THE CHEMICAL COMPOSITION OF PROTOPLASM 71 



plasm" synonymoush'. In this book, however, the terms will be used as has 

 been indicated earlier in tliis chapter. 



The Chemical Composition of Protoplasm. — Since protoplasm is a dy- 

 namic system of substances it is not possible to subject it to chemical analysis 

 without destroying it. In the strict sense, therefore, it is impossible to 

 discover the chemical composition of protoplasm. It is possible, however, to 

 examine the substances present after the protoplasmic sj'stem has been de- 

 stroyed and to determine their chemical composition and relative abundance. 

 A number of such studies have been made. 



Water is the chief component of all physiologically active plant proto- 

 plasm usually making up more than 90 per cent of the system. The water 

 content of the protoplasm of dry seeds, on the other hand, may be less than 

 10 per cent. 



Most attempts to determine the chemical composition of plant protoplasm 

 have been made upon species of the myxomycetes. At certain stages in their 

 life history these organisms consist of naked masses of labile protoplasm. 

 They are often found "flowing" over rotten logs in damp woods. The fact 

 that the mj'xomycetes provide relatively large quantities of protoplasm entirely 

 free from cell wall material has made them a favorite object for chemical 

 analysis. Even in such organisms, however, not all of the constituents of 

 the plant body can be regarded as integral parts of the protoplasm. Distrib- 

 uted throughout the protoplasmic mass are particles of foods and other inert 

 materials which cannot be separated from the protoplasm. The results of a 

 chemical analysis of the dry residue of a myxomycete Plasmodium are shown 

 in Table 12. 



As shown in this analysis proteins and other nitrogen-containing com- 

 pounds constitute the bulk of the organic matter in the Plasmodium of this 

 species. Many different \aricties of proteins are known to occur in the proto- 

 plasm of plant cells. They are compounds of enormous molecular weight 

 (Chap. XXVI), and undoubtedly make up a large proportion of the labile 

 structural framework of the protoplasm. 



Lipids (Chap. XXIII) constitute a smaller fraction of the protoplasm 

 than the proteins. Three types of lipids occur in the protoplasm ; the true 

 fats (oils), the phosphatides (phospholipids) and the sterols, of which phyto- 

 sterol is an example. The oils are generally suspended in the protoplasm in 

 the form of minute globules. They are probably more important as food 

 reserves than as actual constituents of the protoplasm. The phospholipids 

 and sterols, on the other hand, are believed to be essential constituents of the 

 protoplasmic system. 



The water-soluble carbohydrates, amino acids, etc., present in the plas- 



