84 NUTRITION. 



upon as produced by the chemical action of light upon 

 the carbonic acid, but as acting on the vital processes 

 influencing nutrition, as suggested by Link, and so 

 producing the decomposition of that gas ; and the 

 more the vital powers of the plant are hurt, the more 

 strong must be the stimulus to awaken these actions, as 

 shown by those experiments made upon plants more or 

 less deprived of their natural relations, in which a sof- 

 tened or more moderate light causes no evolution of 

 oxygen from the leaves, but the influence of the direct 

 solar rays are required. 



55. From various experiments we are also to con- 

 clude that the quantity of oxygen evolved is equal to 

 the quantity of carbonic acid absorbed by the plant. 



56. The carbon that remains after oxygen is thus set 

 free, contributes to the formation of the various organic 

 products found in the plants, which are either useful to it 

 as nourishment, or are hereafter to be discarded ; but a 

 superfluous portion of it left by the process of digestion 

 at the time, is removed from the plant by means of the 

 function of respiration. 



57. That nitrogen enters the plant by means of the 

 sap, no doubt can exist ; but that it is likewise obtained 

 for digestion by means of the leaves from the air is more 

 doubtful, though late experiments seem to prove that 

 some plants do so obtain it, whilst it is inappreciable in 

 others. 



58. The exhalation of moisture from the leaves du- 

 ring digestion is continually going on, but the quantity 

 given off is much modified by circumstances, and which 

 regulate likewise the quantity inhaled. During the 

 night exhalation is very slight, or indeed ceases, and is 

 greatest during the day-time whea the atmosphere is 

 hot and dry, though light seems to have more influence 

 upon it than does temperature itself. According to 

 some, two kinds of transpiration are to be seen in 

 plants ; the first is that which plants have in common 

 with all humid bodies, and is termed insensible waste 

 (deperdition insensible), whilst the other is dependent at 



