TRANSPIRATION 61 



of each ■watch glass. If none are visible, carefully cut off the leaf 

 and place it for a few minutes in a box with a piece of ice or 

 put it out of doors in a cold place. Report the results. Examine 

 the upper and lower epidermis with the microscope and explain 

 the results noted. 



Eefeeenoe. Osterhout, 13. 



EXPERIMENT XXXVH 



Through which side of a leaf of Ficus elastica does transpiration occur ? The 

 student may already have found (See. 43) that there are no stomata on 

 the upper surface of the Ficus leaf which he studied, i That fact makes this 

 leaf an excellent one for the study of the relation of stomata to transpiration. 



Take two large, sound Ficus leaves, cut off pretty close to the stem of the 

 plant. Slip over the cut end of the petiole of each leaf a piece of small 

 rubber tubing, wire this on, leaving about half of it free, and then double the 

 free end over and wire tightly, so as to make the covering moisture proof. 

 Warm some vaseline or grafting wax until it is almost liquid, and spread a 

 thin layer of it smoothly over the upper surface of one leaf and the lower 

 surface of the other. Hang both up in a sunny place in the laboratory and 

 watch them for a month or more. 



What difference in the appearance of the two leaves becomes evident ? 

 What does the experiment prove ? 



Reference. Darwin and Acton, 11. 



EXPERIMENT XXXVIII 



Amount of water lost by transpiration. * * Procure a thrifty hydrangea^ and 

 a small plant of Ficus elastica,^ each growing in a small flowerpot, and with 

 the number of square inches 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.* 



1 This is also true oi many other leaves, as those of the oleander, the lilac, and 

 most begonias, and any of them may be used for the experiment. 



'^ The common species of the greenhouse, Hydrangea Hortensia. 



8 Commonly known as India-rubber plant. 



* The quickest and most accurate method of procedure is to defer calculating 

 the leaf area until the conclusion of the experiment, and then to cut off all the 



