82 THE NATURE AND WORK OF PLANTS 



the petiole inserted in a bottle of red ink or aniline 

 in water. Examine a day later. The color will 

 have marked out the conducting system of the leaf. 

 This will be found to distribute the liquid to the 

 remotest and smallest parts of the lamina. 



101. Transpiration. — Clustered around the con- 

 ducting tubes are the soft, thin-walled mesophyl 

 cells, which draw water from the veins. The liquid 

 is constantly evaporated from the walls of these cells 

 into the air between them, and this connects with 

 the outer air by means of the thousands of minute 

 openings in the lower side of the leaf. The sun 

 shines on the upper side of the leaf, heating it and 

 the delicate cells underneath, and they constantly 

 evaporate water into the air spaces, and this air laden 

 with water pours out of the stomata and is replaced 

 by drier air from the outside. 



102. The vapor transpired. — That watery vapor 

 is thrown off by the leaf may be shown if the petiole 

 of a leaf is passed through a small hole in a large 

 piece of cardboard and immersed in a tumbler of 

 water. Put clay or wax around the petiole to pre- 

 vent any water vapor from coming up in this way 

 through the hole in the cardboard. Now invert a 



