308 



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 

 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, b). 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. 



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). 



