1G o- 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 ;1 cover the cut end of each stem with a 
bit of grafting wax.2 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 beside 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 ?3 
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 supplhed. 
In many cases, noticeably in the cabbage, the epidermis is 
1 Hydrangea, squash, melon or cucumber is best ; many other kinds will answer 
very well. 
2 Grafting wax may be bought of nursery men or seedsmen. 
3 Tf the student is in doubt whether the jar filled 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. 
#4 See Kerner and Oliver’s Natural History of Plants, vol. I, pp. 273-362. 
