THE FUNGI WHICH CAUSE PLANT DISEASE 



63 



similar to that of the parent. The most usual mode is for the 

 mycelium to rise directly from the spore. In other instances the 

 spores produce zoospores which migrate, come to rest, then develop 

 a mycelium. In still other cases a short mycelium, promycelium, 

 is formed and from this small conidia, sporidia, are made. 

 Figs. 217, 240. These conidia give, direct rise to the mycelium. 

 Spores of some species may by genmiation lead a more or less 

 prolonged existence without return to the mycelial stage. 



Heat and Moisture Relation. Like all living things these 

 organisms cannot develop without heat and moisture. The 

 necessary degree of each varies with different species. Some 



4 



Fig. 39. — ^A perithecium with asci. After Reddick. 



species are strictly aquatic, and must be surrotmded with water; 

 others can grow in comparatively dry situations. Generally 

 speaking, however, dampness favors fungous development, and 

 the growth of most fungi is more vigorous in a damp atmosphere 

 than in a drier one. Similarly moderate warmth, as that of summer 

 heat, favors fungous growth. Hmnidity and warmth combined 

 are proverbial as producers of mold and mildew. So conspicuous 

 is the coincidence of these conditions with fungous growth, that 

 in the minds of many a warm damp air is the cause rather than the 

 condition of fungous development. 



Respiration with the fungi as with other plants and animals 

 consists in oxidation, involving intake and consumption of oxygen 

 accompanied by the giving off of carbon dioxide and water, and 



