PLANT STRUCTURE AND COMPOSITION 181 



most cases a nimibci of these high polymers are involved in the forma- 

 tion of the cell covering. These polymers include such components 

 as cellulose, pectic materials, hemicellulose, and pentosans. Cell walls 

 of the woody and stemmy portions of the plant contain an additional 

 strengthening agent, a complex organic substance called lignin. Al- 

 though the cell wall is rigid, it is by no means impervious to water. 

 Pores in the structure permit the ready passage of smaller molecules 

 from the protoplasm of one cell to that of a neighboring cell. Plant 

 physiologists believe that the protoplasmic membrane separating the 

 cytoplasm from the cell wall constitutes the semipermeable membrane 

 rather than the essentially inert cell wall. There is little or no evi- 

 dence pointing to the existence of extensive enzyme activity in the 

 cell wall. Once laid down, the wall is relatively inert and the con- 

 stituents remain outside the active metabolic pool. 



It is hoped that this brief discussion of the plant cell demonstrates 

 that this unit of the plant is not merely a homogeneous bag of enzymes 

 and substrate. Each structural entity of the protoplasm carries its 

 complement of enzymes in a highly organized manner. The proper 

 functioning of a cell as a whole requires synchronization of the 

 enzyme systems of the various discrete units as Vv'ell as the soluble 

 enzyme systems. Investigations of the interrelationships of these 

 various cytoplasmic components occupy a large portion of the re- 

 search effort at the present time. 



STRUCTURAL MATERIALS 



The various chemical constituents of the plant can be considered 

 from the standpoint of utility to the cell, that is, structural materials, 

 food reserves, metabolic machinery, and special substances. This last 

 group includes such items as essential oils, flower pigments, alkaloids, 

 and other chemical entities for which no present function is known. 

 As the science of biochemistry advances, many of these items will un- 

 doubtedly be reclassified into one of the above functional classes. 



Polysaccharides constitute approximately 75 per cent of the dry 

 weight of higher plants. Most of these polysaccharides are com- 

 ponents of the cell wall. In young cells the quantity of such structural 

 materials is very low. On the other hand, in mature cells the walls are 

 often so thick as to leave only a small cavity within the cell. Such 

 cells frequently die and then serve only in supporting and protecting 

 the attendant softer tissues. 



