72 LECTURE V. 



intercellular spaces through which gases can pass. That this 

 is the case is proved by the fact that a very slight pressure 

 suffices to force air through them. The lenticels are not, 

 however, permanently open. If an attempt be made in the 

 winter to force air through them, it will be found that none 

 will pass, and microscopical examination will shew that they 

 are closed by a compact layer of cork-cells. It appears that 

 in the autumn, the cork-cambium beneath the lenticel pro- 

 duces, instead of loose lenticel-tissue, corky layers which 

 interrupt the communication between the air and the interior 

 of the plant. In the spring these corky layers are ruptured 

 by the pressure exercised upon them by the lenticel-tissue 

 which is now being formed in consequence of the renewed 

 activity of the cork-cambium, and thus the communication 

 is restored. 



Unger made the interesting discovery that the lenticels of stems are 

 developed at points which correspond in position to the stomata of the 

 epidermis. Lenticels also occur very generally on roots. 



It is often thought that the stomata are of primary im- 

 portance in relation to the absorption and exhalation of gases 

 by leaves. Boussingault has found, however, that the upper 

 surface of the leaves of various plants with which he ex- 

 perimented (Cherry-Laurel, Poplar, Chestnut, Peach) absorbed 

 carbon dioxide more actively than the lower surface, al- 

 though the upper surface had scarcely any stomata whereas 

 they were very numerous on the lower. The stomata have 

 evidently no effect upon the absorption of this gas : it would 

 be interesting to know if this holds good also with regard to 

 the absorption of oxygen. Barthelemy regards the stomata 

 as affording rather a means of exit than of entrance to gases; 

 he concludes that, under normal conditions, a slight rise of 

 pressure in the plant is sufficient to cause an escape through 

 the stomata of the gases in the intercellular spaces. The 

 part played by the stomata in the interchange of gases is 

 not, however, their primary function ; their chief physiological 

 significance is in relation to the exhalation of watery vapour 

 (transpiration). We will therefore defer a detailed account 

 of their mode of action until that process is under discussion. 



