THE RELATION OP THE PLANT TO WATER 165 



the free or exposed surfaces of the cells of such plants as the 

 algae, but less noticeable where the cells are in mass and the 

 pressure of each is counteracted by the pressure of its neighbors. 

 When the cells are overfilled with water and the walls dis- 

 tended, they are said to be turgid. The rigidity of the leaves 

 and fleshy parts of the plants is due to the turgor of the cells. 

 When the cells lose their turgidity the parts become flaccid and 

 are said to be wilted. This accounts for the wilting of the foliage 

 and for the shriveling of stored fruits, tubers and roots. If we 

 study a wide range of plants we find a great variation in the 

 turgidity of the cells. In our ordinary- cultivated plants turgor 

 in the epidermal and parenchyma cells is equivalent to that of a 

 7 per cent, sugar solution or about 4.5 atmospheres. It is much 

 higher in some plants. 



Hydrostatic Rigidity. — When the plant is in an active condi- 

 tion of growth, the cells are well supplied with water and the 

 walls are distended; i.e., turgid. This is well illustrated in 

 fleshy plants and in the fleshy parts of woody plants, which, are 

 firm and erect because of the turgidity of the individual cells of 

 which they are composed Even the most delicate plants are 

 capable of sustaining relatively heavy loads when the cells are 

 distended with water. This condition is known as hydrostatic 

 rigidity. When the water content is lowered so that the cells 

 are no longer distended the plants are wilted. 



Plasmolysis and Wilting. — Plasmolysis is the opposite of 

 turgor. If the soil solution surrounding tbe root-hairs is of a 

 greater density than the sap within the cells, there will be an 

 outward movement of the liquid within the cell (exosmosis), and 

 the protoplasm will contract and withdraw from the cell wall 

 (Fig. 97). The plasmolysis of the individual cells results in 



