PLANT NUTRITION. 29 
Feeding by Leaves,—We are now in a position to 
understand the nutritive process as it is carried on by 
the leaves, and our first enquiry is as to what they feed 
on—what is the nature of the food they take in? In 
the first place, it is clear that they can take in no solid 
matter. The pores or stomata already alluded to are the 
only openings by which such matter could get into the 
interior of the leaf ; and we know, from experience, that 
if these pores get blocked, the leaf suffers rather than 
gains. Moreover, the cells, bounding the aperture, open 
and close according to the condition of moisture of the 
atmosphere, and at any rate, when closed, they could 
admit no solid matter. 
Absorption of Water.—As to fluids, it is proved that 
leaves, under certain circumstances, and when there is 
no structural provision to prevent it (as there often is) 
can and do absorb, not only watery vapor, but the finid 
itself. This happens more especially when the plant is 
flagging from the rapid exhalation of moisture, and from 
deficient root supply, and it affords an explanation of 
the benefit plants derive from the deposit of dew after a 
hot and drying day. Still, the absorption of water by 
means of the root seems to be generally of more conse- 
quence than that by the leaf, so that the entrance of 
water by the leaf may, for our purposes, be passed over 
without further mention. 
Absorption and Exhalation of Gases.—There remains 
gaseous food. It has been shown that the root absorbs 
gases, as oxygen ; but in this respect, as also in the ab- 
sorption of other gases, the root is surpassed by the leaf. 
The paramount function of the leaf is the absorption and 
assimilation of carbon. Carbon, as such, does not exist 
in the atmosphere, unless, indeed, as an impurity in the 
air of towns, and a very prej udicial one to plants. It is 
