290 HOW CROPS GEOW. 



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. 



