FUNCTIONS OF LEAVES 131 



EXPERIMENT XIV 



Transpiration. — Take two twigs or leafy shoots of any thin-leaved 

 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 and leave in a warm room, screw the top on, put the other 

 beside it, and allow both to remain some hours. Examine the rela- 

 tive 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 open- 

 ing it) into a refrigerator, or, if the weather is cold, out of doors for 

 a few minutes, and examine the appearance of the inside of the jar. 

 What does this show ? ^ 



156. Uses of the Epidermis.* — The epidermis, by its 

 toughness, tends to prevent mechanical injuries to the 

 leaf, while by the transformation of a portion of its outer 

 layers 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 reservoirs 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 covered with a waxy coating which doubtless increases 

 the power of the leaf to retain needed moisture, and which 

 certainly prevents rain or dew from covering the leaf- 

 surfaces, especially the lower surfaces, so as to prevent 

 the operation of the stomata. Many common plants, like 



^ Hydrangea, squash, melon, or cucumber is best; many other kinds will 

 answer very well. 



2 Grafting wax may be bought of nurserymen or seedsmen. 



8 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. 



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



