66 PHYSIOLOGY OF THE FUNGI 



Organic materials are dried before analysis. On the average about 75 per 

 cent of the fresh weight of mycelium is water, while spores contain only 

 about 40 per cent water. It is probable that the water driven off when 

 fungus cells are dried to constant weight is in part free water and in part 

 water bound to various colloidal cell constituents. 



Ultimate analyses of mycelium and spores always reveal the presence of 

 carbon and nitrogen. On the average about 45 per cent of dry mycelium 

 is carbon. This high content of carbon makes it certain that carbon is an 

 essential element. The percentage of nitrogen found is quite variable. 

 Phosphorus, potassium, magnesium, calcium, sodium, sulfur, and iron are 

 found in the ash that remains after burning mycelium and spores. More 

 refined methods of analysis reveal that fungus ash contains still other 

 elements. Richards and Troutman (1940) investigated the composition 

 of yeast ash by spectrographic analysis and found the following elements : 

 iron, sodium, boron, bismuth, barium, magnesium, manganese, copper, 

 zinc, tin, lead, tellurium, silver, chromium, potassium, gold, and lan- 

 thanum. However, the mere presence of an element in fungus cells does 

 not necessarily mean that it is essential. 



Since many of these elements in fungus ash occur in minute traces only, 

 it is desirable to approach the problem of essentiality in another way. 

 This is done by omitting from the medium the element in question. 

 Raulin (1869) was apparently the first to use this method. He found 

 that the omission of phosphorus, sulfur, magnesium, zinc, or iron from the 

 basal medium allowed very little growth of Aspergillus niger. These ele- 

 ments are thus shown to be essential by the two methods of investigation. 

 In general, the experimental work in which specific elements have been 

 omitted from the medium is more convincing than the method of ultimate 

 analysis. This is the preferred method of testing the essentiality of ele- 

 ments required in small amounts. 



Functions of the essential elements. Thatcher (1934) has attempted 

 to classify the essential elements into groups: structural, functional, and 

 those utilized in the transfer of energy. This classification has some 

 validity and may serve to fix attention upon the more salient biological 

 features of an element. However, most, if not all, of the essential ele- 

 ments play many roles in the life processes of the fungi. In general, the 

 nonmetallic elements may be classified as structural elements. This 

 means that the compounds which make up the structural units such as 

 the protoplasm are largely composed of the nonmetallic essential ele- 

 ments: carbon, nitrogen, hydrogen, oxygen, sulfur, and phosphorus. The 

 functional uses of these elements by the fungi are no less important. The 

 essential metallic elements may be classified as functional elements, but 

 this does not mean that these metallic elements have no structural 

 functions. 



