INFLUENCE OF THE SHADE ON PLANTS. 31 



In Pinus Larix, P. Abies, and P. picea, the excess 

 of hydrogen amounts to y, and in Tilia europcea 

 to |. The quantity of hydrogen stands in some 

 relation to the specific weight of the wood ; the 

 lighter kinds of wood contain more of it than the 

 heavier. In ebony wood (Diospyros Ebenum) the 

 oxygen and hydrogen are in exactly the same pro- 

 portion as in water. 



The difference between the composition of the 

 varieties of wood, and that of simple woody fibre, 

 depends, unquestionably, upon the presence of con- 

 stituents, in part soluble, and in part insoluble, such 

 as resin and other matters, which contain a large 

 proportion of hydrogen : the hydrogen of such sub- 

 stances being in the analysis of the various woods 

 superadded to that of the true woody fibre. 



It has previously been mentioned, that moulder- 

 ing oak wood contains carbon and the elements of 

 water without any excess of hydrogen. But the 

 proportions of its constituents must, necessarily, 

 have been different, if the volume of the air had 

 not changed during its decay, because the propor- 

 tion of hydrogen in those component substances 

 of the wood which contained it in excess is here 

 diminished, and this diminution could only be 

 effected by an absorption of oxygen. 



Most vegetable physiologists have connected the 

 emission of carbonic acid during the night, with 

 the absorption of oxygen from the atmosphere, and 

 have considered these actions as a true process of 



