MINUTE STRUCTUKE OF LEAVES. 



115 



of leaf-surface in the two plants not too widely different. Calculate the 

 area of the leaf-surface for each plant, by dividing the surface of a piece 

 of tracing cloth into a series of squares one-half inch on a side, holding 

 an average leaf of each plant against this and counting the number of 

 squares and parts of squares covered by the leaf. This area, multiplied 

 by the number of leaves for each plant, will give approximately the total 

 evaporating surface for each. 



Water each plant with about all the water the earth will retain, then 

 tie each pot up in a sheet of gum- 

 rubber, such as is used by dentists, 

 bring the rubber up about the stem 

 and tie rather tightly, as shown in 

 Fig. 99.1 A thistle-tube, such as 

 is used by chemists, is also to be 

 tied in with the stem of the plant 

 or (much better) at one side. The 

 mouth of this should be kept corked 

 when the tube is not in use for 

 watering. 



Weigh each plant separately on 

 a balance that is sensitive to one 

 or two grams. Record the weights, 

 allow the plants to stand in a sunny, 

 warm room for 24 hours and re- 

 weigh. 



Add to each plant 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 average loss per 100 square inches of leaf surface for 

 each plant. 



* It will be mucli more convenient to tie the hydrangea if one has been chosen 

 that has but a single main stem. Instead of the hydrangea the common cineraria, 

 Senecio cruentws, does very well. As its stem is too soft to be tied very tightly, it 

 may be put in a glass battery jar covered at top with sheet lead slit to admit the 

 stem and then brought together and sealed round the edges and next to the stem 

 with grafting wax. 



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

 measuring it in a cylindrical graduate. 



Fis. 99. — An " India -Kubber Plant" 

 (Fig), in pot tied up in sheet rubber, and 

 provided with thistle-tube for transpira^ 

 tion experiment. 



