[Chap. XLIV THE FUNGI 543 



aggregate of hyphae growing on, or within, the substrate is called the 

 mycelhim (Greek for fungus). The underground hyphae of some species 

 grow in small compact masses, forming tuber-like bodies. Those of some 

 other species form compact hard strands of many hyphae that grow as a 

 collective unit and push their way through firm substrates. 



Fig. 235. Stages in the development of the common edible pink-gilled mushroom 

 (P.salliota campestris) . Note the underground vegetative body of the plant. 



The cell walls of hyphae are composed of such substances as cellulose, 

 pectose, callose, and chitin. The foods found in fungi are carbohydrates 

 (including sugars and glycogen), fats, and proteins. Various kinds of 

 enzymes are produced by fungi, and these are important in transfomiing 

 substances not only within the fungous cells, but outside in the host or in 

 the substrate. Fungous hyphae may grow into the tissues of the host or 

 other substrate bv mechanical pressure similar to that of growing roots, 

 or the substrate may be digested by excreted enzymes just ahead of the 

 lengthening hypha. 



Fungi mav be found wherever the environment is not inimical to the 

 establishment of a mvcelium, and where there are substances they can 

 digest and use as food. They are found on land and in water, growing 

 free as saprophvtes or as parasites within, or upon, many different hosts. 

 Aquatic fungi are not as numerous as terrestrial ones, and are far less 

 abundant than the algae and bacteria. The aquatic species live as para- 

 sites on fresh- water and marine organisms, or as saprophytes on the dead 

 bodies and residues of such organisms. They are most numerous in well- 



