THE LIVING ENVIRONMENT. 113 



the loss of light suffered by the shaded foliage, 

 but the weed-action is often increased by the com- 

 petition of their roots e.g. briars ; and in the case 

 of woody climbers the gradually increased pressure 

 of the woody-coils round the thickening stems 

 compresses the cambium and cortex of the sup- 

 port and induces strictures and abnormalities which 

 may be fatal in course of time. 



Epiphytes, or plants which support themselves 

 wholly on the trunks, branches, or leaves of other 

 plants, also injure the latter more especially by 

 shading their foliage e.g. tropical Figs, Orchids, 

 Aroids, etc. ; and similar damage is done by our 

 own Ivy, the main roots of which are in the soil, 

 but the numerous adventitious roots of which cling 

 to the bark. 



When the climber or epiphyte is also parasitic, 

 as in the case of the Dodder, Loranthus, Mistletoe, 

 etc., the direct loss of substance stolen from the 

 host by the parasite comes in to supplement any 

 effect of shading that the latter may bring about 

 if it is a leafy plant. 



Of Cryptogams, apart from a few epiphytic 

 ferns, and the intense weed -action of certain 

 Equisetums, the rhizomes and roots of which are 

 as troublesome as those of twitch and other phan- 

 erogamic weeds, it is especially the fungi which 

 act as agents of disease, and which, as we now 

 know, 2irQ par excellence the causes of epidemics. 



The action of fungi may be local or general ; 

 and restricted, slow and insidious, or virulent and 

 rapidly destructive. 



H 



