THE STRUCTURE OF PLANTS. 13 



take up substances contained in the soil, in a form soluble 

 in water, and in such proportions 

 as are required by each individual 

 species of plant, which differ widely 

 for different orders, and even some- 

 times for closely - related species. 

 As each plant requires a certain 

 fixed number of mineral ingredients, 



and also a fixed percentage of each, 



c ° lip of Root, showing 



the absence (or insufficience of uncovered cells and fibrils, 



supply) of even a single one of which absorb the moisture 



, ..... and salts from the ground. 



them prevents their blooming or 



seeding, and causes ultimate death. This explains why, 

 for example, wheat yields less and less seed on fields 

 cultivated for a number of years. Some of the mineral 

 substances of the soil assisting in its structure have become 

 exhausted. Thus we see how important some knowledge 

 of the structure of plants is to every one. Want of infor- 

 mation may bring about the ruin of thousands. The only 

 remedy is replenishment of the wanting substances. 



The juices or sap collected by the roots ascend towards 

 the top ; those gathered by the leaves descend by means of 

 the forces mentioned above, not in separate channels, like 

 < arteries and veins, but penetrating each other in every 

 individual live cell, each appropriating its own share of the 

 nutriment passing. This process is most actively carried 

 on by the cells and fibres of the inner white bark next 

 to the wood in dicotyledonous plants and the inner soft 

 layers of monocotyledons, and explains why the former 

 grow gradually thicker and the latter not ; layers of cells 

 being continuously formed under the covering of the 



