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MINUTE STRUCTURE 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 twograms. Record the weights, 
allow the plants to stand in asunny, 
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 _F16. 99.—An “India-Rubber Plant” 
(Fig), in pot tied up in sheet rubber, and 
<2 dpsed ae maln, if possible, the provided with thistle-tube for transpira- 
effect upon transpiration ofvarying tion experiment. 
amounts of water in the atmosphere. 
Calculate the average loss per 100 square inches of leaf surface for 
each plant. 
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 common cineraria, 
Senecio cruentus, 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. 
* The addition of known amounts of water may be made most conveniently by 
measuring it in a cylindrical graduate. 
