Chapter VI 



WATER AS A PLANT COMPONENT — INTRACELLULAR 



DISTRIBUTION OF WATER 



Introduction : — Because water occurs so commonly, it is often taken 

 for granted and little attention is paid to its properties. Especially is this 

 true in biology where water may make up the bulk of the living organism. 

 The content and character of water are too often considered subsidiary to 

 the properties of other structural components and its role in the structure 

 and function of the organism neglected. 



The foregoing chapters have stressed the unusual properties of water, 

 not only as an individual chemical compound but as a solvent of other com- 

 pounds. In the living organism water must play a vital role in determining 

 the nature of the physiological environment wherein metabolism takes place 

 and various functions are carried on. The intermolecular forces between 

 individual water molecules and between water molecules and molecules of 

 other structural materials such as cellulose, pectin, lignin, etc., must have 

 an important influence in the design of the physical mechanism of such 

 complex organisms as plants. In fact, water, as the liquid medium for 

 growth, provides an essential continuity for the transport of solutes, the 

 transmission of impulses, and the coordination of correlative influences that 

 distinguish the organized plant from a mass of individual cells. 



The continuity of the protoplasm may rightfully be considered the 

 critical mechanism in the above coordination. However, certain structural 

 features such as the moist cell walls upon which CO2 molecules are ab- 

 sorbed in leaves and the thoroughly wet pectic surfaces of root hairs that 

 provide intimate contact with colloids in the soil for ion exchange, con- 

 stitute connections between the protoplasm and these colloids that cannot be 

 neglected. 



And, finally, the very character of life itself may depend upon the unique 

 properties of water and the interrelations between water molecules and 

 those of proteins, lipoids, carbohydrates, and the various mineral elements 

 that go to make up the living organism. 



As a beginning to the presentation of the water relations of plants it 

 seems proper to consider the water of cells, its distribution, the forces 

 determining its absorption, movement and loss, and the dynamics of balance 

 between these forces. 



Anatomy of Cells : — It would be impossible to give detailed consideration to the 

 water relations of the great number of cell types found in plants, a few of which are 

 shown in Figure 17. The complexity of form and function as related to specialization, 

 adaptation, and division of labor among cells and tissues makes generalization difficult. 

 As a compromise an attempt will be made to treat in detail a common cell type with 

 occasional reference to important exceptions. The type selected is the mature, active, 

 vacuolated parenchyma cell common to the pith, phloem, and cortex, and also occurring 

 in the xylem of succulent organs. Though more highly specialized, the mesophyll 

 cell of the leaf may be considered to fall within this same category. Such a cell is 

 conveniently described under three phases : cell zvall — the enclosing membrane, some- 

 what elastic but having considerable tensile strength; protoplasm — the semi-fluid living 

 substance including cytoplasm, nucleus, plastids, and other inclusions ; and vacuole — the 

 central region, filled with cell sap. 



In certain water storage cells, such as those making up most of the leaf and stem 

 tissue of succulents, the vacuoles may be relatively large, the protoplasm and cell wall 

 consisting of thin layers. In others, for example the meristematic cells of stem and 

 root tips, the vacuoles are small and scattered. In these the protoplasmic phase pre- 



