184 BOAED OF AGRICULTURE. • 



client of maple sap is cane-sugar, that of birch sap is grape- 

 sugar, and that of vine sap is mucilage or gum. 



These three carbo-hydrates, cane-sugar, grape-sugar and 

 gum, are doubtless chiefly formed out of the starch which 

 has descended to the root of the plant as the result of the 

 previous season's growth. It seems probable that these trans- 

 formations occur in the sap after the period of activity begins 

 and in the following order, viz. : Insoluble starch, whether 

 deposited in the cells of jihe root or previously transferred in 

 solution to those of the stem, and there stored during the 

 period of repose, becomes soluble gum, gum becomes uncrys- 

 tallizable grape-sugar, and this under favorable circumstances 

 l)ecomes cane-sugar. Why then do we find cane-sugar in the 

 maple and not in the birch, and why only gum as the chief 

 ingredient in the sap of the vine, and of those trees which do 

 not acquire the power of active absorption until the develop- 

 ment of their buds ? Probably because these transformations 

 require time, and the maple alone is gorged with sap during 

 the six months intervening between the fall of the leaf and 

 the beginning of growth in the spring. This aflbrds ample 

 time for chemical changes, and seems to have some connec- 

 tion with the fact that the maples are the only trees from 

 which crystallizable cane-sugar can be profitably extracted. 



For a similar reason, since we find the birches filled with 

 sap for several weeks before a bud begins to expand, we may 

 reasonably expect the formation of grape-sugar at least in 

 them, and in the north of Europe a sweet syrup is obtained 

 by the evaporation of their sap. 



The spring sap of the vine at the beginning of its motion, 

 about the first of May, contains no sugar of any kind, but 

 three weeks later it often acquires a sweetish taste and then 

 we may find a trace of grape-sugar. At this period the be- 

 ginning of vegetable growth is attended by the rapid exhala- 

 tion of the water of the crude sap, and the assimilation of its 

 gum, in the formation of cellulose, and this is precisely the 

 transformation which ordinarily occurs in plants at the begin- 

 ning of the vegetating season. 



In regard to the circumstances which aflfect the flow of sap 

 from the sugar-maple, the following results have l)een ar- 

 rived at. A careful comparison of the daily weight of sap 



