STATING THE CASE. 



25 



posing the sheatJi is formed over it. As fast as the 

 latter are worn away, fresh ones take their places. 

 This goes on until the root has penetrated as far as 

 is necessary. The sheath-cells then die, for their 

 special work is done ; and now the fresh, uninjured 

 cells of the root begin to collect and dissolve the 



Fig. 7.— Section of the Root-tip ot a Plant, showing the Sheath at the end, 

 which protects the Plant. 



mineral salts. The acid nature of the sap assists in 

 this process. The dissolved salts pass from one cell 

 to another, through the intermediate walls, as if they 

 were strained through. We hardly need say that 

 this upflowing fluid is called sap, and that it is to 

 plants what the arterial blood is to animals. 



