l68 LECTURE X. 



by means of which the communication of the cortical intercellular spaces with the 

 atmosphere during the period of rest is interrupted. At the beginning of the new 

 period of vegetation, however, new looser lenticel-tissue arises from the phellogen of 

 the lenticel, which splits the closing layer of cork and opens the lenticel during the 

 period of vegetation. The origin of the lenticels is not always connected with the 



Fig. 172.— Transverse section through a lenticel oi Belula alba. <• epidermis ; Ja stoma. Beneath 

 this is tlie loose tissue of the lenticel, and further inwards the phellogen ; at the margin of the lenticel the 

 development of periderm is beginning (after Ue Bary). ■ 



existence of stomata, however. In the formation of internal layers of periderm, 

 lenticels are developed quite independently, but to a certain extent as local growths 

 of the new layer of periderm ; and since internal layers of periderm are necessarily 

 connected with the formation of bark, lenticels thus arise within the bark. This is 

 particularly clear after the fall of the bark scales in Plaiaims, on the fresh exposed 

 surface of the cortex, which is now beset with corky warts. 



What has hitherto been said concerning growth 

 P V J. f^ in thickness, is true especially of the shoot-axes. 



f^ ^ As already mentioned, however, the roots of woody 



plants also undergo a subsequent growth in thick- 

 ness, which only differs in a few points from that 

 of the shoot-axes. Here, as there, the first origin 

 of the cambium is connected with the vascular 

 bundles. These, as we know already, form an 

 axial cylinder in the roots ; in which the phloem- 

 ,A portions are placed at the periphery, alternating 

 "^ with the xylem. The cambium arises therefore as 



r- "" an annular layer with sinuosities, so running that 



here again the phloem comes to lie on the outside, 

 and the xylem on the inside of the cambium. The 

 formation of secondary wood and cortex then 

 follows exactly as in the shoot-axis; except that, 

 in general, the peculiarity prevails in the root that 

 the secondary wood springing from the cambium-ring (seen in transverse section) 

 does not fit on to the xylem-portions of the axial string, but is developed between 

 them, — i.e. on the inside of the primary phloem-portions. In the growth in thick- 

 ness of the root, also, the production of parenchymatous tissue very commonly 



