48 TREES AND TREE-PLANTING. 
tude, exhibit this phenomenon at all do so when clothed 
in foliage. The striking and extraordinary differences 
thus evidenced are not accounted for by any peculiarity 
of structure or habitat. The soft and spongy wood of 
the willow and elm, growing in moist ground, seem spe- 
cially suited to absorb and pour forth water before the 
expansion of their leaves or flowers in the spring; but 
examination shows that they contain no unusual amount 
of sap at that time. Of more than sixty species of trees 
and shrubs tested by Professor Clarke, only six— Betula, 
which includes the birch; Acer, the maples; Vitis, the 
vines; Ostrea, the hornbeam; Juglans, walnuts ; and Ca- 
rya, the hickories—showed any tendency to bleed. The 
genus Carya exudes but very little, and possibly Fagus, 
the beech ; and Carpinus, the hop hornbeam, may do the 
same, though no satisfactory test was applied. 
It was found that each species had its own time of 
beginning the flow of sap; that the flow then steadily 
increased in quantity and force until the maximum was 
reached, when it gradually declined; and that the com- 
position of the sap of the several species differed remark- 
ably, both according to the date of the flow and the time 
of its beginning. 
This singular periodicity demonstrates that the absorp- 
tion of water by the rootlets is not caused by osmose or 
any other merely physical force, but is the result of that 
specific life which imparts to every plant its distinctive 
characteristics. 
The sugar maple, which begins its flow in October, 
reaches its maximum about the first of April, and ceases 
about the first of May. The black birch begins the last 
of March, reaches its maximum in a single month, and 
stops entirely about the middle of May. The wild sum- 
mer grape-vine commences the first of May, arrives at its 
maximum by the twenty-fifth of the same month, and 
ceases early in June. Differences in the season of flow- 
ing are of course accompanied by corresponding differ- 
