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, i 



The structure of the fig leaf has already been studied. That of the 

 hydrangea is looser in texture and mere 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. Long pasture-grass has been estimated to give off 

 106 tons of water to the acre in twenty-four hours. 



These large amounts of water are absorbed, carried through 

 the tissues of the plant, and then given off by the leaves 

 simply because the plant-food contained in the soil-water is 



^ When the experiments on tlie hydrangea have been finished, it should be kept 

 moderately watered and left sealed up until it is needed for a later experiment, § 157. 



