

380 M or pho genetic Factors 



and Burkholder (p. 30) found that the more rapid elongation here was 

 the result of increase in cell length rather than in cell number. 



Most tropisms have now been shown to be due to greater growth on the 

 convex side because of longer cells there (Fig. 18-5). In both photo- 

 tropism and geotropism, however, the role of auxin is different in root and 

 shoot. If a young plant, for example, is placed in a horizontal position, 

 auxin can be shown to accumulate on the lower side, though how this 

 occurs is not well understood. In the stem, this results in a bending 

 upward of the axis. The young primary root, however, will bend down- 



THALLUS 



HOLDFAST 



NO. HOLDFASTS 



CONCEN TRATION 



Fig. 18-4. Effect of different concentrations of indoleacetic acid on the growth of 

 Fucus, as measured by length of thallus and length and number of holdfasts, in com- 

 parison with untreated controls. ( From Davidson. ) 



ward, the cells elongating faster on its upper side. Evidently the same 

 concentration which stimulates cell elongation in the stem checks it in 

 the root, another example of the difference in the effect of auxin under 

 different conditions. Geiger-Huber and Huber (1945) found that by 

 continually decapitating a root its auxin content was markedly reduced 

 and that it now bent upward instead of downward. 



How much of the difference in cell size among the various tissues of 

 the plant is due to auxin it is hard to say but, directly or indirectly, this 

 substance is probably involved in most of the differences in degree of 



