3i° 



PH \'SIOLOGY 



camera drawings of them, comparison after treatment with an appro- 

 priate solution shows the shrinkage of the wall to its unstretched size. 

 If the outside solution remains more concentrated after a loss of water 

 from the cell just sufficient to permit the return of the wall to its 

 unstretched condition, water continues to leave the cell. As a conse- 

 quence of the diminished volume of cell sap in the vacuole the proto- 

 plast is dragged away from the wall, if this is rigid enough (as it often is) 

 to support itself; or if not, the whole cell, wall and all, is collapsed. 

 Usually only extreme shrinkage from loss of water, resulting in separa- 

 tion of protoplast from wall, is called plasmolysis; but obviously, plas- 

 molysis has two phases, inseparable except arbitrarily. It begins with 

 the first emigration of water, and up to the complete recovery of the cell 

 from previous stretching, it can be detected best by measurement. In 

 its second phase the further emigration of water is made evident by the 

 more or less extensive collapse of the protoplast. 



Rigidity from turgor. — The emigration of water which takes place 

 when a turgid cell is surrounded by a more concentrated solution is only 

 one way by which turgor is reduced or plasmolysis produced. The 

 evaporation of water may produce the same effects. When a flexible 

 organ, like a leaf or a young shoot, loses water to such an extent that its 

 cells are no longer turgid, the parts bend by their own weight; the edges 

 of the leaf and the tip of the shoot droop. To the touch they are less 

 rigid than before. This observation shows one effect of turgor. Thin- 

 walled cells in masses, such as form the greater part of young shoots, 

 leaves, and young roots, are rendered much more rigid by the strains 

 set up in the mass by turgor. Turgor tensions in the smaller and in the 

 less differentiated plants, as well as in the younger parts of all plants, are 

 thus important in maintaining bodily form; whereas in the older parts, 

 especially of large plants, mechanical tissues, characterized by thickened 

 and altered cell walls, provide the requisite rigidity. 



Growth and turgor. — Besides its rule in maintaining bodily form, 

 turgor has important relations to the growth of cells, especially in the 

 phase when enlargement is the marked feature (see p. 420). At this 

 time water is entering in relatively large amount, and turgidity is pre- 

 requisite to the permanent enlargement. The cells of flaccid tissues do 

 not grow larger. Whether stretching is merely a mechanical necessity 

 for such growth, or whether growth is dependent upon the increase in 

 solutes, which would likewise determine the increase in turgor, or 

 whether both conditions are necessary, is not certainly known. 



