THE AEEATION OF PLANTS 



111 



tissues. A direct proof can be afforded by a simple experi- 

 ment. If the lamina of a 

 leaf is immersed in water, 

 air can be driven through 

 it by subjecting the cut end 

 of the petiole to gaseous 

 pressure by means of an 

 air-pump, or even by the 

 effort of the lungs of the 

 observer, and can be seen 

 to emerge from the surface 



of the leaf on which the stomatal apertures are situated. 

 If a petiole is passed into a glass bottle through a tightly 



FIG. 79. SECTION OF A LENTICEL. 

 I, lenticel ; per, cork layer. 



FIG. 80. APPARATUS TO SHOW CONTINUITY OF INTERCELLULAR 

 SPACES IN THE LEAF. (After Detmer.) 



fitting cork, and covered with water, while the lamina 

 remains in the air outside (fig. 80), bubbles of gas can be 



