CH. IV.] THE LEA\'ES. 147 



row just noticed, were not apparently affected. 

 Thin leaved plants consume daily from five to ten 

 times their own bulk of carbonic acid gas, whilst 

 fleshy leaved plants, such as the cacti, aloes, agaves 

 and mesembiyanthemums, do not consume more 

 than their owa or double their ovm. bulk of that gas. 

 Plants and their leaves if excluded from light 

 become of a white or pale yellow colour, in which 

 state they are said to be blanched or etiolated. This 

 is occasioned by their being neither able to decom- 

 pose the water they imbibe, nor to inhale cai'bonic 

 acid. In the dai'k, plants can only inhale oxygen, 

 and thus, deprived of free hydrogen and carbon, 

 on the due assimilation of wliich by the leaves all 

 vegetable colours depend, and satm*ated with oxygen, 

 they of necessity become wliite. An excess of 

 oxygen has uniformly a tendency to whiten vege- 

 table matters ; and, to impart it to them is the prin- 

 ciple upon which aU bleaching is conducted. An 

 over-dose of oxygen causes in them a deficiency of 

 alkaline, or an excess of acid matter, and light 

 enables plants to decompose the acid matter and 

 to restore that predominancy of alkalinity on which 

 their green colour depends. Sennebier and Davy 

 found most carbonic acid in etiolated leaves ; and all 

 green leaves contain more alkahne matter than the 

 rest of the plant which bears them. Ever}- cook 

 knows that a little alkali, carbonate of soda, added 



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