in plants are turgor movements involving 

 receptor-conductor-effector structures similar 

 to those of Mimosa. Accordingly, (1) the 

 drooping of leaves during rainfall, (2) the 

 movements by which leaves and flowers fol- 

 low the course of the sun, (3) the leaf-closing 

 movements by which insectivorous plants 

 trap their living prey (Fig. 14-2), and (4) the 

 sleep movements of many plants (Fig. 14-3) 

 are all considered as typical turgor move- 

 ments. 



GROWTH MOVEMENTS 



An example of growth movement is the 

 bending of young stems toward a source of 

 light (Fig. 14-4). Such growth movements 

 proceed so slowly that about an hour is re- 

 quired to appreciate the change. Growth 



The Responses of Higher Plants - 261 



movements tend to be irreversible. The bend- 

 ing depends upon a faster lengthwise growth 

 of the cells on one side of the stem, and the 

 convexity of the curvature always lies on the 

 side of faster growth (Fig. 14-4). 



The fundamental difference between 

 growth and turgor movements was not clari- 

 fied until 1910, when evidence began to ac- 

 cumulate proving the existence of growth 

 substances, which regulate the growth of 

 cells in different parts of the plant. Previ- 

 ously it was known that cutting off the meri- 

 stem tissue at the tip of a stem stops growth 

 in the cells of the stump, where normally 

 the cells continue to elongate until they be- 

 come fully differentiated. The situation re- 

 mained obscure, however, until about 1930. 

 Then as a result of the researches of Boysen- 

 Jensen, in Denmark, and Frits Went, in 



Fig. 14-2. Venus's-flytrap (Dionaea), catching and digesting a fly. This move- 

 ment is a turgor movement. (Copyright, General Biological Supply House, Inc.) 



