Chapter VI — 71 — Intracellular Distribution 



ume of the vacuole is a measure of its water content ; the method is more questionable 

 in the case of the cell wall and cytoplasm. 



In many tissues the protoplasm of mature vacuolated cells forms a thin parietal 

 layer, comprising only a small portion of the total volume. Values between 10 and 30 

 per cent are common in storage parenchyma ; palisade cells of leaves have a greater 

 proportion of protoplasm. 



Equilibrium Between Phases : — Finally, attention should be drawn 

 to the fact that, at least in active cells, there is a constant trend toward an 

 equilibrium between the forces attracting water to the three cellular phases. 

 We use the word trend because an actual equiUbrium probably never ob- 

 tains, due to varied activities of the hving organism of which the cell is a 

 part. With Krogh (1939), we consider "steady state" a better term, but 

 even this is not completely valid. 



The most useful concept in a consideration of the mechanics of inter- 

 phasal water balance, and for the balance between various tissues and or- 

 gans of the plant is that of Meyer ( 1938), namely diffusion pressure deficit. 

 As explained in Chapter V, water, regardless of its state, location, or tem- 

 perature, has a definite diffusion pressure. When this diffusion pressure 

 is decreased, as by addition of solutes or imbibants, by decrease in turgor 

 or temperature, or by any other means, it develops, with respect to its own 

 previous state, or with respect to water in any phase which has not under- 

 gone change, a difference in diffusion pressure. The difference may be 

 termed a deficit if the previous state is designated as standard. For con- 

 venience, this term is contracted to DPD. 



With this concept in mind it is not difficult to explain the maintenance of 

 a dynamic equilibrium in plant cells. If water is evaporated from the cell 

 wall, the DPD of water in the wall will increase, and water will move in 

 from the protoplasm to make up at least a part of the deficit. In turn, water 

 will leave the vacuole to satisfy the DPD of water in the protoplasm. And 

 if water is moving through an adjacent tracheid there will be a movement 

 to satisfy the developing deficits in all three cellular phases. 



Other activities which may change the DPD of water in cells and shift 

 the equilibrium are: 1) water exchange with other cells; 2) solute meta- 

 bolism, including condensation, hydrolysis, accumulation, or loss; 3) water 

 metabolism, anabolic {e.g., photosynthesis) or catabolic {e.g., respiration) ; 

 4) changes in turgor; and 5) changes in hydrophily of the protoplasm. 

 Under certain conditions the volume of the protoplasm may increase at the 

 expense of water in the vacuole, a phenomenon known as vacuolar contrac- 

 tion. As a consequence the solute concentration in the vacuole increases ; 

 presumably water moves into the cell to satisfy the DPD gradient. Con- 

 traction of the protoplasm may occur under other conditions. Chloro- 

 plasts of Spirogyra readily contract to give up water to the rest of the cell 

 (OsTERHOUT, 1945). This may occur normally, or it may be induced ex- 

 perimentally through the action of certain salts. 



The DPD concept emphasizes the fact that water in the free state under 

 atmospheric pressure has a greater diffusion pressure than does solvent 

 water in vacuoles, imbibed water in cell walls and protoplasm, or water 

 under tension in the xylem. Thus water tends to move along gradients of 

 diffusion pressure from regions where it is relatively free to those where 

 it is highly bonded or relatively deficient. Considering the individual cell 

 and its aqueous medium, such movement is opposed by turgor within the 

 cell and hydrostatic tension outside, and between these forces the dynamic 

 equilibrium known as water balance is in constant flux. Scarcely an ac- 



