PLANT PHYSIOLOGY 131 
Method.—Dissolve some cobalt chloride in water till 
the solution is a fairly deep pink, then soak the filter 
papers in it, and dry them thoroughly. When quite 
dry they turn a bright blue colour, but the least drop 
of moisture will turn them pink again. Place a leaf 
between two pieces of this cobalt paper and wrap them 
in a duster so that no moisture from the air may reach 
them. ‘Two other pieces, without a leaf should be 
placed in a duster for comparison. Leave for a few 
minutes (three or four) and then examine. 
Result.—The paper next the underside of the leaf has 
turned pink where the leaf touched it, that next the 
upper side is either unchanged, or only slightly pink. 
Conclusion.—The under side of the leaf gives out most 
moisture. 
99. Experiment 9.—To measure the rate of trans- 
piration :— 
Apparatus.—Glass bottle, cork to fit, leaves. 
Method.—Bore a hole in the cork, put the stalks of the 
leaves through it, fill the bottle with water, push in the 
cork so that the stalks are in water, and stop up the 
hole with wax. Since the hole is waxed up no evapora- 
tion can take place, and any loss of weight must be 
almost entirely due to transpiration. Weigh the whole 
apparatus, and weigh again atthe end of anhour, and at 
the end of twenty-four hours. Hence calculate the per- 
centage loss per hour and per day. Fit up two other 
bottles in the same way, and in one place a leaf whose 
lower surface has been covered with vaseline, in the 
other a leaf in which both surfaces have been covered. 
Compare the loss per a each case with that of the 
