156 PLANT PHYSIOLOGY 



when negative chemotropism is exhibited, they will draw away 

 from the openings. The Japanese physiologist Miyoshi has used 

 leaves, previously injected with various solutions, for the same 

 purpose. With this method, positive chemotropism caused the 

 hyphae to penetrate through the stomata into the inner tissues 

 of the leaf, a picture very similar to that observed in cases Avhen 

 the leaf is infected by certain parasitic fungi. 



The specific effect of various substances will depend upon 

 their concentration. Excessive concentrations may change 

 positive chemotropism to negative. Fungal hyphae may exhibit 

 a strikingly high sensitivity to some substances. Thus Mucor 

 will show distinct curvatures when subjected to the effect of a 

 sugar solution of as low a concentration as 0.01 per cent. 



The growth of pollen tubes through the style to the ovule 

 presents a picture of chemotropism similar to that of fungal 

 hyphae. They invariably find their way to the embryo sac, 

 reacting doubtless, to the substances secreted by the ovule. If 

 an ovule is placed in a drop of water with some pollen in it, the 

 pollen tubes will grow toward the ovarian tissues. 



The chemotropism of roots has not been studied sufficiently. 

 The root responses to chemicals present considerable difficulties, 

 as the same substance diffusing into the environment of the root, 

 may produce a double effect on it. It may have a direct influence 

 on the growth rate of that part of the root that comes in contact 

 with the substance, and it may produce also an irritation in the 

 highly sensitive root tip. The two effects may be antagonistic 

 to each other. A toxic substance affecting the side of the root 

 with which it comes in contact produces a local inhibition of 

 growth and, consequently, a positive curvature. The same 

 substance affecting the tip of the root may produce therein a 

 condition of stimulation, which may be transmitted to the 

 growing zone and may result in the formation of a curvature in 

 the opposite direction. It has been possible to separate these 

 two effects by means of a unique method of procedure suggested 

 by Porodko (1911). He placed the seedlings of various plants 

 in a layer of gelatin gel, in which the diffusion of a substance 

 producing a chemotropic effect was known to be free from con- 

 vection currents. In some of the experiments, the tip of the 

 root was left outside the gelatin; in others, the growing zone was 

 outside. These experiments have led Porodko to conclude that 



