SECTION 11. 



THE CARBOHYDRATES. 



Under the general heading of carbohydrates are included the 

 sugars, starches, gums and celluloses, all of which substances 

 play an important part in the economy of the plant. Many, 

 such as starch, sugars, inulin and glycogen, are all-important 

 as food materials, and are stored up for future use in various 

 tissues. Speaking generally, the carbohydrate reserves are 

 at a maximum in the autumn and sink to a minimum in the 

 early summer, after the expansion and growth of the leaves 

 and young shoots. Other carbohydrates, such as the celluloses, 

 play an important part in the mechanics of the plant, since 

 they are largely concerned in the formation of cell-walls ; some 

 have merely a transitory existence, such as maltose ; and 

 finally, some possibly may be degradation products — e.g., 

 mucilages. Notwithstanding the differences in the physio- 

 logical significance of the various types of carbohydrates in 

 the plant, these substances are all closely related chemically, 

 being composed of the same elements — carbon, hydrogen and 

 oxygen — united together in a similar fashion. 



The term carbohydrate originated through the erroneous 

 conception that these substances were compounds of carbon 

 with water, since the proportion of hydrogen to oxygen in all 

 of them is the same as in water, as may be seen from the 

 formula for grape sugar, which is CgHi-^Oe, but which might 

 be written C,i6H^O. 



CLASSIFICATION OF CARBOHYDRATES. 



On purely physical grounds such as appearance, solubility 



in water, taste, etc., the carbohydrates may be roughly divided 



into sugars and non-sugars; the systematic classification of 



the carbohydrates is, however, based upon their behaviour 



