THE CELL WALL 41 



Although several reports since 1921 have challenged von Wettstein's 

 generalizations, more recent work with the X-ray diffraction method 

 has confirmed them. These results are summarized in Table 3. From 

 this compilation it can be seen that chitin is found in most of the true 

 fungi, except for the Oomycetes, which are characterized by cellulose. 

 The Chytridiales have not been studied by modern methods; individ- 

 ual species have been claimed to form chitin (111, 186). 



Two points need to be made in connection with the yeasts. The 

 X-ray diffraction data of Table 3 indicate that they divide on family 

 lines, only the mycelial yeasts having chitin. The principal cell wall 

 material of the unicellular yeasts appears to be a polyglucose distinct 

 from cellulose (8) with a second membrane containing a mannan- 

 protein complex (45, 119). However, microchemical tests suggest that 

 there is a small amount of chitin in unicellular yeasts; glucosamine 

 can be isolated (45, 145). The amount present in these yeasts is prob- 

 ably so small as to escape detection by the X-ray diffraction method. 



The amount of chitin produced is lower in a medium with nitrate 

 than in one with ammonium ion as the nitrogen source (7, 152); this 

 may be a pH effect. Autolytic loss of chitin is said to occur in nitrate 

 but not in ammonium media (7). 



Although it is probable that other materials are more or less closely 

 associated with chitin in the cell wall, unambiguous evidence of such 

 association is lacking. Pectic materials, protein, lipids, cellulose, cal- 

 lose, and minerals have all been suggested (47, 103, 104, 178-181). 

 The cellulose problem is considered by Castle (23) and by Farr (46). 

 In Herpomyces stylopygae, parasitic on insects, histochemical studies 

 indicate the presence in the basal shield of a chitin-protein layer, and 

 a process of sclerotization similar to that in the development of insect 

 cuticle may take place (141). 



Chitin appears not to occur in the actinomycetes (57a); staining re- 

 actions of Streptomyces spp. are the basis of a suggestion that a lipid 

 layer is associated, perhaps loosely, with the spore wall (43). 



The sheath surrounding the stalk of Dictyostelium discoideum is 

 composed of cellulose, laid down extracellularly (137). 



Callose, described by Mangin (105) and by Thomas (179), appears 

 to be an acidic polyglucose; its relation to the cell wall itself is uncer- 

 tain and cannot at present be accepted. 



A material identified as lignin occurs in relatively large amounts in 

 several fungi (128, 177). The identification rests on solubility char- 

 acteristics alone, and it seems very doubtful that the substance is chemi- 

 cally the same as the lignin of higher plants (69, 155, 165). 



