CYTOST IN THE PLANT WORLD 247 



that phototropic, stereotropic and chemotropic re- 

 sponses in plants depended upon the release of some 

 endocellular agent by light, contact, pressure, or cau- 

 terization. This view was championed by the late 

 Jacques Loeb and the earlier literature on this sub- 

 ject is reviewed in his book Forced Movements, 

 Tropisms and Animal Conduct. (1918.) More re- 

 cently, Cholodny (1927) has reviewed this problem, 

 with particular reference to plants. He points out 

 that all the various tropistic responses of plants, which 

 are essentially growth changes, can best be explained 

 upon the assumption that, due to pressure, injury, or 

 light, the plant cells give rise to a "Wachshormone" 

 which, by diffusion to various parts of the plant, 

 brings about alterations in growth, the asymmetrical 

 growth characteristic of tropic curvatures being due 

 to an uneven distribution of the growth-promoting 

 substance. 



A few years ago Paal (1918) presented evidence 

 for the presence of such a substance in the tips of 

 coleoptiles of grasses. Later Went (1928a) succeeded 

 in extracting such substances from the coleoptile tips 

 and devised a quantitative method of estimating the 

 activity of such extracts in accelerating the growth 

 of plants. The latter author (Went, 1928b) has also 

 demonstrated that the quantitative differences in 

 growth on the two sides of a seedling which follow 

 unilateral illumination must be due to changes in the 

 concentration of the growth-promoting substance on 

 the illuminated side. 



When plants are subjected to continual friction, 

 fungus infection, or chemical irritants, they frequently 

 give rise to pathological outgrowths or intumescences. 



