146 PLANT PHYSIOLOGY 



and closing of the stomata are due to changes in the tur- 

 gor,* consequently in the form and dimensions, of the 

 guard-cells. These changes in turgor may be due wholly 

 to the absorption and withdrawal of water by means only 

 remotely if at all controlled by the living cells, or they may 

 be due to stimulations of the living cells which cause them 

 to absorb or to expel water or otherwise to change the 

 density of their cell-sap with consequent changes in turgor. 

 The accompanying figures illustrate a stoma without aux- 

 iliary cells (Fig. 3) and with auxiliary cells (Fig. 4). 

 From figures 5 and 6 the differences in thickness of the 



FIG. 3. FIG. 4. 



Figure 3. Lower epidermis from leaf of Vicia faba showing stomata 

 without auxiliary cells. Figure 4. Lower epidermis from leaf of Tradescan- 

 tia zebrina showing stoma with auxiliary cells. 



different walls of the guard-cells are evident. The thin walls 

 joining the guard-cells to their neighbors permit rapid 

 osmotic transfer, the thin walls opposite are pliant. If, for 

 any reason, the guard-cells absorb water, the volume of the 

 cell cavities will increase, the form of the cells will change, 

 and they will occupy the position indicated by the heavy 

 outline in figure 5 ; the stoma will be open. If the reverse 

 is the case, if the amount of water in the cell-sap is dimin- 

 ished by evaporation or expulsion, the volume of the cell 

 cavities will decrease, the thin adjacent walls of the guard- 

 cells will bend out, approach each other and finally come 

 into close contact; the stoma will be closed. From the 

 figures it will be noticed that the guard-cells, provided 



* Mohl. H. von. Welche Ursachen bedingen die Erweiterung und Vereo- 

 gung der Spaltofmungen ? Bot. Zeitung. 1856. 



