190 Plant Tissue Culture 



function in root tissues. Root cultures, although 

 immersed in nutrient fluid, nevertheless form well 

 differentiated vascular strands (Fig. 55). The 

 existence of such strands suggested the possibility 

 that they might be serving some real function, and 

 means were devised which made possible the 

 demonstration of the existence of a unidirectional 

 flow of liquid through these immersed organs 

 (White, 1936, 264). Further study showed that 

 this flow was not continuous but had a diurnal 

 rhythm and that it was maintained even when 

 opposed by very great external pressures (Fig. 

 53) (White, 1938, 267-269). The existence of a 

 rhythmic secretion was verified by Grossenbacher 

 (1938, 243, 1939, 244), and the unidirectional 

 character of the flow and the magnitude of the 

 force were verified by Rosene (1937, 259, 1941, 

 260). A correlative diurnal rhythm in respira- 

 tion rate has likewise been demonstrated in grow- 

 ing excised roots (White, 1942, 37, 38). The tech- 

 niques required for study of this phenomenon are 

 far from simple, so that investigations thereon 

 have been intensive rather than extensive (Figs. 

 56-58). The problem is, however, an impor- 

 tant one in its bearing on the question of water 

 movement in plants and of glandular activity in 

 general. It serves as an excellent example of the 

 unforeseen applications which a new technique 

 may find. 



