134 ESSENTIALS OF BOTANY 
Water moderately and weigh on a balance that is sensitive to one 
or two grams. Record the weight, allow the plant to stand in a 
sunny, warm room for twenty-four hours, and reweigh. 
Add just the amount of water which is lost, and continue the 
experiment in the same manner for several days so as to ascertain, if 
possible, the effect upon transpiration of varying amounts of water 
in the atmosphere. 
Calculate the loss per 100 square inches of leaf-surface throughout 
the whole course of the experiment. 
Try the effect of supplying very little water, so that the hydrangea 
will begin to droop, and see whether this changes the relative amount 
of transpiration. Vary the conditions of the experiment 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.? 
EXPERIMENT XVI 
Rise of Sap in Leaves. — Put the freshly cut ends of the petioles 
of several thin leaves of different kinds into small glasses, each con- 
taining red ink to the depth of one-quarter inch or more. Allow 
them to stand for half an hour, and examine 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? 
159. Amount of Transpiration. —In order to prevent 
wilting, the rise of sap during the life of the leaf must 
1 The addition of known amounts of water may be made most conveniently 
by measuring it in a cylindrical graduate. 
2 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. 
