TRANSPIRATION 91 
two dry sheets of glass of somewhat larger area. In a very 
short time, often in less than a minute, the paper in con- 
tact with the lower side of the leaf becomes pink, while the 
other piece remains blue for a considerable time. 
The amount of water given off by transpiration varies 
in different plants. In the sunflower (Helianthus) the 
amount has been stated to be 74; cubic inch of water per 
square inch of surface in twelve hours. V. Héhnel has 
computed that a birch-tree with about 200,000 leaves may 
transpire 60 to 80 gallons of 
water during a very hot day. 
Doubtless, however, individual 
plants show a considerable variety 
in the amount. This copious 
evaporation readily explains why 
the bleeding of plants from wounds 
can seldom be observed when the 
leaves are expanded and active. 
When transpiration is exces- 
sive the leaves and branches lose 
their turgescence, become flaccid, 
and droop. A branch which has 
reached this condition may be / 
revived by forcing water into it, 
which can be done by fastening Fic. 65.—APPARATUS TO SHOW 
it into one arm of a U-tube con- DEPENDENCE oF WITHERING 
taining water (fig. 65), and pour- 
ing mercury into the other. The restoration of the water 
restores the turgescence of the tissues, and the branch 
regains an erect position. : 
The exhalation of the water accumulated by root- 
pressure in the closed system of the vessels leads to a 
diminution of the pressure of the air which they contain 
in addition to the water. Indeed it is by such a suction 
that the air is originally enabled to enter the vessels, being 
drawn into them from the intercellular spaces. Conse- 
quently, while transpiration is active, there is a negative 
L rer | 
