ESSENTIAL NONMETALLIC ELEMENTS 97 



Phosphorus enters into the composition of the nucleoproteins, which are 

 found in the nucleus and cytoplasm of every cell. The nucleoproteins are 

 conjugated proteins which consist of a protein moiety in combination 

 with purine or pyrimidine nucleotides (nucleic acids). These nucleotides 

 are important functional compounds and may be classified according to 

 their heterocyclic components. 



The preliminary hydrolysis of purine and pyrimidine nucleotides 

 involves the removal of phosphoric acid and the formation of nucleosides. 

 Nucleosides on hydrolysis yield sugars, purines (adenine, guanine) or 

 pyrimidines (cytosine, thymine, uracil). The nucleotides are also classi- 

 fied according to the sugar moiety, i.e., D-ribose or D-desoxyribose. 



The nucleoproteins which contain D-ribose are mainly found in the 



cytoplasm, while D-desoxyribose characterizes the nucleoproteins of the 



nucleus. The Feulgen stain is used by cytologists to detect the presence 



of D-desoxyribose nucleic acid. Viruses, chromosomes, and genes consist 



largely of nucleoproteins. For a review of the role of nucleoproteins see 



Mirsky (1943). 



NITROGEN 



This essential element is used by fungi for functional as well as struc- 

 tural purposes. The cell wall of many species, with the exception of the 

 Oomycetes and yeasts, appears to be composed of chitin (Brian, 1949). 

 Chitin is a linear polymer, similar to cellulose, of D-glucosamine. The 

 amino group of glucosamine in chitin is acetylated. This substance 

 makes up the exoskeleton of insects and Crustacea. It is interesting that 

 the chitin formed by fungi, insects, and Crustacea appears to be the same 

 substance. Protein, the basis of protoplasm, is composed of nitrogenous 

 substances. Purines, pyrimidines, and some of the vitamins are also 

 nitrogen-containing compounds. 



Not all nitrogen sources are equally suitable for all fungi. Fungi may 

 be specific in the nitrogen sources they utilize. Our information on this 

 subject, while extensive, is far from complete. The reports in the litera- 

 ture which indicate that specific fungi are able to grow on a given source 

 of nitrogen may be accepted with confidence, but the reported negative 

 results are to be viewed with caution. Failure of a fungus to grow upon a 

 given nitrogen source may mean only that the medium used did not con- 

 tain the necessary growth factors, as in the case of Ophioholus graminis 

 (See Chap. 2). 



Classification according to nitrogen sources used. Robbins (1937), 

 Steinberg (1939, 1950), and others have classified the fungi according to 

 their ability to utilize different sources of nitrogen. In the main Rob- 

 bins's classification is as follows: (1) fungi able to utihze atmospheric 

 nitrogen, nitrate nitrogen, ammonmm nitrogen, and organic nitrogen; (2) 

 fungi able to utilize nitrate nitrogen, ammonium nitrogen, and organic 



