404 



BOTANY OF THE LIVING PLANT 



Many of these grow on decaying humus, and like the Common Mush- 

 room are saprophytes. Others are parasites, hlie the large shelf- 

 fungi [Polyporus], which grow out from the trunks of trees, and 

 are the cause of the perishing of the heart-wood in hollow timber ; 

 or Uke the Honey-Agaric [Arniillaria mellea), which kills forest trees 

 by attacking their soft and nutritious cambium (Fig. 340). But 



Fig. 339. 



Harvevella inirahilis, growing as a colourless parasite on the thallus of Rhodontela, 

 one of the Red Algae. Longitudinal section of Rkodotnela bearing the parasite, with 

 a mature cystocarp, the fertile filaments of which are black. The cells of the 

 host with food-material are dotted ; those which are e.xhausted are left blank. {After 

 Sturch.) 



apart from these there are multitudinous smaller Fungi, such as 

 the parasitic Mildews and saprophytic Moulds, while the unicellular 

 Yeasts show the simplest structure of them all. However complicated 

 and various their structure may be, it is based upon the simple or 

 branched filament, or hypha. The whole system of such hyphae is 

 called a mycelium. Such filaments may grow singly, as in the Moulds 

 and Mildews, or they may be massed together so as to form the 

 complex bodies of the larger Fungi. When closelv appressed the 

 septate filaments may seem to form a definite tissue ; but it is in 



