ABSORPTION AND MOVEMENT OF WATER 147 



with chlorophyll grains, are capable of manufacturing for 

 themselves substances which are osmotically active, con- 

 tributing to the turgor as well as to the nutrition of the 

 cell. The other epidermal cells of most plants, being devoid 

 of chlorophyll, must absorb from their neighbors the osmoti- 

 cally active substances upon which their turgescence depends. 

 But their turgescence will vary according to the proportion 

 sustained between the amounts of water which they absorb 

 and give off. If, as is shown in figure 4, the shape, size, 

 position, and other characters of some or all of the epider- 

 mal cells adjoining the guard-cells are different from the 



Fig. 5. Fig. 6 



Figure 5. Diagram (after Schwendener) to illustrate changes in form of 

 guard-cells in opening and closing stomata. Figure 6. Cross section of 

 stoma of TradeBoantin zebrina, showing relations of auxiliary cells to me- 

 chanics of opening and closing. 



other epidermal cells, their turgidity will vary at times, at 

 rates, and in degrees different from the ordinary epidermal 

 cells. With these changes in the turgor of the ordinary 

 epidermal cells, of the auxiliary, and of the guard-cells, there 

 will necessarily be changes in the size of the openings, in the 

 degree to which the stomata are open or closed. As re- 

 cently re-stated by Darwin,* in opposition to the extreme 

 view prevailing of late that the guard-cells alone effect the 

 opening and closing of the stomata, "the pressure of the 

 guard-cells and that of the surrounding epidermis should be 

 looked at as correlated, not as opposed and independent 



* Darwin, Francis. Observations on Stomata. Philos. Trans. Eoyal 

 Society of London, Series B , vol. 190, 1898. 



