330 TEXTBOOK OF PLANT PHYSIOLOGY 



jected to a unilateral exposure. These tropisms, however, have 

 been studied very little. Moreover, their role in the life of a plant 

 is comparatively small. Therefore, their discussion will be limited 

 to only a short review. 



Chemotropic curvatures are caused by an unequal distribution 

 of certain substances in the environment of a plant. Chemotrop- 

 ism is of the greatest importance in the life of fungi and other sa- 

 prophytic and parasitic plants. It assists them to direct their 

 hyphae and haustoria to the source of nutritive substances. There 

 are two kinds of chemotropisms, positive and negative. The 

 former is caused chiefly by various nutritive substances, namely, 

 sugar, peptone, asparagine, phosphates and ammonia compounds. 

 Negative chemotropism manifests itself when acids and alkalies 

 are applied. It may be produced also by certain salts, such as 

 potassium nitrate and magnesium sulphate. A good method for 

 the study of chemotropism in fungi is to sow their spores upon a 

 perforated plate of mica, and placing it over a layer of gelatin 

 which has been previously soaked in the substance that is being 

 investigated. In the case of positive chemotropism, the fungus 

 hyphae will penetrate through the perforations, but when negative 

 chemotropism is exhibited they will draw away from the openings. 

 The Japanese scientist, 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 when the leaf is infected by a 

 parasitic fungus. 



The specific effect of various substances will depend on their 

 concentration. Excessive concentrations may change positive 

 chemotropism to negative. Fungal hyphae may exhibit a strik- 

 ingly 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 pre- 

 sent a similar picture of chemotropism to that of fungal hyphae. 

 They invariably find their way to the embryo sac, being guided, 

 doubtless, by the substances secreted by the maturing egg cells. 

 If a bit of an ovary be added to a drop of water with some pollen in 

 it, the pollen tubes will be directed toward the ovarian tissues. 



The chemotropism of roots has not been studied sufficiently. 



