GROWTH OF PLANTS 155 



in itself decreases the amount of the growth-promoting substance 

 in the apex. This explains the retarding influence of light on 

 growth. Moreover, under the influence of unilateral iUumina- 

 tion, there takes place an electrical polarization of the cells that 

 produce and transmit the growth-promoting substance. The 

 illuminated side receives a negative, the shaded side a positive, 

 charge. Under the influence of this polarization, the current 

 of growth-promoting substance is shifted to the shaded side. 

 The cells of this side elongate more rapidly and more strongly 

 than do those of the opposite side, and as a result a curvature 

 of the seedling toward the light appears. 



Not all the rays of the solar spectrum produce an equal photo- 

 tropic effect. Red rays, as a rule, are least effective. Toward 

 the blue end of the spectrum, there is an increased effect, which 



o 



attains its maximum in the indigo-blue rays (4650 A.) and then 

 again gradually decreases toward the ultraviolet region. 



36. Chemo-, Hydro-, and Thermotropism. — Besides the force 

 of gravity and light, there are a number of other factors that may 

 cause curvatures in the growing organs of plants, if they are 

 subjected 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, discussion of them 

 will be limited to only a short review. 



Chemotropic curvatures are caused by an unequal distribution 

 of certain chemical substances in the environment of a plant. 

 Chemotropism is of the greatest importance in the life of fungi 

 and other saprophytic and parasitic plants. It directs the 

 growth of their hyphae and haustoria toward the source of 

 nutritive substances. There are two kinds of chemotropisms, 

 positive and negative. The former is caused chiefly by various 

 nutritive substances, viz., sugar, peptone, asparagine, phos- 

 phates, and ammonia compounds. Negative chemotropism 

 manifests itself when acids and alkalies or salts of heavy metals 

 are applied. Chemotropism 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 on a perforated plate of mica, placing it over a 

 layer of gelatin that 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 



