TEANSPIEATION 



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 y-^-g- cubic inch of water per 

 square inch of surface in twelve hours. V. Hohnel 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 

 it into one arm of a U-tube con- 

 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 



FIG. 65. APPARATUS TO SHOW 

 DEPENDENCE OF WITHERING 

 UPON Loss OF WATER. 



