Chap, i Processes of Absorption of Water 261 



to be accompanied by regulating changes of volume of the 

 guard cells which are situated there, the power of the latter 

 to appreciate the changes of the surrounding air with regard 

 to light and temperature, and perhaps to hygrometric con- 

 dition, was suggested as a factor in the mechanism that 

 must be taken into account. 



In 1881 the mode of action of the guard cells was the 

 subject of a very important investigation by Schwendener, 

 who made more complete and minute research into both 

 structure and behaviour than Von Mohl had done. He 

 showed, with very great wealth of illustration, how the 

 distribution of thickening of the walls of the guard cells, 

 and the consequent tensions set up in them, secures definite 

 and purposeful changes of form when the pressure of turgor 

 is increased or diminished. 



Sachs supplemented the views of Schwendener by re- 

 stating the ideas of Von Mohl and suggesting that these 

 variations of turgor are attributable to osmotic interchanges 

 between the guard cells and the other cells of the epidermis 

 abutting on them, under the influence of light; he like 

 Von Mohl pointed to the chloroplasts in the guard cells as 

 indicating a means whereby osrnotically active substances 

 can be produced in them as conditions become favourable. 



Schwendener's views met with general acceptance for 

 a few years, but in 1886 another contribution was made 

 by Leitgeb to the subject which caused some controversy. 

 According to Schwendener, the mechanism is supplied by 

 the guard cells alone ; they are independent of pressure 

 of liquid in the other epidermal cells, possessing their own 

 forces of distension and recoil, which suffice for all they 

 have to do. They work under the influence of light and 

 temperature certainly, but these affect them directly. 



Leitgeb's hypothesis was very different ; according to 

 this, the chief factor in the closing of the stomatal orifice 

 is the action of the other epidermal cells upon the guard 



