Rotation of Crops 



37 



into the cell, and some of the sap being excreted into the soil. 

 As, however, the sap is much denser than the water without, 

 the flow of this latter is more rapid. By the same means the 

 food material thus absorbed is transferred up the root from cell 

 to cell towards the stem. The sap is densest in the upper cells, 

 owing to the evaporation which is going on, and hence the flow 

 is maintained. This passage of food takes place especially by 

 means of the prosenchymatous cells or fibres, whilst absorption 

 goes on most actively in those cells which are near the growing 

 point and by means of the hairs. 



The various food materials are not, however, taken up in- 

 discriminately by the plants. It is found, if plants are grown 

 in water containing equal amounts of the salts necessary for 

 food, that the quantities removed are very different. 



There is what is known as selective power, each plant remov- 

 ing from the soil those substances which are more especially 

 necessary for its life, leaving other foods behind. Thus legu- 

 minous plants remove especially lime salts ; potatoes and 

 turnips, compounds of potash ; cereals and grasses, silica ; and 

 so on. 



If the same plant be grown year after year in the same soil 

 it gradually impoverishes the ground, whilst food materials 

 needful for other plants will be accumulated. Hence is brought 

 about what is known as rotation of crops — that is, growing a 

 well-chosen selection of plants in succeeding years in the same 

 soil. 



The exact plan of rotation varies with the nature of the 

 soil and the plants desired to be grown. The following is an 

 example of what is known as the Norfolk or four-course rota- 

 tion: — 



That is, the farm will be broken up into four portions. The 

 first will undergo thorough tillage and be planted with root 



