404 



BOTANY OF THE LIVING PLANT 



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

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

 fungi {Polyponcs), which grow out from the trunks of trees, and 

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

 or like the Honey-Agaric {Armillaria mellea), which kills forest trees 

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



Fig. 339. 



Harveyella mirabilis, growing as a colourless parasite on the thallus of Rhodomela, 

 one of the Red Algae. Longitudinal section of Rlwdomela 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 exhausted 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 compHcated ! 

 and various their structure may be, it is based upon the simple or 1 

 branched filament, or hypha. The w^hole system of such hyphae is ; 

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

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

 complex bodies of the larger Fungi. When closely appressed the 

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



I 



