28 INTRODUCTION TO CYTOLOGY 



Inorganic salts are present in considerable variety, as shown by the 

 presence of the following elements in the ash of Fuligo protoplasm: 

 chlorine, sulphur, phosphorus, potassium, magnesium, sodium, calcium, 

 and iron. 



Enzymes, or organic catalysts, are of great importance in protoplasm 

 because of their power of initiating and altering the rate of reactions. 

 Their chemical nature is a matter of considerable dispute. Urease, 

 isolated from jack-bean meal, is a globulin,^ and there is evidence that a 

 number of other enzymes also are proteins. The high specificity 

 characterizing the action of enzymes indicates the importance of their 

 chemical composition, and it is also ev-ident that such action depends 

 largely upon the peculiar energy relations existing at the surfaces they 

 occupy or present.^ 



The amount of water in protoplasm varies greatly under different con- 

 ditions, but usually it is present in large proportions. It makes up from 

 85 to 95 per cent of the weight of actively streaming protoplasm such as is 

 seen in Tradescantia, and in actively functioning cells it rarely drops 

 below 70 per cent. In dry spores, however, it may be reduced to 10 or 15 

 per cent, the protoplasm then becoming very viscous. The body of a 

 jellyfish may consist of more than 99 per cent by weight of water. The 

 approximate composition of the human body is as follows: water, 65 

 per cent; protein, 15 per cent; fat, 14 per cent; salts, 5 per cent; other 

 organic compounds, 1 per cent. A considerable portion of the water in 

 the cell or body is physically "bound" in the colloidal structure of the 

 protoplasm and must be considered an integral part of the living system. 



No single substance is of greater significance in the life of the organism 

 than water. In the words of Henderson (1913): 



. . . the physiologist has found that water is invariably the principal con- 

 stituent of living organisms. Water is ingested in greater amounts than all 

 other substances combined, and it is no less the chief excretion. It is the vehicle 

 of the principal foods and excretion products, for most of these are dissolved as 

 they enter or leave the body [across the wall of the intestine and across the 

 epithelia of kidnej^s, lungs, and sweat glands]. Indeed, as clearer ideas of the 

 physico-chemical organization of protoplasm have developed, it has become 

 evident that the organism itself is essentially an aqueous solution in which are 

 spread out colloidal substances of great complexity [Bechhold, 1912]. As a 

 result of these conditions there is hardly a physiological process in which water 

 is not of fundamental importance. 



It is not only by virtue of its great solvent power that water is so 

 effective in promoting chemical interaction. Its exceptional ionizing 

 power is especially noteworthy in this connection, while its high surface 

 tension has a notable influence on reactions involving adsorption. There 



' Sumner (1926), Sumner and Hand (1928). 

 8 See Gortner (1929; Chap. XXXV). 



