4 STRUCTURE AND CLASSIFICATION OF FUNGI 



from the thymonucleic acid found characteristically in the nucleus 

 itself. It is easily demonstrated by vital staining with neutral red. 

 Growth and Differentiation of Mycelium. Although in many cases 

 any part of a thallus may grow and give rise to a new individual if 

 artificially transferred to a new medium, normally fungi are repro- 

 duced by specialized cells called spores. These spores give rise to 

 a new mycelium by germination, i.e., the protoplasm within the spore 

 absorbs water and swells when in a favorable environment, bursts 

 through the spore wall; and extends outward as a long filamentous 

 process called a germ tube. Several of these may be formed from 

 one spore. Growth is mainly at the apex of this germ tube. At first 



the mycelium is non-septate, 

 but after it reaches a certain 

 size, septa are formed in those 

 varieties which possess them, 

 beginning in the oldest part of 

 the mycelium and proceeding 

 toward the periphery. Thus in 

 a mold which is still growing 

 there may be no septa at the 

 tips of the filaments of myce- 

 lium. Several stages of spore 

 germination are sho^\Tl in Fig. 2. 

 With most multicellular fungi there can be seen a differentiation 

 of the mycelium into two parts: a vegetative portion which burrows 

 into the substrate, digests and absorbs it, and a reproductive portion 

 which usually extends into the air and forms and discharges the re- 

 productive bodies or spores. The reproductive mycelium and its 

 spores are widely diverse in different kinds of fungi and serve to 

 classify and identify them. Not infrequently one finds molds grow- 

 ing on artificial culture media which do not form any reproductive 

 bodies. They can develop only imperfectly on such a medium. Such 

 molds are called Mycelia sterila, and usually cannot be identified 

 with any degree of satisfaction. 



Oidia and Yeast-like Cells. In addition to reproduction by special- 

 ized bodies or spores, many species of fungi can also propagate by 

 the separation of cells from any part of the mycelium, including the 

 vegetative. Such methods, however, are not peculiar to any class of 

 fungi and do not serve to classify. The same type may be formed 

 by widely diverse species. But in some species they occur so regu- 

 larly or are so prominent that they may be of great value in iden- 

 tification. 



Fig. 2. Germination of spores (conidia) 

 of a species of Aspergillus. 



