134 TEXT-BOOK OF FUNGI 



however, intermediate forms. Some fungi, usually sapro- 

 phytic, can under certain conditions become true parasites ; 

 such are termed facultative parasites. Those fungi which, 

 so far as is known, can only develop and grow on living 

 organisms, are known as obligate parasites. In some 

 instances certain groups of fungi that were considered to 



FIG. 43. Puccinia pringsheimiana, a typical obligate parasite, i, 

 portion of a gooseberry leaf with three aecidia or ' cluster-cups ' fully ex- 

 panded ; 2, section through an aecidium, showing, a, the spores produced 

 in chains, which afterwards become free and fill the cup with yellow 

 powder, t, the protective wall or peridium, c, the epidermal cells of the 

 leaf, d, the middle cells of the leaf, e, the mycelium of the fungus running 

 between the cells of the leaf; 3, portion of the leaf of a sedge, bearing 

 sori or clusters of uredospores and teleutospores ; 4, a portion of fig. 3 

 enlarged ; 5, teleutospores ; 6, uredospores. Fig. 3, nat. size ; all the other 

 figs. mag. 



be strictly obligate parasites have not proved to be so 

 under all conditions. Brefeld has succeeded in growing 

 several species of Ustilago quite independently of the host- 



