CONCLUSIONS. 135 



open according as the leaf as a whole is badly or well supplied with water, 

 either owing to the variation in the psychrometric conditions or in water 

 supply." The secondary stomatal characters and those of the cuticle in 

 many cases conserve the water supply so that wilting does not progress 

 rapidly, and, further, the mechanical conditions found in the leaf may be 

 such that wilting may progress so far that the stomata are closed before the 

 effect is rendered visible in flaccidity. These phenomena are apart from 

 the main issue, however, which is the relation of psychrometric conditions 

 on the physiology of the guard-cells proper, and there is no evidence that 

 this relation is any different for the stomatal than for the epidermal or 

 chlorenchyma-cell. 



Concerning the function of the oil in the guard-cells little at present may 

 be said. The earlier mentioned suggestion, which was prompted by the 

 apparent correlation of the starch and oil, their behaviors being in inverse 

 relation, that there might be some causal connection between the oil and 

 stomatal movement, is borne to the ground by the force of the experimental 

 evidence. A change of a diffusible carbohydrate into a hydrocarbon is a 

 possible mechanism for bringing about closure, but this is not in accord with 

 the observed facts. At low temperatures, such as occur in December and 

 January, the oil fails to appear in the guard-cells at the usual time, though 

 this does not seem to result from external conditions, as oil does occur in 

 the chlorenchyma at all times during the winter. We might regard it as a 

 by-product, as has been done, but this does not save us from our ignorance. 



