156 FOUNDATIONS OF BOTANY 
not readily shown in ordinary class-room experiments, but 
some things can readily be demonstrated in the experi- 
ments which follow. 
EXPERIMENT XXIX 
Transpiration. —Take two twigs or leafy shoots of any thin-leafed 
plant ;! cover the cut end of each stem with a bit of grafting wax? 
to prevent evaporation from the cut surface. Put one shoot into a 
fruit jar, screw the top on, and leave in a warm room; 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, put it out 
of doors for a few minutes, and examine the appearance of the inside 
of the jar. What does this show ?? . 
168. Uses of the Epidermis.t— The epidermis, by its 
toughness, tends to prevent mechanical injuries to the 
leaf, and after the filling up of a part of its outer por- 
tion with a corky substance it greatly diminishes the loss of 
water from the general surface. This process of becom- 
ing filled with cork substance, suberin (or a substance 
of similar properties known as cutin) is essential to the 
safety of leaves or of young stems which have to with- 
stand heat and dryness. The corky or cutinized cell- 
wall is waterproof, while ordinary cellulose allows water 
1 Hydrangea, squash, melon, or cucumber is best; many other kinds will 
answer very well. 
2 Grafting wax may be bought of nurserymen or seedsmen. 
3 If 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. 
