THE CELL WALL 



3. THE CELL WALL 



39 



The most serious defect in our knowledge of cell wall composition 

 in the filamentous fungi is the fact that no "pure" wall material is yet 

 available. The major sources of error are the inclusion of extraneous 

 substances in the preparation, and the loss of wall material during 

 purification. We do not have, therefore, for any fungus even a quali- 

 tative picture of the chemical components of the wall. This section, 

 consequently, can only review work in which one or the other major 

 constituent has been studied. When we speak of a chitin wall, we 

 mean a wall with detectable chitin, not necessarily or even probably 

 a wall which is exclusively chitin. Morphologically, in fact, the wall 

 may be a multiple structure with layers of different composition (15, 

 46). 



In most fungi the basic material of the cell wall is chitin, a polymer 

 of A T -acetylglucosamine, identical chemically with the chitin of arthro- 

 pods. The history of the study of chitin in fungi is reviewed by 

 Schmidt (152) and by Frey (56). Much confusion exists, however, 

 because of the use of methods which are not critical. The most ac- 

 ceptable method of qualitative identification of chitin is determina- 

 tion of the X-ray diffraction pattern (56, 66, 79, 87, 91). However, if 

 chitin is present only in small amounts, the X-ray method may not 

 reveal it. Other methods — microchemical tests, isolation of derivatives 

 from a hydrolyzate, and determination of the nitrogen in a purified 

 preparation — are usually in agreement with X-ray results, but are not 

 sufficiently discriminating to stand by themselves. Microchemical 

 methods for chitin and cellulose are particularly unsatisfactory (56). 



The quantitative determination of chitin requires purification, usu- 

 ally by the method of Scholl (153), and determination of the nitrogen 

 of the product. In a survey of 25 fungi, chitin contents from 2.6 to 

 26.2 per cent of the dry weight have been reported (18); other quanti- 

 tative data are found in Table 1 above, and in the paper of Schmidt 

 (152). The possibility that some protein nitrogen, from resistant pro- 

 teins, is included in these analyses cannot be ignored. 



On the basis of microchemical tests for chitin, von Wettstein (186) 

 proposed that large taxonomic groups of the fungi differ in the consti- 

 tution of the cell wall. Specifically, he found chitin in the Zygomy- 

 cetes and in the higher fungi — ascomycetes and basidiomycetes; repre- 

 sentative Myxomycetes, Oomycetes and Monoblepharidales were 

 found not to form chitin. This supports a polyphyletic origin of the 

 phycomycetes. 



