iQii] Ernest D. Clark 73 



of the carbohydrate is in the process of downward transformation, 

 whereas in the plant carbohydrate is synthesized in large quantity 

 and much of it is stored in complex forms which may readily be 

 identified. Besides acting as energy Stores, some carbohydrates 

 probably supply radicals and atomic groups for the intracellular syn- 

 thesis of proteins and fats. The carbohydrates seldom figure as 

 conspicuous parts of animal cells but in plants the contrary is true ; 

 in fact, the walls of most plant cells are composed of celltilose or 

 related carbohydrate materials. 



Free carbohydrates. Of the carbohydrates which exist imcom- 

 bined in the plant cell, the most abundant and best known are the 

 starches. Starch exists in the storage organs in the form of char- 

 acteristic grains which often fill the cells of such tissue with gran- 

 ules of various sizes. In the cells of the leaf where the starch is 

 being formed, the grains are attached to the chloroplasts, which are 

 the active agents in photosynthesis. Inulin is another storage car- 

 bohydrate somewhat like starch but differing in the fact that inulin 

 remains dissolved in the cell-sap of the storage organs of the Com- 

 positae and is precipitated in sphero-crystals in the cell upon the 

 addition of alcohol. Glycogen which is so important as a carbohy- 

 drate Store in the animal body is not often found in the higher 

 plants but it occurs in bacteria, forms granules in yeast cells, and 

 also appears among the fungi in the form of highly refractive mi- 

 croscopic masses. In certain oral bacteria, grains have been noted 

 which stained blue with iodine, but it has never been proven that 

 these grains were really composed of ordinary starch. Among the 

 carbohydrate reserves in other plants are sucrose, especially con- 

 spicuous in the sugar-cane and sugar-beet; also rarer sugars, like 

 raffinose, rhamnose, and gentianose, all of which are found free in 

 the cell-sap of the plants after which many of them are named. 

 Maltose is present most noticeably in the cells of germinating seeds 

 where the starch has been partially digested by the diastase. 

 Glucose, friictose, and other hexoses occur in the cells of most 

 plants, functioning either in the destructive metabolism of the plant 

 or as conitruction units for the other substances synthesized by the 

 plant. It is an interesting fact that the pentoses rarely occur free 

 in cells but only in the form of nucleoproteins, cell-wall constituents 

 and the like. 



