38 



Elementary Botany 



crops, which need especially potash and lime, and, having short 

 roots, take their food near the surface, or are surface-feeders. 

 Division No. 2 has barley, which takes up very little lime and 

 potash but much silica, and is also a surface-feeder. Clover, 

 in division 3, takes much the same food, as the root crops, but is 

 a subsoil-feeder— that is, sends its roots deeply into the ground. 

 The wheat in division 4 is also a subsoil-feeder, but like barley 

 takes up much silica. 



The next year the position of the crops is changed. Barley 

 is grown on division i, clover on division 2, wheat on 3, and 

 so on ; so that once every four years each- part of the soil has 

 each kind of plant growing on it, and the full rotation is thus 

 shown : — 



At the same time various manures are employed to supply 

 the place of the different materials removed from the soil. 



3. The root is also, as we have seen, an organ of excretion; 

 some of the sap, by osmosis, passes out into the soil, whilst the 

 food enters the root. This is most important, as the sap is 

 found to possess, as a rule, an acid reaction (that is, it turns 

 vegetable blues, such as litmus, red), and hence it aids the root 

 by dissolving up some of the materials present in the soil. 

 Many of these substances, which are very needful for the food 

 of the plant, are insoluble in water, but the acid sap dissolves 

 them readily. 



This can be easily shown by means of an experiment. Take 

 a piece of perfectly smooth marble, and cover it with sand to 

 the depth of about a quarter of an inch ; sow in this some seeds 

 of mustard or cress, and place in a position favourable for 

 germination. AVhen the young plants have grown for a short 

 time, clear the whole off, and it will be found that the rootlets 

 will have eaten their way into the marble, dissolving the sub- 

 stance, and forming minute grooves where they had been, 



