612 The Outline of Science 



Root-Fungi 



The roots of many other plants, such as the beech, the pine, 

 and the heaths, are associated with symbiotic fungi (Mycorhiza) , 

 and in some cases there is evidence that the fungus supplies its 

 host with nitrogenous compounds. The best-authenticated case 

 is that of the heaths. These plants inhabit barren moorland sta- 

 tions. In such situations relatively few species of plants are able 

 to flourish, and the vegetation is usually dominated by a few which 

 are specially endowed on the common by the whin, on the moor 

 by the heather. The whin has its root-bacteria ; the heather has a 

 fungus which spreads all through it and enables it to utilise the 

 peaty soil. Thus the heather is in large measure an impostor; 

 it thrives because it is interpenetrated by a partner fungus. 



3 



Insectivorous Plants 



On wet moors there grow three genera of plants which show 

 yet another method of supplementing the food supplies obtain- 

 able from the soil. These are the sundews, the butterworts, and the 

 bladderworts, several species of each of which occur in Britain. 

 These plants catch small flies and crustaceans, and absorb the 

 products of their decay, or even actively digest them. They all 

 possess chlorophyll, and it is in the supply of proteins, and pos- 

 sibly of salts, which the victims offer, that we find the explanation 

 of so curious a habit. 



The rosy leaf of the sundew is covered with little club-shaped 

 tentacles, short-stalked at the centre, long-stalked round the 

 margin. On the thickened end of each is a drop of sticky fluid, 

 which, glistening in the sun, gives the plant its name, and which 

 serves to catch "incautious, deluded insects." The entangled 

 midge struggles, comes in contact with more tentacles, and is held 

 more firmly. Stimulated by contact with the solid body, stimu- 

 lated too by the chemical action of the insect, the marginal ten- 

 tacles of the plant, even though they are not touched, soon begin 



