38 PLANT PHYSIOLOGY 



higher plants, the cells of which are covered with cellulose w^alls 

 and nearly always contain reserves of starch and similar sub- 

 stances, carbohydrates usually form over one-half of the total 

 plant substance. They represent a very large and extremely 

 important group of organic products. The name of this group 

 of substances is based on their being composed of three elements, 

 carbon, hydrogen and oxygen, the two last elements being in the 

 same proportion as in water; i.e., for each atom of oxygen, there 

 are 2 atoms of hydrogen. The empirical composition of carbo- 

 hydrates may be expressed by the formula CnH.2nOn- In the 

 primary sugars of the type of glucose, with the formula C6H12O6, 

 the number of atoms of oxygen is equal to that of carbon. In 

 more comphcated sugars, e.g., in sucrose, C12H22O11, it is 1 atom 

 less; for these sugars are formed from the primary sugarp by the 

 removal of 1 molecule of water. With regard to their chemical 

 properties, carbohydrates contain either aldehyde, — CHO, or 



ketone, C=0, groups. They may be separated into two large 



divisions : primary carbohydrates or monosaccharides (monoses) ; 

 and complex carbohydrates or polysaccharides (polyoses), 

 representing a combination of several molecules of monosac- 

 charides and formed by the removal of water. Depending upon 

 the number of molecules of monosaccharides connected, there 

 may be distinguished disaccharides, trisaccharides, etc. When 

 the number of monose molecules forming the polyoses is not 

 large, the latter retain some of the properties of the primary 

 sugars, viz., the capacity of crystallization and solubility in 

 water; that is why they are often united into one group of sugars 

 with the monoses. With an increase of the number of molecules, 

 up to 6 or more, the polysaccharides obtain properties charac- 

 teristic of colloids and do not give true solutions in water. 

 Starch is one of the most important representatives of this group 

 of colloidal polysaccharides. 



According to the number of carbon atoms participating in the 

 composition of the monoses, there may be distinguished trioses 

 with 3 carbon atoms and the empirical formula CaHeOa; tetroses, 

 C4H8O4; pentoses, C5H10O5; hexoses, C6H12O6; etc. Plants 

 most frequently contain pentoses and hexoses. The former 

 group contains xylose, arabinose, rhamnose, and others, while 



