80 THE PLANT CELL WALL 



terium diphtheriae capsules which also yield galactose and hexose- 

 amine, contain pentose instead of rhamnose. In Diplococcus 

 pneumoniae, differences among the capsules of various cultural 

 types may also be profound. Pneumococcus capsule Type I contains 

 28 per cent D-galacturonic acid, whereas type TT is 70 per cent 

 D-glucose. Types III and IV both consist of D-glucose and D-glucur- 

 onic acid, but in the type III capsule they occur exclusively as cellobiu- 

 ronic acid units in which the glucose is joined to the glucuronic 

 acid by a 1,3-linkage, and the glucuronic acid to the next glucose 

 by a 1,4-linkage. In type VIII, the polyscacharide chain yields 

 D-glucose units in addition to D-glucose-containing cellobiuronic 

 acid units. 



The actinomycetes, which are a somewhat controversial group 

 taxonomically, have cell walls consisting of hexose, hexosamine, 

 and amino acid units (including aminosuccinoyl-lysine), the latter 

 in only small amounts. The aerial, but not the vegetative, mycelium 

 has an outer layer of lipid. Grossly, therefore, such walls resemble 

 those found in Gram-positive bacteria. This similarity in wall 

 building units has led to arguments relating the actinomycetes 

 to the bacteria rather than the fungi. The many variations which 

 exist among the bacteria proper and the existence of hexose- 

 hexosamine-protein wall systems in some fungi, suggest that 

 the comparative analysis of cell wall components be applied with 

 circumspection in taxonamic classification. 



The cell wall chemistry of the fungi has long been a subject 

 of controversy. The presence of chitin, for example, has been 

 claimed and questioned for many years. It has also been suggested 

 that cell wall components vary along broad taxonomic lines. The 

 phycomycetes contain cellulose mainly; the ascomycetes and 

 basidiomycetes, chitin. 



Perhaps much of the uncertainty and controversy associated 

 with cell wall constitution lies in the lack of rigorous analytical 

 procedures and specific microchemical tests. A few examples will 

 illustrate the utility of comparatively recent analytical methods 

 in providing a more definitive picture of the cell wall in the fungi. 



Uncertainties as to the presence of chitin in the fungal wall 

 have been settled in some forms at least by X-ray diffraction studies. 



