THE BLOOD OF TREES. 49 
ences in the temperature of the soil and atmosphere, as 
also in the chemical condition of the sap. The principal 
ingredient of maple sap is cane sugar; of birch sap, grape 
sugar; and of vine sap, mucilage or gum. 
But why do we find cane sugar in the maple, and not 
in the birch? and why only gum in the vine? Possi- 
bly because these several transformations of the starch 
(which descended to the root of the plant and was depos- 
ited in its cells, or in those of the stem, as the result of 
the previous season’s growth) require different periods 
of time. The maple is the only one gorged with sap 
during the six months which intervene between the fall 
of the leaf and the beginning of spring growth. This 
affords ample time for the necessary chemical changes, 
and may account for the fact that the maple is the only 
indigénous tree from which crystallizable cane sugar can 
be profitably extracted. Birches arenextin order. Being 
filled with sap for several weeks before a bud begins to 
expand, we may reasonably expect to find in them the 
formation at least of grape sugar; and in the north of 
Europe a sweet syrup is obtained from their sap by evap- 
oration. At last the vine. The beginning of the motion 
of its sap is deferred until about the first of May, at 
which time it seems to contain no sugar of any kind. 
Three weeks later it acquires a sweetish taste, and we 
may then find a trace of grape sugar. At this period 
the beginning of vegetable growth is attended by the 
rapid exhalation 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 oc- 
curs in plants at the beginning of the vegetating season. 
A careful comparison of the daily weight of the sap from 
several sugar-maple-trees with the meteorological obser- 
vations of the same period, conclusively proves that 
while the general flow corresponds with the season—ris- 
ing to a maximum and declinimg—the daily and hour- 
ly flow varies with the weather. Steadily and severely 
3 
