3 o8 
PHYSIOLOGY 
3. TITRGOR AND ITS CONSEQUENCES 
Immigration of water. The fact that there are formed within the 
cells certain substances to which the cytoplasmic membranes are nearly 
impermeable, and that they may accumulate to a considerable extent, 
insures the entrance of water into such cells either directly from the out- 
side or indirectly from adjacent cells in which the solutions are less con- 
centrated. The mode of this movement may be conceived thus. It is 
known that the presence of any solute reduces the vapor pressure of 
water ; which, in terms of current theory, means that there are fewer 
molecules of water per unit volume over a solution than over pure water 
under the same conditions. Thus in fig. 622, p. 305, if A be pure solvent 
and B the watery solution, the actual pressure of water vapor in b will 
be less than in a. If the partition between a and b be removed, the dif- 
ference in pressure would cause more particles of 
water vapor to move into b in a unit of time than 
would diffuse in the reverse direction. If the whole 
partition were permeable to water and not to the 
solute, the same movement would take place through 
the partition; this occurs, it may be conceived, 
because the presence of the solute particles reduces 
the internal pressure of the water, whose particles 
thus diffuse, in the common fashion, from regions of 
higher to regions of lower pressure. The conditions 
determining the movement of the water are created, 
be it noted, by the number and nature of the solute 
particles. 
Turgidity. As a consequence of the migration of 
water into the vacuole, the protoplast is forced out- 
ward against the cell wall, which, being elastic, is 
stretched thereby, unless the pressure is balanced by 
an equal pressure from an adjoining cell. Superficial 
a 
d 
FIG. 624. The 
cell walls of a Clado- 
phora : a, a young tip 
of a filament ; b, a 
division wall in the 
middle of a filament ; 
c, a division wall next 
to a dead cell (d). 
cells, without exception when healthy, have the free wall convex out- 
ward. The filamentous algae have the free end often very convex 
(fig. 624, a), but the partitions between cells at a little distance from 
the end are practically plane (fig. 624, &). If the filament be broken 
or a cell dies, the adjacent walls, previously plane, at once bulge out (fig. 
624, c) on account of this internal pressure. When a cell is surrounded 
on all sides by those of equal internal pressure, its walls are plane. 
