100 A TEXTBOOK OF BOTANY [Ch. Ill, 13 



2. THE RESERVE FOODS. While much of the photo- 

 synthetic sugar is used directly as food by the various living 

 cells throughout the plant body, a large quantity is trans- 

 formed into reserve materials, which accumulate in special 

 parts, to be used later in growth, especially that of the next 

 season. The places of such accumulation are buds, bulbs, 

 tubers, and seeds ; and it is to the presence of these accu- 

 mulated foods that the swollen form of those parts is due. 

 These reserve foods are of three general classes, — carbohy- 

 drates, fatty oils, and proteins. 



The Carbohydrates are minor transformations of grape 

 sugar into substances which retain the food value of the 

 sugar, though with different phj^sical properties. They in- 

 clude the sugars, starches, and hemi-celluloses. 



The Sugars are of several kinds. The photosynthetic 

 sugar itself is a mixture of two kinds, grape sugar or glucose 

 (also called dextrose) and fruit sugar or fructose, these 

 two being the simplest and most stable of the sugars. They 

 have an identical formula, CeH^Oe, and differ only in the 

 arrangement of the atoms wdthin the molecules. Both are 

 present, the former more abundantlj^, dissolved in the sap 

 of practically all plants. The glucose, with some fructose, 

 accumulates in stems, as in the Sugar Cane, where it con- 

 stitutes most of the molasses, and in Corn, whence it is 

 taken for use as the clear syrup called " glucose." Both occur 

 also in fruits, where, however, the fruit sugar is usually the 

 more abundant ; and they form also the sugar of nectar, which 

 is the basis of honey, chief food of many insects. Far better 

 known, however, is Cane sugar, or sucrose (saccharose), 

 which accumulates in Sugar Cane, Beets, and the Sugar Maple. 

 Its formula is C12H22OU, implying a close relation to glucose 

 and fructose (2 CeHijOe -H2O = C12H22O11), to which it is read- 

 ily converted back, into a molecule of each, in various ways. 

 And several other sugars, differing httle from these, occur also 

 in plants, though none are especially prominent. Grape and 

 fruit sugars can be made artificially in the chemical laboratory. 



