue Culture 



number which it should be possible to attack by 

 this means is legion. It seems desirable therefore 

 to outline some of the possibilities. (See Harri- 

 son, 1912, 415, 1933, 416 ; Fischer, 1922, 405, 1930, 

 409, 1931, 410; Fischer u. Parker, 1929, 411; 

 Parker, 1934, 427, 1936, 428, 429). 



That the developmental and behavior pattern 

 in higher animals is greatly influenced by meta- 

 bolic products known as "hormones" is today 

 generally recognized. Of recent years, there has 

 grown up a considerable literature on "plant 

 hormones" as well. The first plant hormone, 

 "auxin," was originally studied as a factor in 

 geotropic and phototropic, hence purely orienta- 

 tion, responses (Paal, 1914, 367, 1919, 368; Boysen- 

 Jensen, 1910, 344). It has subsequently come to 

 be looked upon as a true morphogenetic hormone, 

 since it seems to induce the differentiation of roots 

 from tissues that ordinarily rarely, if ever, pro- 

 duce roots (Went, 1929, 380 ; Bouillenne et Went, 

 1933, 277 ; Thimann and Went, 1934, 229 ; Laibach, 



1935, 218; Thimann and Koepfli, 1935, 376; 

 Cooper, 1935, 345, 1936, 346; Zimmerman et al., 

 1933, 384, 1935, 385, 386), to control the formation 

 and development of buds (Thimann and Skoog, 

 1933, 377, 1934, 378; Snow, 1925, 372; LeFanu, 



1936, 359; van Overbeek, 1938, 366), to influence 

 the abscission of leaves and of fruits (Gustafson, 



