FUNGI 6 



Structure. The vast majority of fungi consist vegetatively of more or 

 less elongated, septate or nonseptate filaments. These are called indi- 

 vidually hyphae (singular, hypha) and collectively the mycelium. They 

 may be uniform in thickness or tapering from broad to slender portions 

 in the same hypha or in different portions of the same mycelium. They 

 may be branched or unbranched (simple). In thickness they may be less 

 than 0.5 jj. up to over 100 n (in some Saprolegniales). In size the whole 

 mycelium may be only a few microns in length or it may produce great 

 sheets or strands that extend many meters. 



The composition of the cell wall is very variable among the different 

 fungi and sometimes in the same individual at different stages of ma- 

 turity. Basically the chief components appear to be various types of 

 carbohydrates or mixtures of these: cellulose, pectose, callose, etc. Mixed 

 with these and probably often in chemical combination with them there 

 may be other substances. Cellulose predominates in many of the Phyco- 

 myceteae so that the characteristic cellulose reaction is shown upon treat- 

 ment with chloriodide of zinc, but sometimes where it makes up the bulk 

 of the wall it does not respond to this reagent until certain fatty deposits 

 in the outer portion of the cell wall are first dissolved away, as in Mono- 

 hlepharis. In a great many fungi, especially the Ascomyceteae, Basidio- 

 myceteae and the higher Phycomyceteae, either cellulose is entirely 

 lacking, being replaced by some other carbohydrate, or the considerable 

 amount of chitin with which the wall is impregnated prevents the cellulose 

 from showing its presence. Chitin is never alone in the wall but it may 

 form a considerable portion of the component substances. Aside from the 

 foregoing substances calcium carbonate or other salts may be deposited 

 upon or within the wall. Although von Wettstein (1921) and others have 

 identified the chief component of the cell wall of many fungi, apart from 

 the carbohydrates, as chitin, identical with the chitin of the Arthropods, 

 Dous and Ziegenspeck (1926) after a careful comparative study of the 

 animal chitin with the fungus chitin conclude that these are parallel 

 compounds derived from different basic substances, but with much the 

 same general characters. Throughout this book, wherever the word chitin 

 is used it should be understood as referring to this fungus chitin, not to 

 the true animal chitin. 



There are two main types of mycelium ; in one the hyphae are cellular 

 and in the other, coenocytic. A cellular hypha usually contains either one 

 or two nuclei per cell and the division of the cell is initiated by the division 

 of the nucleus or by the simultaneous division of both nuclei, respectively. 

 In a coenocyte there are many nuclei and the formation of septa occurs 

 without immediate reference to any preceding nuclear division. A coeno- 

 cytic hypha may be "tubular," i.e., lacking septa, or septate. In the 

 latter case each segment is multinuclear. A tubular coenocyte, such as is 



