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 ; 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, 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 ? 3 



168. Uses of the Epidermis. 4 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. 



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. 



