112 ELEMENTS OF BOTANY. 



chestnut, Fig. 75, the black walnut, Fig. 58, and other trees, 

 are merely the spots at which the leaf-traces passed from 

 stem to petiole. 



Experimental Study of Functions of Leaves. — The most 

 interesting and profitable way in which to find out what work 

 leaves do for the plant is by experimenting upon them. Much 

 that relates to the uses of leaves is not readily shown in ordi- 

 nary class-room experiments, but some things can readily be 

 demonstrated in the experiments which follow. 



140. Experiment 25. Transpiration. — Take two twigs or leafy 

 shoots of any thin-leafed plant ; i coyer the cut end of each stem with a 

 hit of grafting wax.^ To prevent evaporation from the cut surface, put 

 one shoot into a fruit jar, and leave in a warm room ; screw the top on, 

 put the other heside it, and allow both to remain some hours. Examine the 

 relative appearance of the two, as regards wilting, at the end of the time. 



Which shoot has lost most ? Why ? Has the one in the fruit jar lost 

 any water ? To answer this question, put the jar (without opening it) 

 into a refrigerator ; or, if the weather is cold, out of doors for a few min- 

 utes, and examine the appearance of the inside of the jar. What does 

 this show ? ' 



141. Uses of the Epidermis.^ — The epidermis, by its tough- 

 ness, tends to prevent mechanical injuries to the leaf, while 

 by the transformation of a portion of its outer portion into a 

 corky substance it greatly diminishes the loss of water from 

 the general surface. In most cases, as in the india-rubber 

 tree, the epidermal cells (and often two or three layers of cells 

 beneath these) are filled with water, and thus serve as reser- 

 voirs from which the outer parts of the leaf and the stem 

 are at times supplied. 



In many cases, noticeably in the cabbage, the epidermis is 



1 Hydrangea, gguasli, melon or cucnmber is best ; many other kinds wiU answer 

 very well. 



2 Grafting wax may be bought of nursery men or seedsmen. 



3 If the student is in doubt whether the jar illled with ordinary air might not 

 behave in the same way, the question may be readily answered by putting a sealed 

 jar of air into the refrigerator. 



* See Kerner and Oliver's Natural History of Plants, vol. I, pp. 273-362. 



