

AND OTHER PARTS OF PLANTS. 13 



3. The leaves absorb carbonic acid from the air while the 

 sun is above the horizon, and give off oxygen in nearly equal 

 bulk. In the dark, on the other hand, they give off carbonic 

 acid and absorb oxygen. They also alternately absorb or 

 exhale watery vapour, according as the atmosphere in which 

 they live is saturated with moisture or otherwise. The quan- 

 tity which escapes from them during the hot weather of summer 

 is thus often very great. Nitrogen is also given off by the ; 

 leaves of plants in variable, but sometimes, it is said by Draper, 

 in very considerable quantity, and the petals or leaves of their 

 flowers habitually disengage this gas to a sensible extent. Such 

 are the mechanical functions of the leaf. 



Their direct chemical functions are obscure. Carbonic acid 

 contains its own volume of oxygen ; and as the leaves give 

 off nearly as large a bulk of oxygen as they take in of carbonic 

 acid, it has been supposed that the leaves actually decompose 

 the carbonic acid directly giving off 'its oxygen into the air, 

 and working up its carbon into the starch, sugar, gum, cellulose, 

 &c., found in the sap and in the solid substance of the plant. 

 The changes, however, are deeper seated and more complicated 

 than this opinion implies, and for the present we must be 

 content to confess that we do not, as yet, half understand 

 them. 



4. The bark. The sap descends through the inner bark, 

 and during its passage those chemical changes take place 

 which give rise, in trees, to the deposits of young wood within 

 the inner bark. The air, which penetrates through the outer 

 bark, probably has an influence upon these changes. 



5. The pith. The pith is said to nourish the young buds 

 till they are able to procure nourishment for themselves, after 

 which " it is of no further importance, and dies." (Lindley.) 

 For this purpose it is often filled with starch. Through the 

 medullary rays, which, like the pith, consist of vessels or rows 

 of cells, laid horizontally, it communicates with the bark and 

 the external air. But whether it actually admits and absorbs 

 air, or itself, by means of these vessels, gives off gaseous matter, 

 is, I believe, unknown. 



