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; (8) 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. 
