94 JOUENAL OF THE ROYAL HORTICULTCEAL SOCIETY. 



tion, due to light. From my own experiments I have found very 

 marked differences in the amount of water lost in the same times and 

 conditions, as well as in the rapidity with which desiccation takes 

 place under the two processes. Such, at least, obtains when one-half 

 of a living leaf transpired under sunlight ; while the other half, having 

 been suddenly killed hy scalding, could only evaporate. An important 

 paper indirectly bearing upon this subject is one by M. Maquenne, 

 entitled * ' Eecherches sur la determination des pouvoirs absorbants et 

 diffusifs des feuilles " (Annales Agronomiques, torn. 6, 1880, p. 321). 



A discovery of Maquenne 's (which would seem to furnish an impor- 

 tant aid to distinguish between the two processes evaporation and 

 transpiration) is that older leaves absorb more heat than the younger 

 ones of the same kind, and evergreens more than deciduous leaves ; yet, 

 according to Deherain, it is exactly the reverse which transpire the 

 most. Hence, if this function be identical with the purely physical 

 process of evaporation, it would be difficult to reconcile the above fact 

 with it. 



In order to test the relative effects of transpiration upon colourless 

 plants, I experimented with mushrooms and blanched sea-kale, so as 

 to avoid the effects of chlorophyll. 



The mean results of four specimens under coloured glasses were 

 for the mushroom : — " 



R. Y. G. V. CI. Dk. 



•030 -027 -028 -030 -037 '027 



These results agree with those from plants possessing chlorophyll, 

 so far as the maxima under red, violet, and clear glasses are concerned ; 

 while yellow, green, and total darkness give minima. 



If one neglects the third place of decimals we may write the above 

 thus : — 



•03 -03 -03 -03 -04 •OS 



This result shows that the differences between the effects of any 

 particular colour is really almost inappreciable, whereas the augmenta- 

 tion under clear — i.e. colourless, glass is more pronounced. 



One great point of difference between transpiration and evaporation 

 consists in the relatively greater quantity of water lost, and that with 

 a greater rapidity, by evaporation as compared with transpiration. 

 Then — e.g. the mean loss by transpiration under all lights of a plant 

 of Echeveria was 0'33 gr. per hour; while the same plant, scalded to 

 death, evaporated "068 gr. per hour under a very similar range of 

 temperature. The inference is, that living protoplasm has a powxr of 

 controlling and regulating the loss of water which a dead organic 

 structure does not possess. 



The third important function of plant-life is respiration or breath- 

 ing, a process consisting of an oxidation of carbohydrates, as sugar, 

 &c., thereby liberating energy for the use of the plant's growth, as 

 well as carbon dioxide and water as waste products. It is precisely 



