5/6 LECTURE XXI. 



activity of transpiration, for we have seen (p. 109) in how 

 high a degree light promotes this process. But the question 

 as to how light promotes transpiration has yet to be con- 

 sidered. It seems probable that the effect of light on transpi- 

 ration is due in no small degree to changes in the properties 

 of the protoplasm of the transpiring cells. It was pointed out 

 at the time when we were studying this process that transpi- 

 ration is something different from mere evaporation, that it is 

 a process essentially dependent upon the life of the plant. The 

 account which, from our point of view, we give of the effect of 

 light in promoting transpiration is that the permeability of the 

 protoplasm of the transpiring cell is increased, and that water 

 is consequently more readily forced to the surface of the cell, 

 where it is evaporated. This account is supported by the fact, 

 to which attention was drawn at the appropriate time, that 

 the transpiration of leaves is temporarily increased by violently 

 shaking them, a fact which clearly points to the possibility of 

 molecular change in the protoplasm of the transpiring cells. 



Exposure to light acts, then, as a stimulus to the growing 

 organ, in consequence of which the protoplasm of the cells 

 undergoes a molecular change which leads to an increase of 

 its permeability and thus induces a diminution in the turgidity 

 of the cells. So long as the exposure continues the protoplasm 

 remains in this condition, a result which we must attribute to 

 an interference with the motility of the protoplasm. The 

 stimulating effect of sudden exposure to light or of sudden 

 withdrawal from its influence, in a word, of rapid variations in 

 its intensity, is only manifested in a marked manner by organs 

 possessing a high degree of irritability. Instances of these 

 are afforded by the flowers and leaves of certain plants men- 

 tioned in a previous lecture (p. 400), which are excited by 

 such variations to perform movements ; movements closely 

 resembling those performed by leaves provided with pulvini, 

 which we have just been discussing. With regard to the 

 mechanism of the movements of growing floral and foliage 

 leaves, it is essentially the same as that of leaves which move 

 by means of pulvini. Neither the movement of opening 

 which follows on an increase in the intensity of light, nor the 



