XII] SIGNIFICANCE OF TRANSPIRATION 125 



chloro-vaporisation. That it is really the light rays, and 

 not merely the increased temperature, which thus promotes 

 the escape of aqueous vapour from the cells containing 

 chlorophyll, is shown by the experiments of the observers 

 referred to, who have demonstrated that the maximum 

 effect is in the blue and in the red-orange rays of the 

 solar spectrum i.e. the rays most actively absorbed by 

 the chlorophyll, and that the process rises and falls with 

 the process of assimilation. 



In any case it is clear that the function of transpiration 

 is admirably adapted for bringing water to the mesophyll- 

 cells, especially at times when the leaves are receiving 

 most light, and experiments show that this water comes 

 through the vascular bundles of the venation, petiole, and 

 wood-system of the stem and root, and therefore laden 

 with soluble mineral salts derived from the soil. Since 

 there is no question of any volatilisation of these mineral 

 substances, it is obvious that they remain behind in 

 the cells of the leaf, and it is of the utmost importance 

 to understand that the whole meaning of transpiration 

 is that it is a provision for bringing these necessary 

 mineral ingredients from the soil to the living cells of 

 the plant. The land plant has to transpire much water 

 in order to obtain even a small supply of mineral salts, 

 and thus, important as the function of transpiration is, it 

 is nevertheless subsidiary to the principal function (photo- 

 synthesis) of the organ. 



A second function performed by the living leaf is that 

 of oxygen-respiration, common to all organs composed, as 

 it is, of living cells. Much confusion has been caused in 

 the past, partly by bad terminology and partly by mis- 

 conceptions as to the real meaning of the process itself, 

 and the student should clearly apprehend the significance 

 of the following experiments. 



