ASSIMILATION. 115 



paste which envelopes them is excessively thin. If the 

 leaves be examined _ when more perfectly developed, the 

 blue tint is more obscure, because the gelatinous bed is 

 thicker. That the formation of this green gelatinous en- 

 velope is posterior to that of the starch granules which it 

 encloses, is evident from the fact, that when grains of 

 pure starch are formed in organs which afterwards become 

 subjected to the light, they become covered with a bed of 

 green matter, and are thus converted into chlorophyl. 

 Thus potatoes and roots will assume a green appearance if 

 the earth be removed from them, which results from con- 

 version of the starch granules into chlorophyl. 



Gwm, or rather mucilage, which is a solution of gum, is 

 one of the most common vegetable products, and is found 

 plentifully in the cells of aU young and growing parts. It 

 is one of the forms through which nutritive matter passes 

 before it is assimilated. Grum consists of C^H^O", its 

 chemical composition being precisely the same as that of 

 starch. It is in the form of gum that starch passes through 

 the walls of the cells in which its granules were originally 

 generated, when assimilated by the plant. Prom the bark 

 of many trees, gum is- procured in the form of an exuda,- 

 tion. Two well-marked varieties of gum have been distin- 

 guished. Arabine, soluble in cold water, and constituting 

 the principal ingredient of gum-arabic procured from va- 

 rious species of acacia ; and cerasine, insoluble in cold 

 water but soluble in boiling water, constituting the gummy 

 secretion of the cherry and plum.- 



Sugar occurs abundantly in the sap of plants. There 

 are two marked varieties of it. Cane sugar procured from 

 the sugar-cane, sugar maple, beet, carrot, and many other 

 plants ; and grape sugar occurring in numerpUs fruits, as 

 grapes, gooseberries, currants, peaches, and apricots. Sugar, 



