56 Plant Tissue Culture 



immersed in a nutrient substratum (the soil solu- 

 tion), the essential feature of which is a mobile 

 liquid solution so that placing them in a liquid 

 nutrient does not involve any marked change in 

 their environment, provided the nutrient is suffi- 

 ciently aerated in some way (Cannon, 1932, 240; 

 Zimmerman, 1930, 270; Steward, Berry, and 

 Broyer, 1936, 262; Hoagland and Broyer, 1936, 

 246). This substratum is, in nature, a fairly com- 

 plex solution of salts, humic acids, and other sim- 

 ple organic compounds. The major problem in 

 duplicating the environment of a root is in pro- 

 viding the equivalent of the organic materials 

 ordinarily supplied by the parent plant, and a 

 glance at the anatomy of a rapidly growing root 

 shows that these materials, coming from the 

 phloem, must travel for considerable distances by 

 diffusion through other cells before they can reach 

 the dividing cells themselves. It might, therefore, 

 be anticipated that these organic materials would 

 prove to be relatively simple substances, as indeed 

 they have (see later). The problem of growing 

 excised roots should, therefore, both from a me- 

 chanical and a physiological point of view, be rela- 

 tively simple. Kotte's (1922, 51, 52) and Robbins' 

 (1922, 57) early work supported this supposition. 

 Excised root tips have, in fact, served as the ex- 

 perimental material par excellence for the pre- 



