314 HOW CROPS GROW. 



the one hand it permits, and to a certain degree facili- 

 tates, 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 that freely penetrates it, 

 certain gases which furnish the principal materials for 

 the construction of vegetable matter. We have seen that 

 the plant consists of elements, some of which are volatile 

 at the heat of ordinary fires, while others are fixed at 

 this temperature. When a plant is burned, the former, 

 to the extent of 90 to 99 per cent of the plant, are con- 

 verted into gases, the latter remain as ashes. 



The reorganization 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 surface and their indispensability to the welfare of 

 the plant. 



CHAPTEK IV. 

 REPRODUCTIVE ORGANS OF PLANTS. 



1- 

 MODES OF REPRODUCTION. 



Plants are reproduced in various ways. The simplest 

 cellular plants have no evident special organs of repro- 

 duction, but propagate themselves solely by a process of 

 division which begins in the protoplasm, as already de- 

 scribed in case of Yeast, p. 253. The lower so-called 

 flowerless plants (Cryptogams), including molds, blights, 

 mildews, mushrooms, toadstools (Fungi), mosses, lichens, 



