CHAPTER II 



CHEMICAL COMPOSITION OF PLANTS AND THEIR 



BASIC METABOLISM 



9. Chemical Composition of Protoplasm and of Other Parts 

 of the Cell. Reserve and Structural Substances. The Main 

 Groups of These Substances. — Protoplasm, the principal agent 

 in vital phenomena, has long been the subject of chemical inves- 

 tigation, the purpose of which is to gain a more complete 

 and detailed comprehension of its composition. Still, in spite 

 of the efforts of many outstanding chemists, information con- 

 cerning the chemical composition of protoplasm is very incom- 

 plete. This insufficient knowledge of one of the most important 

 questions of biology is due to the extreme complexity and 

 variability of the composition of the living matter. Life is a 

 continuous exchange of substances and is impossible without 

 these processes, and therefore the carrier of life, the proto- 

 plasm, must continuously change its composition, which greatly 

 increases the difficulty of exact analysis. Besides, there are 

 always found in the protoplasm various reserve substances that 

 serve as a sort of fuel for the living factory and as raw material 

 for the continuously created products. It is very often difficult 

 to establish which of the substances revealed by analysis are 

 substances constituting the protoplasmic essentials and which 

 are only reserve or waste products. One of the essential diffi- 

 culties in studying the composition of the protoplasm of plant 

 cells is that it usually occupies a very small part of the total 

 volume of the cell and is very difficult to separate from other 

 substances present in the cell sap. The most convenient objects 

 for the study of protoplasm are the plasmodia of shme molds 

 (M3rxomycetes), which are large aggregations of naked proto- 

 plasm. A careful analysis, performed comparatively recently 

 by Kiesel (1925), gives the percentage composition of the Plas- 

 modium of one of these molds shown in the table on p. 35. 



In this instance, of course, not only the active protoplasm but 

 also a number of reserve substances have been included, as is 



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