236 STRUCTURAL BOTANY 



ing the family Erysiphece, so named after its largest 

 genus Erysiphe) are all parasites. One species (8. 

 Castagnei) is exceedingly common on Hops, and produces 

 a very serious disease, the mildew, which causes great 

 loss to the hop-growing industry. Another species (S. 

 pannosa) is equally abundant on the leaves of Roses. 

 All these Fungi are remarkable for being external para- 

 sites, that is to say their much-branched mycelium forms 

 a web on the surface of the leaves and other organs of 

 the host-plant. The presence of the parasite is quite 

 evident to the naked eye, owing to the dirty- white colour 

 of its mycelium, which obscures the natural green of the 

 leaf. The popular name " mildew " (equivalent to mealy 

 dew) refers to this appearance, the leaves looking as if 

 they had been powdered with flour. It must have been 

 to Fungi of this kind that the name mildew was origin- 

 ally applied, though now it is extended in popular usage 

 to other diseases of plants, presenting quite different 

 symptoms. 



The mycelium which spreads over the leaf, when 

 examined microscopically, is found to be fixed to the 

 host by means of enlarged hyphse, producing broad, root- 

 like organs of attachment, which anchor themselves by 

 growing out into short plugs wedged in between the 

 epidermal cells of the host. These rhizoids produce 

 other branches, which have a still more important 

 function, for they penetrate into the interior of the cells 

 of the epidermis, forming suckers or haustoria, which 

 absorb the organic substances in the cells attacked, and 

 thus supply the whole Fungus with its food, at the 

 expense of the living tissue of the host. The mycelium, 

 as in all the higher Fungi, is multicellular, consisting of 

 a single chain of cells, each of which in this case has a 



