FUNCTIONS OF LEAVES 161 



the jar with a piece of sheet lead, slit to admit the stem of the plant, 

 invert the jar and seal the lead to the glass with a hot mixture of 

 beeswax and rosin. Seal up the slit and the opening about the 

 stem with grafting wax. A thistle-tube, such as is used by chem- 

 ists, is also to be inserted, as shown in Fig. 122. 1 The mouth of this 

 may be kept corked when the tube is not in use for watering. 



Water each plant moderately and weigh the plants separately on 

 a balance that is sensitive to one-fifth gram. Record the weights, 

 allow the plants to stand in a sunny, warm room for twenty-four^ 

 hours and reweigh. 



Add to each plant just the amount of water which is lost, 2 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 average loss per 100 square inches of leaf-surface for 

 each plant throughout the whole course of the experiment. Divide 

 the greater loss by the lesser to find their ratio. Find the ratio of 

 each plant's greatest loss per day to its least loss per day, and by 

 comparing these ratios decide which transpires more regularly. 



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 con- 

 ditions 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. 



The structure of the fig (India-rubber plant) 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. 116). 



What light does the structure throw on the results of the pre- 

 ceding experiment ? 



1 It will be much more convenient to tie the hydrangea if one has been 

 chosen that has but a single main stem. Instead of the hydrangea, the com- 

 mon cineraria, Senecio emeritus, does very well. 



2 The addition of known amounts of water may be made most conveniently 

 by measuring it in a cylindrical graduate. 



