Grafts et al. — 72 — Water in Plants 



tivity takes place in the plant which does not in some way influence this 

 balance. 



Summary: — Water plays a vital role in determining the nature of the environ- 

 ment wherein various cell functions are carried on. It forms a continuum in which 

 transport of solutes and stimuli occur. The protoplasm also constitutes an inter- 

 connected unit termed the symplast. 



Water makes up as much as 75 to 90 per cent of the total mass of many common 

 plants. Water is retained in cells by osmotic, imbibitional, and chemical forces, all 

 of which lower its free energy. At their extremes and in simple systems these three 

 types of forces are distinct and are easily distinguished ; in complex systems, however, 

 they overlap. Various types of bonds are involved in the hydration of cells among 

 which the hydrogen bond is probably predominant. 



Plant cell walls may consist of pectins, lignins, cellulose, and a number of other 

 infiltrating substances. Cellulose and pectin have strong affinities for water. Water 

 also exists in the microcapillaries of the cell walls. Much of this water is relatively 

 free, and it may translocate during absorption and movement of water in the plant. 



Protoplasm may consist largely of water. Yet it may regulate the movement 

 and accumulation of solutes and hence of water. Certain evidence indicates that a proto- 

 plasmic water content of around 30 to 35 per cent is critical ; water held at lower con- 

 tents is less mobile. Proteins are pictured as forming interconnected chain structures 

 similar to, but more complex than, those of cellulose. Many types of radicles, both 

 hydrophilic and hydrophobic, may be included in protein structure. Hydrated protein 

 may have sol or gel structure with the molecules in various states of aggregation. 

 Protoplasm may have similar but more complex structure. It may shift from reticulate 

 to corpuscular structure, or from a lattice having a high degree of coordination to a 

 fluid containing holes resulting from abnormal coordination. Capillary forces may 

 also come into play, holding water in the structural meshwork of protoplasm. 



Vacuolar sap is high in Vi^ater but it also contains sugars, salts, acids, pigments, 

 tannin, protein, gums, resins, fats, etc. Water is held osmotically by the solutes in the 

 vacuole. The osmotic pressure also reflects an equilibrium with inbibitional forces 

 in those cells in which vacuolar colloids occur. Active forces have been postulated by 

 some to account for the high water content of vacuoles in certain plants. 



Vacuolar sap from large algal cells has been used in studies on sap composition 

 and solute uptake. Sap from smaller cells has been withdrawn micrurgically for 

 analysis. Sap obtained by pressure may contain materials from cell walls, protoplasm, 

 and vacuole, and it does not give an accurate indication of concentration in any of the 

 three phases. 



A tendency toward water equilibrium in the three cell phases is maintained by a 

 tendency for diffusion pressure equilibrium. Factors that shift this equilibrium are 

 loss or gain of water, solute metabolism, water metabolism, changes in turgor, and 

 changes in the hydrophily of the protoplasm. 



