148 THE LIFE OF THE PLANT 



can exist only on soil already containing organic matter : 

 every attempt to grow them in a medium devoid of 

 such matter proves fruitless. Plants feeding on other 

 plants by attaching themselves to their stems or roots 

 belong to the same category : e.g. the broom-rape, 

 which grows on the roots of hemp ; the dodder (Cuscuta) , 

 which twists round the stems of hop, flax, and clover, 

 and clings to them until it finally exhausts them alto- 

 gether. All these plants have either ugly scales of 

 some other colour than green in place of leaves, or else 

 do not possess leaves at all. They are therefore incap- 

 able of independent existence, but suck the sap of other 

 plants. Such plants are called parasites. All of them, 

 and especially the minute fungi, which cause various 

 diseases in plants, give farmers much trouble and 

 frequently rob them of entire crops. 



