I40 PLANT LIFE. 



by decaying vegetation and manure ; {b) Parasites are those 

 fungi that grow upon and obtain their food from a living 

 body, which may be either plant or animal, most frequently 

 the former. The diseases of plants and animals, as the 

 potato disease, and the numerous pests popularly included 

 under the terms "rust," "smut," "mildew," "mould," etc., 

 also the silkworm disease known as " muscardine,'' are due 

 to parasitic fungi, which are, as a rule, minute or even 

 microscopic in' size, although some timber diseases are 

 caused by large perennial fungi, as the well-known dry-rot, 

 due to the attacks of a fungus called Merulius lacrymans. 



Between the above conditions there are intermediate 

 forms. Some species, that under ordinary conditions live 

 as saprophytes, can exist as parasites for a time, or alto- 

 gether; such are texiatA facultative parasites. On the other 

 hand, those fungi that ordinarily live as parasites, but under 

 certain conditions can live for the whole or part of their life 

 as saprophytes, are c&Wtd facultative saprophytes. 



Parasites attack their host, that is, the plant on which 

 they are parasitic, in different ways. Access to the interior 

 of the host is usually accomplished by the germ-tube from 

 a germinating spore, either piercing the epidermis, or passing 

 in through a stoma. Most species are endophytes, the whole 

 of the vegetative portion of the fungus developing in the 

 tissues of the host, where the sexually-produced reproductive 

 bodies are also formed, as in the species of Pythium. When 

 asexual reproductive bodies are present, they are usually 

 formed outside the host, the sporangiophores not unfre- 

 quently emerging through the stomata, as in some species 

 of Peronospora, or form dense clusters called sort just below 

 the epidermis, which is eventually ruptured, exposing the 

 spores, as in the genera Ustilago, Uredo, etc. A smaller 



