2 STRUCTURE AND CLASSIFICATION OF FUNGI 



and rebranch and intertwine, sometimes even uniting or anastomos- 

 ing, to form a structure called mycelium. This mycelium may form 

 a loose meshwork as in the molds or a compact tissue as in the mush- 

 rooms. Thus, although fundamentally the same in their finer struc- 

 ture, fungi may show extreme variations in their external appear- 

 ance, due simply to the degree of compactness of the mycelium. 



Such variations in external appearance have to some extent been 

 used in the past as a basis for classification, but they are not always 

 correlated with other characters and are unreliable. The same or- 

 ganism may form a loose mold- 

 like mycelium in some circum- 

 stances, and a fleshy solid 

 tissue in others. The true rela- 

 tionships of the fungi are indi- 

 cated by minute cell charac- 

 ters, and more especially by 

 their modes of reproduction. 



In some fungus tissues, how- 

 ever, the cells are not arranged 

 in filaments, but form com- 

 pact masses of round or polyg- 

 onal cells like those of higher 

 plants. Such a tissue is called 

 a pseudoparenchyma; its cells 

 have their origin in mycelium. 

 Sometimes such masses of fleshy tissue become quite dry and firm, 

 developing thick, hard walls, and may have the property of main- 

 taining vitality in a dormant state for long periods. 



In some fungi the mycelium is not divided into individual cells 

 by crosswalls or septa. The entire mass of mycelium forms one large 

 cell containing many nuclei. This absence of septa makes possible 

 a flowing of protoplasm. Such a structure (described as being co- 

 enocytic) also occurs in the filaments of certain green algae, and 

 has been considered (together with other evidence) to indicate a rela- 

 tionship between these algae and those fungi which possess it. Con- 

 sequently the latter have been called Phycomycetes, or alga-like 

 fungi. The other fungi possess a septate mycelium. In these, each 

 cell, separated from the others by crosswalls, may contain one or more 

 nuclei. In some of the Basidiomycetes and Ascomycetes the my- 

 celium a^ pertain stages of its development is made up of cells, each 

 with two nuclei. Streaming of protoplasm has been shown by Buller 

 and others to occur in septate hyphae through central pores of the 



Fig. 1. 



a, Coenocytic mycelium; h, sep- 

 tate mycelium. 



