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BURTON EDWARD LIVINGSTON 



evaporational water loss. In such cases, as in the epidermal cells 

 of plant leaves, evaporation of water from the exposed wall 

 increases the imbibitional force in that region and water is drawn 

 from the protoplasm, from the vacuole, and from the unexposed 

 parts of the wall. The increase in imbibitional and osmotic 

 forces thus initiated results in a withdrawal Of water from neigh- 

 boring cells in which there may be a lower attraction for water. 

 Thus the process of water removal is transmitted through the 

 tissue to some more or less permanent source of supply. Most 

 plants continuously remove water from their subterranean envi- 

 ronment, through their roots. Many plants, and all land animals, 

 receive water from the surroundings at more or less irregular 

 intervals. In this respect the cactus which absorbs sufficient 

 water in a rainy period to allow usual transpiration, growth, etc., 

 for a year or more, is not essentially different from the camel, 

 which also can continue its usual transpiration and excretion 

 for rather long periods of time, without intake of water from the 

 environment. The obvious point here is simply this, that evapor- 

 ational water loss must be checked altogether, or there must be 

 adequate entrance of water to the transpiring cells, otherwise, 

 ultimate desiccation must ensue. 



As in every phenomenon depending upon supply and demand, 

 the factors which condition the state of the organism at any given 

 time are not those of static equilibrium, but are relative rates of 

 change. Thus a peripheral leaf cell may, at a given instant, be 

 receiving water from other cells at a certain rate and may be los- 

 ing water to the air also at a given rate. If these two rates are 

 numerically equal, the cell may remain in its present state, as 

 far as water content is concerned. If the rates are not numerically 

 equal, however, the cell must be either gaining or losing in its 

 absolute water content. This proposition is rendered extremely 

 complicated by the facts already mentioned, that the water con- 

 tent of protoplasm and of cell wall determine, in great measure, 

 the attraction of these bodies for water or their resistance to water 

 loss, and the same is of course true osmotically for the vacuolar 

 solution. Thus the very fact that a cell is gaining in water con- 

 tent is a cause for increase in its rate of loss and for decrease in its 



