290 HOW CROPS GROW. 
or in many instances, as, for example, with a stalk of maize, by simply 
immersing one end in water and blowing into the other. 
On the contrary, roots are destitute of any visible 
pores, and are not pervious to external air or vapor in the 
sense that leaves and young stems are. 
The air passages in the plant correspond roughly to the 
mouth, throat, and breathing cavities of the animal. We 
have, as yet, merely noticed the direct communication of 
these passages with the external air by means of micros- 
copically visible openings. But the cells which are not 
visibly porous readily allow the access and egress of wa- 
ter and of gases by osmose. To the mode in which this 
is effected we shall recur on subsequent pages, (pp. 354— 
366.) 
The Offices of Foliage are to put the plant in commu- 
nication with the atmosphere and with the sun. On the 
one hand it permits, and to a certain degree regulates, the 
escape of-the water which is continually pumped into the 
plant* by its roots, and on the other hand it absorbs from 
the air, which freely penetrates it, certain gases which 
furnish the principal materials for the organization of vege- 
table matter. We have seen that the plant consists of 
elements, some of which are volatile at the heat of ordina- 
ry fires, while others are fixed at this temperature. When 
a plant is burned, the former, to the extent of 90-99 per 
cent of the plant, are converted into gases, the latter re- 
main as ashes. 
The reconstruction of vegetation from the products of 
its combustion (or decay) is, in its simplest phase, the 
gathering by a new plant of the ashes from the soil 
through its roots, and of these gases from the air by its 
leaves, and the ‘compounding of these comparatively sim- 
ple substances into the highly complex ingredients of the 
vegetable organism. Of this work the leaves have by far 
the larger share to perform; hence the extent of their sur- 
face and their indispensability to the welfare of the plant. 
