116 ELEMENTS OF BOTANY. 
Try the effect of supplying very little water to each, so that the 
hydrangea will begin to droop, and see whether this changes the relative 
amount of transpiration for the two plants. Vary the conditions of the ex- 
periment for a day or two as regards temperature, and again for a day or 
two as regards light, and note the effect upon the amount of transpiration. ! 
The structure of the fig leaf has already been studied. ' That of the 
hydrangea is looser in texture and more like the leaf of the lily or the 
beet, Fig. 96. 
What light does the structure throw on the results of the preceding 
experiment ? 
~145. Experiment 27. Rise of Sap in Leaves. —Put the freshly 
cut ends of the petioles of several thin leaves of different kinds into small 
glasses, each containing red ink to the depth of one-quarter inch or more. 
Allow them to stand for half an hour, and examine them by holding up 
to the light and looking through them to see into what parts the red ink 
has risen. Allow some of the leaves to remain as much as twelve hours, 
and examine them again. The red-stained portions of the leaf mark the 
lines along which, under natural conditions, sap rises into it. Cut across 
(near the petiole or midrib ends) all the principal veins of some kind of 
large thin leaf. Then cut off the petiole and at once stand the cut end, 
to which the blade is attached, in red ink. Repeat with another leaf and 
stand in water. What do the results teach ? 
In order to prevent wilting, the rise of sap during the life 
of the leaf must have kept pace with the evaporation from 
its surface. A little calculation will show that the amount 
of water thus daily carried off through the foliage of a large 
tree or the grass-blades on a meadow is enormous. A medium- 
sized elm has been found to have about 7,000,000 leaves. 
presenting a surface of about five acres, and transpiring about 
seven and three-quarter tons in twelve hours of clear, dry 
weather. Tong pasture-grass has been estimated to give off 
106 tons of water to the acre in twenty-four hours. 
_ hese large amounts of water are absorbed, carried through 
the tissués of the plant, and then given off by the leaves 
simply because the plant-food contained in the soil-water is 
* 1 When the experiments on the hydrangea have been finished, it should be kept 
moderately watered and left sealed up until it is needed for a later experiment, § 157. 
