CHEMISTRY AND PHYSICS OF PLANTS. 62 
126. This avidity of protoplasm for water plays an im- 
portant part in the general economy of the plant. By it 
all the cells which contain protoplasm are kept turgid, and 
by the tension thus created the soft parts of plants are 
made rigid. It plays no small part also in keeping up the 
supply of moisture in living tissues when wasted by evapo- 
ration. 
127. Water in the Cell-Walls.— According to Niigeli’s the- 
ory, the wall of the cell is not a membrane which separates 
the water of one cell-cavity from that in the next, but rather 
a pervious stratum, composed of solid particles (molecules) 
which are not in contact, and between which the water 
freely passes. In a living tissue the water is continuous 
from cell to cell, and constantly tends to be in equilibrium 
—i.e., the turgidity of the cells is approximately equal 
throughout the tissue, and likewise the wateriness of both 
cell-walls and cell-contents. 
128. In the simpler aquatic plants the water of the cells 
and their walls is continuous with that in which they grow. 
Likewise the water in the tissues of roots or other absorb- 
ing organs of the higher aquatic plants is continuous with 
that which surrounds them ; and even in ordinary terres- 
trial plants there is a perfect continuity of the water in the 
root-tissues with the moisture of the soil. 
129, The Equilibrium of the Water in the Plant.—The 
water in the tissues of every plant tends constantly to be- 
come in equilibrium, and this state would soon be reached 
were it not for certain disturbing causes which are almost 
as constantly in action. In any cell an equilibrium may 
soon be reached between the two forces which reside re- 
spectively in the cell-wall and the protoplasm, viz., (1) the 
attraction of the surfaces of the molecules for the water, 
