THE MOLECULAR FORCES IN PLANTS. 



167 



distributed in the ground tissue. The ground tissue is further 

 traversed by numerous more or less wide air canals. Into these 

 last project stellate internal hairs, each of which springs from a 

 single cell of the ground tissue limiting the canal. 1 



1 Literature : de Bary, Comparative Anatomy of the Phanerogams and Ferns, 

 and Strasburger, Practical Botany (Hillhouse). 



66. Lenticels. 



To acquaint ourselves with the very widely distributed struc- 

 tures known as Lenticels, we investigate twigs of Sambucus nigra. 

 In transverse sections which have been prepared from young 

 branches, we make out that, immediately below the epidermis, is 

 present hypodermal collenchyma, interrupted only at places by 

 the green cortical parenchyma, which, at such' points,: itself 



Fia. 68. Transverse section through a lenticel of Sambucus nigra. e, epidermis ; pK, 

 phelloderm or cork cambium ; I, packing cells ; pi, cambium of the lenticel ; pel, phelloderm. 

 Magn. 90. (After Strasburger.) 



reaches up to the epidermis. The inner cortical tissue consists 

 entirely of green cells, and surrounds the circle of vascular 

 bundles. Examination of older branches of Sambucus shows that 

 important changes have taken place. Close under the epidermis 

 a cork tissue has developed, the yellowish cells of which shut in 

 the living tissue of the cortex. But this enclosure is not com- 

 plete, since the twigs are provided with numerous lenticels. 

 These may be detected even with the naked eye as brownish spots, 



