394 VEGETABLE GROWTH. 



accompanying figure shows an easy method of demonstrating 

 this mode of governing the direction of growing roots. 



1032. Thermotropism. As might be expected from what has 

 been said regarding the tensions of tissues and the facility with 

 which their balance is disturbed, the effect of warmth in govern- 

 ing the direction of a growing organ must be considerable. Cur- 

 vatures dependent upon temperature are called thermotropic. 



1033. Assumption of definite form during growth depends, of 

 course, chiefly upon inherited tendencies ; but there have been 

 experiments which show that to a slight extent it may be pos- 

 sible by external influences to induce special shapes of growing 

 structures. Among the most interesting of these are the experi- 

 ments by Pfeffer l upon the growth of bilateral organs in some 

 of the lower plants, especially Marchantia ; by De Vries 2 upon 



1 Arbeiten des hot. Inst. in Wiirzburg, 1871, p. 77. 



2 Arbeiten des bot. Inst. in Wiirzburg, 1872, p. 223. 



PIG. 174. Boots of seedlings affected by moisture during their descent. The ap- 

 paratus consists of a network frame filled, with moist sawdust in which the seedlings 

 germinate. (Sachs.) 



