332 TEXTBOOK OF PLANT PHYSIOLOGY 



out. But no hydrotropic curvatures will be secured in an atmos- 

 phere completely saturated with water vapor, as in this instance 

 the whole space will be of equal humidity. Similar to geotrop- 

 ism, hydrotropic sensitivity is centered in the tip of the root. 



An unequal distribution of temperature may also produce 

 bending both of the roots and the aboveground organs, showing 

 also both positive and negative thermotropism. When the tem- 

 perature is below the optimum, plants will curve in the direction of 

 the more heated side; when above the optimum, they will bend 

 toward the colder side. Thermotropic curvatures may be secured 

 with roots growing in moist sawdust. 



The study of the reaction of plants to strong chemical and 

 thermal stimulation is to a considerable extent complicated by the 



Fig. 136. — Hydrotropism of roots. Instead of growing straight downwards, the 

 roots cling to the damp sawdust (after Osterhout). 



phenomenon of traumatropism, which was discovered by Darwin. 

 If the tip of a root is severely injured by means of a unilateral 

 incision or cauterization with an acid, etc., the irritation pro- 

 duced is transmitted to the zone of elongation and produces a 

 negative curvature. Traumatropic sensitivity permits the root 

 to avoid obstacles, which may stand in its way. It is, therefore, of 

 ecological importance. The aboveground organs, particularly of 

 seedlings, also exhibit traumatropic bending, but these are chiefly 

 of a positive nature. 



The so-called autotropism holds an exceptional place among 

 the tropisms. By this term is understood the tendency of a plant 

 to grow in an upright position. This tends to counteract the curv- 

 ing effect of the other tropisms. Thus curvatures formed owing 

 to a transient effect of some external stimulus are again straight- 

 ened out, providing they have not been definitely fixed by growth. 

 Autotropism also serves to straighten the small curvatures of a 

 casual nature. 



