134 ESSENTIALS OF BOTANY 



"Water moderately and weigh on a balance that is sensitive to a 

 tenth of a gram. 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. 



