GS THE PRINCIPLES OF AGRICULTURE. 



Application of these Principles to Plant Growth. — These 

 three principles all have an api)lication in the passage of 

 suhstancos into }>lants from the suil. Roots are covered 

 with a kind of membranous coating'. The moisture of 

 the soil, passing through this coating, and upward into 

 the plant, saturates the plant, or completely Tills it with 

 moisture, hy the force of capillary attraction. The nutri- 

 tive substances dissolved in the moisture likewise pass 

 into the ])lant through its roots. 



By the principle of diffusion, these substances tend 

 to distribute themselves eipially, not only through the 

 moisture of the soil about the roots, but throughout the 

 juice or sap (»f the plant. 



The Absorbings Power of Roots. — The material of which 

 the roots of plants are composed has a very strong at- 

 traction for water. On this account, the moisture of 

 the soil is drawn upward through the roots into the stem 

 and leaves with considerable force. This pressure, by 

 filling all i)arts of the ])lant, assists in keeping it in a 

 firm, upright position. When the supply of moisture is 

 cut off by drought, or by severing the root, the plant 

 withers and droops. 



It has Ijeen found that this force is sufificient to assist in 

 the extension of buds and leaves in their growth. It is 

 supposed to ex])lain also the tall, slender growth of crops 

 in a wet season. The upward pressure of the moisture, 

 which is abundantly sujiplied to roots in such a season, 

 is sutticient to force th(> different i)arts of the plant out 

 of their normal dimensions. 



The Structure of Plants. — The roots, stems, and leaves 

 of phiuts, and in I'act all vegctnble substances, an^ origi- 

 nally ('om])()S('(l of ;i gi-eat number of small cells. These 

 generally consist of little membi'anous vesicles or ))ags, 



