186 



BOTANY 



mechanical elements to flexure is greater the farther they are removed 

 from the centre of an organ, it will be readily seen that, while a 

 flattened, outspread organ can be easily bent, its power of resisting 

 a deflecting force would be increased if it were folded or rolled 



together. In accordance with 

 this principle many leaves become 

 plaited or rolled (Fig. 179), and 

 so acquire a sufficient rigidity 

 without the assistance of any 

 specially developed mechanical 

 tissues. 



In addition to the rigidity 

 of the individual organs of the 

 plant, their arrangement and 

 position are of importance for 

 the stability of the whole organ- 

 ism. The lateral branches, 

 which are normally arranged as 

 symmetrically as possible, may 

 be forced by bending them up- 

 wards or downwards to take on 

 an elliptical cross-section or to 

 form knee-like bends. In Coni- 

 fers the " white wood," which 

 FIG. isi.-Young plant of Lupine, the main root of offers greater resistance tostretch- 



which has become curved. The lateral roots ing forces, and the " red WOod," 

 have arisen on the convex faces of the curves, wh j ch fe d offerg reate ,. 



and act as stays. . . 



resistance to compression, ;ire 



formed on the sides which are respectively stretched or compressed ("). 

 The roots of many tropical trees are so excentrically constructed as to 

 form relatively thin plank-like buttresses to the trunk (Fig. 180). 



Lateral roots originate from the convex flanks of a main root 

 which has been thrown into curves (Fig. 181), and act as stays, 

 preventing the straightening of the root and the loosening of the hold 

 of the root-system which would result from this ( 8 ). 



II. Nutrition 



By nutrition are understood all the processes of METABOLISM, or 

 the chemical transformation and conversion of matter carried on 

 by plants in the production and appropriation of their food supply. 

 Without nourishment and without the supply of new formative 

 material neither growth nor development is possible, nor, indeed, 

 without continuous nutrition can a plant maintain itself in any given 

 stage of its development. The processes of life are connected with 



