SFXT. II 



PHYSIOLOGY 



247 



opposite side becomes shorter or longer, and also from the unequal 

 elongation or contraction of both sides, and similarly from the elonga- 

 tion of one side and the contraction of the other. 



Such curvatures most frequently occur in plants as a consequence 

 of UNEQUAL GROWTH. More rarely they are due to the different length 

 of the opposite sides, resulting from unequal turgor tension. This is 

 principally the case in fully-grown organs, as in leaf-cushions (p. 268) 

 and stamens. A third source of curvature is found in the unequal 

 amount of water taken up by imbibition, and the consequent unequal 

 distension of the cell walls on the opposite sides of an organ. 



1. Hygroscopic Curvatures (Imbibition Movements) 



As the cell walls of actively living cells are always completely 

 saturated with imbibition water, hygroscopic curvatures are exhibited 

 only by dry and, for the most part, 

 dead tissues ; although occasionally 

 they also take place in living tissues 

 which can endure desiccation without 

 injury, as in the cases of Mosses, 

 Lichens, and Selaginella lepidoplujlla 

 (p. 179). The hygroscopic move- 

 ments in any case, however, are due 

 to the physical properties of the cell 

 walls, and have no direct connection 

 with the vital processes, except in 

 so far as the capacity of cell walls 

 to swell and take up large quanti- 

 ties of imbibition water is due to 

 the protoplasm by which they were 

 formed. The activity of the proto- 

 plasm in the formation of the cell 

 walls is likewise manifested in their 

 anatomical structure, in their strati- 

 fication and striation, and in the 

 position of the pits, as well as in 

 the arrangement and disposition of 

 the cells themselves. 



The absorption of imbibition 

 water by cell walls is accompanied 

 by an increase in their volume, and 

 conversely the volume of the cell 

 walls is diminished by the evaporation 

 of the imbibition water. Accordingly, whenever unequal amounts of 

 water are held by the cell walls on the different sides of an organ, either 

 through unequal absorption or evaporation, hygroscopic movements 



A ~B 



Fig. 200. — Fruit of Erodium gruinum. A, in 

 the dry condition, coiled ; B, moist and 



elongated. 



