6 THE STOCKFEEDER'S COMPANION 



and minerals, to work the soil in order to admit air 

 and get a good tilth, as well as to keep a supply of 

 humus in the soil as reserve plant food, and as an 

 absorbent for water. 



Absorption of Minerals. The minerals in the soil 

 which are dissolved in the soil-water soak or diffuse 

 through the very delicate walls of the root-hairs (osmosis). 

 The solution then rises up the wood-vessels (xylem) in the 

 plant till it comes to the leaves, where it is continually 

 being lost by transpiration or vaporisation, with the 

 result that the minerals, which were dissolved in the 

 water, become gradually concentrated in the leaves. 

 A further supply of water containing dissolved minerals 

 is again drawn up into the leaves, and so the process 

 goes on. Generally speaking, the warmer the weather, 

 the more rapidly do the leaves transpire. 



The power which plants possess of drawing water from the 

 roots up to the leaves is most likely due to a number of forces 

 acting simultaneously, viz. : 



(a) Root-pressure, or the force exerted on the liquids in the 



plant by the absorbing action of the roots. The root 

 continues to absorb, with the result that this exerts a push 

 from behind on the liquid previously absorbed. 



(b) The vacua formed in the wood-vessels of the stem of the 



plant, due to the loss of water by transpiration ; as well as 



(c) The osmotic force of the minerals in the leaves, etc. 



All these forces are involved in the distribution of 

 minerals to those parts of the plant where they are 

 required. 



