50 FUNDAMENTALS OF CYTOLOGY 



then vitrifies instead of crystallizing, and the cells survive and resume 

 activity after the temperature is raised very rapidly through the range 

 where crystallization would otherwise occur. This has an interesting 

 bearing on the problem of storing living material at low temperatures. 



The inorganic salts in protoplasm are fairly numerous though small in 

 amount. They occur in part in the free water and in part as ions bound 

 by the organic constituents. They incorporate many of the 40 or more 

 essential elements, some of which are present in extremely small amounts 

 and can be detected only by extremely sensitive methods. The amounts 

 present often do not indicate the amounts necessar}^, but in some instances 

 their ratio of concentration is very significant. For example, sodium 

 lowers viscosit}^ and increases the permeability of membranes, whereas 

 calcium, necessary to membrane formation, has the opposite effects; in 

 combination the two are antagonistic. The ratio of calcium to sodium 

 salts is about the same in sea water, blood, and balanced protein-lipide 

 emulsions, a fact that surely has interesting theoretical implications for 

 the student of evolution. 



Among the carbohydrates the pentoses, hexoses, and their condensation 

 products (polysaccharides) are of special importance in the constitution 

 and activity of protoplasts. The pentoses, C5H10O5, are one of the main 

 components of nuclear chromatin (page 51), while various pentosans, 

 (CsHioOs)^, are the principal constituents of many plant mucilages and 

 gums and are components of pectins. Among stored foods are hexoses, 

 C6H12O6, including glucose, levulose, mannose, and galactose, as well as 

 hexosans, (CeHioOs)^, notably starch in plants and glycogen in animals. 

 Cellulose, a hexosan, is the main constituent of most plant cell walls. 

 Carbohydrates other than the pentoses do not enter directly into the 

 actual constitution of protoplasm but serve as sources of energy and build- 

 ing materials. 



Fats and oils occur in great variety in protoplasts. Although it is often 

 impossible to tell in what degree a given kind is a true constituent or only 

 a product of protoplasm, there can be no doubt that some of them, notably 

 the phospholipides, are among the fundamental constituents. The fragrant 

 essential oils of plants are not fats but belong to other chemical classes. 

 Although of great commercial imi^ortance, their value to the plant is 

 questionable. 



True fats, which contain only carbon, hydrogen, and oxygen, are salt- 

 like combinations (esters) of fatty acids and glycerol (the glycerides) or of 

 fatty acids and other alcohols (the sterols and most waxes) . The fats are 

 of importance as reserve food and together with sterols and waxes function 

 in retarding loss of water at surface membranes. Ergosterol, a sterol 

 found in plants, becomes the antirachitic vitamin D upon irradiation with 

 ultraviolet light. Vitamins A and E are commonly found in tissues high 



