70 BOTANY. 



and (2) the " imbibition-power" of protoplasm. So, too, an 

 equilibrium between cell and cell may soon be reached. 

 This equilibrium once attained, all motion of the water 

 must cease, and it must remain at rest until disturbed by 

 some other force or forces. This condition, or one ap- 

 proximating very closely to it, is reached by many of the 

 perennial plants during the winter or period of rest. 



130. Disturbance of Equilibrium. — During the growing 

 stages of plants the equilibrium of the water is constantly 

 disturbed in one or more ways, viz., (1) by the chemical 

 processes within the cells ; (2) by the " imbibition-power " 

 of the protoplasm and walls of newly formed cells ; (3) by 

 the evaporation of a portion of the water. 



131. The chemical processes within the cell include : 



(1) the actual use of water by breaking it up into hydro- 

 gen and oxygen ; every molecule which is so broken up 

 leaves a vacancy which, sooner or later, must be replaced; 



(2) the formation of substances which are more soluble 

 than those from which they were formed : (3) the forma- 

 tion of substances which are less soluble than those from 

 which they were formed. These processes take place in 

 all cells, even those of the simplest plants. 



132. In plants composed of tissues, wherever new cells 

 are forming and developing, the new protoplasm and cell- 

 walls require considerable quantities of water to satisfy 

 their molecular attraction; this supply is always made in 

 part or entirely at the expense of the adjacent cells. In 

 many aquatic plants there can be little doubt that the needed 

 water in growing tissues is obtained partly by direct ab- 

 sorption from the surrounding water, but this can only be 

 the case with the external cells; the deep-lying ones must 

 obtain their supply from the cells which surround them. 



