336 HOW CROPS GROW. 
sucked in through the spile; as the trunk becomes heated 
again, the gaseous and liquid contents of the ducts ex- 
pand, the flow of sap is renewed, and proceeds with in- 
creased rapidity until the internal pressure passes its max- 
imum. 
As the season advances and the soil becomes heated, the 
root-power undoubtedly acts with increased vigor and 
larger quantities of water are forced into the trunk, but 
at a certain time the escape of sap from a wound suddenly 
ceases. At this period a new phenomenon supervenes. 
The buds which were formed the previous summer begin 
to expand as the vessels are distended with sap, and final- 
ly, when the temperature attains the proper range, they 
unfold into leaves. At this point we have a proper mo- 
tion of sap in the tree, whereas before there was little mo- 
tion at all in the sound trunk, and in the tapped stem the 
motion was towards the orifice and thence out of the tree. 
The cessation of flow from a cut results from two cir- 
cumstances: first, the vigorous cambial growth, whereby 
incisions in the bark and wood rapidly heal up; and sec- 
ond, the extensive evaporation that goes on from foliage. 
That evaporation of water from the leaves often pro- 
ceeds more rapidly than it can be supplied by the roots 
is shown by the facts that the delicate leaves of many 
plants wilt when the soil about their roots becomes dry, 
that water is often rapidly sucked into wounds on the 
stems of trees which are covered with foliage, and that 
the proportion of water in the wood of the trees of tem- 
perate latitudes is least in the months of May, June, and 
July. : 
Evergreens do not bleed in the spring-time. The oak 
loses little or no sap, and among other trees great diversity 
is noticed as to the amount of water that escapes at a 
wound on the stem. In case of evergreens we have a 
stem destitute of all proper vascular tissue, and admitting 
a flow of liquid only through the perforations of the wood- 
