68 THE PRINCIPLES OF AGRICULTURE. 



Application of these Principles to Plant Growth. These 

 three principles all have an application in the passage of 

 substances into plants from the soil. 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 fills it with 

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

 tive substances dissolved in the moisture likewise pass 

 into the plant through its roots. 



By the principle of diffusion, these substances tend 

 to distribute themselves equally, not only through the 

 moisture of the soil about the roots, but throughout the 

 juice or sap of the plant. 



The Absorbing 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 parts of the plant, 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 been found that this force is sufficient to assist in 

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

 supposed to explain also the tall, slender growth of crops 

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

 which is abundantly supplied to roots in such a season, 

 is sufficient to force the different parts of the plant out 

 of their normal dimensions. 



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

 of plants, and in fact all vegetable substances, are origi- 

 nally composed of a great number of small cells. These 

 generally consist of little membranous vesicles or bags, 



