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 

 ains. 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 fluid 

 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 prejudicial one to plants. It is 



