The Various Substances Made by Plants 107 



Class II. The Food and Skeletal Substances, or Carbohydrates 

 Grape Sugar. This substance is formed abundantly in green 

 leaves as the photosynthate, and is common in nearly all parts 

 of all plants. It is, however, much less known than its import- 

 ance would imply, because it has no prominent economic uses, 

 and exists in the plant only in solution in the sap of the cells, 

 which therefore display through its presence no more striking 

 appearance than that represented in the accompanying example 

 (figure 33). However, it sometimes ac- 

 cumulates considerably in fruits, which 

 it helps to make nutritious and attract- 

 ive to animals in connection with dis- 

 semination, a subject to be later dis- 

 cussed in a special chapter devoted to 

 that subject; and in grapes, especially, 

 it is so plenty that it crystallizes out 

 when they are dried, forming the soft 

 sugar abundant on some kinds of raisins. 

 Its many and easy transformations into 

 other substances will be traced in the 

 following pages. It has, however, a 

 second origin and significance in the 

 plant, for it is that into which many other 

 substances are converted in digestion, as we shall presenth^ learn, 

 and is the commonest form in which substances are translocated 

 through the plant. It is white in mass, looks amorphous and not 

 crystalline to the eye, is sweet to the taste, though much less 

 sweet than cane sugar, and is the easiest of all sugars for Yeast 

 to ferment. It is interesting to know that it has been made 

 artificially in the chemical laboratory. Chemically its correct 

 name is dextrose, though often also called glucose, and its formula 

 is C6H12O6. 



Fruit Sugar. This substance is extremely like grape sugar, with 

 which until lately it was more or less confounded, and with which 



Fig. 33. — Appearance in opti- 

 cal section, highly magnified, 

 of a cell in which sugar is 

 stored in the sap. 



Inside the wall is a lining of liv- 

 ing protoplasm which encloses 

 the large sap cavity wherein 

 is water containing the dis- 

 solved sugar. 



