292 STUDIES OF AMERICAN FUNGI. 



ring to Table I, it will be seen that nitrogen constituted 5.79 per cent, 

 of the total dry substance of Coprinus comatus. This high nitrogen 

 content, which is common to the mushrooms in general, was formerly 

 taken to indicate a very unusual richness in proteid materials. It is 

 now known, however, that there were several sources of error in 

 this assumption. 



Much of the nitrogen is present in the form of non-proteid sub- 

 stances of a very low food value. Another and very considerable 

 portion enters into the composition of a substance closely related to 

 cellulose. A third source of error was the assumption that all the 

 proteid material was digestible. It is now known that a very con- 

 siderable portion is not digestible and hence not available as food. 

 Thus, notwithstanding the 5.79 per cent, of nitrogen in Coprinus 

 comatus, we find but .82 per cent, in the form of actually available 

 (i. e., digestible) proteids, or approximately one-seventh of what 

 was formerly supposed to be present. 



The digestibility of the proteids varies very greatly with the spe- 

 cies. Morner found the common field mushroom, Agaricus campes- 

 /m, to have a larger amount of proteids available than any other 

 species studied by him. Unfortunately, the digestibility of the 

 American plant has not been tested. There is great need for further 

 work along this line. Enough has been done, however, to demon- 

 strate that mushrooms are no longer to be regarded as a food of the 

 proteid class. 



The energy for the muscular exertion and heat is most economi- 

 cally derived from the foods in which the carbohydrates and fats 

 predominate. 



The common way of comparing foods of the first two classes 

 scientifically is to compare their heat-giving powers. The unit of 

 measurement is termed a calorie. It represents the amount of heat 

 required to raise a kilogram of water 1 Centigrade. (This is 

 approximately the heat required to raise one pound of water 4 Fahr- 

 enheit.) A man 24. moderately hard muscular labor requires daily 

 enough food to give about 3500 calories of heat-units. The major part 

 of this food may be most economically derived from the foods of the 

 second class, any deficiency in the .28 lb. of digestible protein being 

 made up by the addition of some food rich in this substance. 



In the following table the value of ten pounds of several food sub- 

 stances of the three classes has been worked out. Especial atten- 

 tion is called to the column headed ** proteids " and to the last column 

 where the number of heat-units which may be purchased for one 

 cent at current market rates has been worked out. 



