COMPAEED WITH THOSE OP ANIMALS. 63 



soil, is impregnated with various earthy matters necessary 

 to the health and life of the plant. These earthy matters 

 are deposited in the cells of plants from the very com- 

 mencement of their growth,- but not to any great extent 

 until they have acquired their full development. As the 

 secondary deposit forms on the cell walls, the cells acquire 

 rigidity, and growth is therefore necessarily arrested. 



It has been shown that the fibro-vascular system of 

 plants is subordinate in function to the cells of the 

 parenehyma, and that it subserves the simple physical pur- 

 pose of conveying to them the nutrient fluid or sap. The 

 fibro-vascular system of the stem terminates in the leaves, 

 where it takes a horizontal spread, and is attenuated into 

 a plexus or network of capillaries, which anastomose with 

 each other in the same manner a,s the capillary vessels in 

 man. i 



The design of nature in forming the leaves of plants, is 

 to spread the fluid over a horizontal surface, so that it may 

 be the more readily exposed to the air and light. The 

 anatomical structure of the leaf proves this. The leaves of 

 plants are simply horizontal expansions of the fibro-vascular 

 and cellular tissues of the wood and bark of the stem, with 

 which every part of the leaf directly communicates. The 

 sap appears to be transferred laterally through the walls of 

 the capillaries, and to be imbibed by the parieties of the 

 parenchymatous cells amongst the meshes of the capillary 

 network. It is in the leaves then that the principal 

 changes in the sap take place. 



Now the current of sap is kept continually flowing 

 through the fibro-vascular portions of the stem, owing to 

 the constant evaporation which is going on at the surface 

 of the leaves. Earthy matter is therefore necessarily 

 rapidly accumulated in those cells through which the fluid 



