168 BOTANY. 



water which is occasionally found in their cavities has little 

 or no physiological relation. 



218. The Equilibrium of the Water in the Plant. The 

 water in the tissues of every plant tends constantly to become 

 in equilibrium, and this state would soon be reached were it 

 not for certain disturbing causes which are almost as con- 

 stantly in action. In any cell an equilibrium may soon be 

 reached between the two forces which reside respectively in 

 the cell-wall and the protoplasm, viz., (1) the attraction of 

 the surfaces of the molecules for the water, and (2) the 

 " imbibition power " of protoplasm. 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 approximating very closely to it, is 

 reached by many of the perennial plants during the winter 

 or period of rest. 



219. 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. 



The chemical processes within the cell include : (I) the 

 actual use of water by breaking it up into hydrogen 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 formation of sub- 

 stances which are less soluble than those from which they 

 were formed. These processes take place in all cells, even 

 those of the simplest plants. 



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 (paragraphs 215 and 216 above) ; 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 meristem tissues 

 is obtained partly by direct absorption from the surround- 



