LEAVES ON THE ATMOSPHERE. 1 63 



than a regular chemical production of their constitu- 

 tion when in health ; for Sennehier found the quantity 

 of oxygen emitted was in proportion to the thickness 

 of the leaf, or quantity of parenchyma. Yet the 

 parenchyma must be in its original organized state, for 

 when bruised its functions are destroyed. 



Possibly such an alternation in the functions of vege- 

 tables between day and night may afford a necessary 

 repose to their vital principle, wliose share in them 

 we know to be of primary importance. Whatever may 

 happen to plants in the dark, there can be no doubt 

 of their principal business in the oeconomy of nature 

 being what we have described. The most luminous 

 and compendious view of the whole subject is given 

 by Dr. Thomson of Edinburgh in the fourth vol. of 

 his Chemistry, which is well worth the attention of 

 those who wish to enter more aeeply into all the va- 

 rious chemical examinations respecting it than suits 

 our purpose. It is only necessary to add a short view 

 of Dr. Darwin's hypothesis which Dr. Thomson has 

 not mentioned, probably on account of its insufficiency. 

 That lively writer thought the watery perspiration of 

 leaves, acted upon by light, gave out oxygen for the 

 use of the plant itself, such oxygen being immediately 

 absorbed by the air-vessels. This is by no means ade- 

 quate to explain any of the phiTnomena, but rather 

 contradictory to most of them, and is totally superseded 

 by the observations and expei'iments of other writers. 



There can be no question of the general purpose 

 M 2 



