314 CHEMICAL ACTION OF 



to be excessive V luxuriant in vesretation. 

 PluDts also give out azotic gas : but M. rle 

 Saussure is of opinion that this proceeds from 

 their internal substance ; and it appears by 

 his experiments to be rather a sign of disease 

 or approaching decay, than a regular che- 

 mical production of then' constitution when 

 in health ; for Sennebier found the quantity 

 of oxygen emitted was in proportion to the 

 thickness of the leaf, or quantity of paren-^ 

 chyma. Yet the parenchyma must be in its 

 original organized state, for when bruised its 

 functions are destroyed. 



Possibly such an alternation in the func- 

 tions of vegetables between day and night 

 may afford a necessary repose to their vital 

 principle, whose 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 

 ceconomy of nature being what we have de-r 

 scribed. The most luminous and compen- 

 dious view of the whole suljject is given by 

 Dr. Thomson of Edinburgh in the fourth vol. 

 of his Chemuir?/^ which is well worth the 

 attention of those who wish to enter more 



