194 



COMPARATIVE MORPHOLOGY OF FUNGI 



surface of the leaves. This external mycelium does not penetrate the 

 epidermal cells by haustoria, but puts forth special branches of limited 

 length through the stomata into the mesophyll (Fig. 121, 2) where they 

 secure nourishment. 



The ectoparasitic stage includes all the other Erysiphaceae. These 

 are entirely extramatrical and form a loose white covering on the surface 

 of the host, so that it seems dusted with flour. The hyphae creep about 

 the epidermis and from time to time form short branches. Where these 

 come into contact with the wall of the epidermis, they spread out into a 

 simple or lobate appressorium. The epidermal wall begins to swell and 

 stains deeply (Fig. 120, 1). The very slender hyphal tip bores through 



Fig. 121. — 1. Uncinula Salicis on Salix discolor. Haustoria which have penetrated the 

 hypodermal cell layers. 2. Phyllactinia corylea on Cornuss tolonifera. Hyphal branch 

 penetrating the mesophyll. 3. Erysiphe graminis on Poa pratensis. Haustoria. (1, 2 

 X 600; 3 X 1,200; after G. Smith, 1900.) 



the cuticle and the cell wall into the interior of the epidermal cell (Fig. 

 120, 2 to 4) and expands there into a saccate haustorium (Fig. 120, 5 

 and 6). This simple tuberiform or saccate knot is the usual type of 

 haustorium in this group. Only in Erysiphe graminis, the haustoria 

 grow laterally to long filiform lobes (Fig. 121, 3). Generally the haus- 

 toria remain limited to the epidermal cells; only in Uncinula Salicis they 

 may penetrate to the subepidermal cells but never further (Fig. 121, 1). 



In spite of this extreme ectophytism which only differs from pure 

 epiphytism in the formation of haustoria, and in spite of their hyaline 

 walls, the Erysiphaceae are very resistent to external influences; e.g., 

 they can thrive on parts of plants which are in full sunlight the whole 

 day. They are found from the polar regions to the tropics but 



