74 



NATURE OF LENTICELS 



majority of plants special devices are required to bring the 

 living cells of the stem into more direct communication with the 



FIG. 51. FIG. 52. 



FIG. 50. Later development of a lenticel : c, cork cambium; i, intercel- 

 lular space. 



FIG. 51. Surface view of lenticels : A, lenticels on branch of horse- 

 chestnut appearing as minute brownish swellings. B, old lenticels on 

 white birch appearing as dark lens-shaped streaks. 



air. This work is effectively accomplished in herbaceous stems 

 by the stomata, but in stems characterized by the formation of 

 cork it is noticed that the cells just below the stomata begin to 



FIG. 52. Cross-section of a stem of castor bean showing the formation 

 of the cambium between two vascular bundles : x, xylem ; ph, phloem ; c, 

 cambium of the bundle. The faint lines, ic, are the first divisions of the 

 parenchyma cells between the bundles that result in the formation of the 

 cambium zone. 



<*' "*' 



divide and form a jratfier loose mass of cells that lift up and 

 finally rupture the tissues about the stomata, thus forming a 

 small, lens-shaped outgrowth on the surface of the young stem, 



