104 THE BIOLOGY OF FLOWERING PLANTS 



nearly uniformly thickened, and to such an extent that the 

 lumen is reduced to a narrow slit. When the cell is turgid, 

 the thin-walled ends expand and the middle portions, which 

 remain straight, are pulled apart. In this open condition 

 the guard cells are dumb-bell shaped. This type of 

 mechanism seems to be less delicate than the other, since 

 the grass stomata show relatively slight activity of movement. 



The bulging out of the guard cells takes place against the 

 resistance of the neighbouring epidermal cells. It is there- 

 fore possible only if the guard cell can develop a higher 

 turgor pressure than these, and to do this the cell sap must 

 have a higher osmotic pressure. Iljin (1915) has found, for 

 steppe plants, that the osmotic pressure of the guard cells 

 of open stomata may reach the very high value of 90 atmo- 

 spheres, while that of the neighbouring epidermal cells is 

 only about one-quarter of this. Wiggans (1921) obtained 

 similar though smaller differences for the leaves of meso- 

 phytes. Thus the guard cells of Cyclamen had a maximum 

 osmotic pressure of 29*49, those of the beet of 31*5 atmo- 

 spheres ; the values for the epidermal cells were 10*09 

 and I2'55 atmospheres respectively. Ursprung and Blum 

 (1916, 1918) found the suction force of the guard cells 

 about 2 atmospheres higher than that of the epidermal 

 cells in the beech, and about 3 atmospheres higher in 

 the ivy. 



The collapse of the guard cells, resulting in stomatal 

 closure, may be due to two causes. Reduction of water 

 content by excessive transpiration may lead to loss of 

 turgor by all cells of the leaf, including the guard cells, when 

 collapse and closure will occur ; or a fall in the osmotic 

 pressure of the guard cell may so far reduce turgor pressure 

 that collapse takes place. It will be seen that the first cause 

 of movement depends on the water relations of the leaf as 

 a whole. It is unlikely that conditions favouring high 

 transpiration — low humidity, high temperature, etc. — affect 

 the guard cell independently. Its rate of transpiration may 

 be increased, but, while it maintains a high osmotic pressure, 

 it must also tend to withdraw water from the neighbouring 



