212 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[35. 



This bark is stretched and torn by the increasing size of the 

 trunk, and the scales of it may be shed, as is the case in the 



Plane, or they may 

 adhere one upon the 

 other, as in the Pines 

 and Larches, or remain 

 connected by the bast- 

 fibres in long strips, as 

 in Robinia. When, on 

 the other hand, the 

 primary periderm has 

 been formed in the 

 deeper layers of the 

 cortex, the secondary 

 periderru often forms 

 complete concentric 

 rings ; thus hollow cy- 

 linders of the cortex 

 are transformed into 

 bark (ringed bark). 

 The longitudinal rup- 

 ture of this kind of 

 bark is effected by the bastrfibres enclosed in it (e.g. Vine, Clematis, 

 and Thuja). 



There are frequently in the periderm of both stems and roots, 

 organs corresponding to the stomata of the epidermis, serving, 

 like them, to admit air to 

 the living internal tissues ; 

 these are the Lenticels. 

 They are usually circum- 

 scribed circular areas of 

 the periderm where the 

 cork-cells formed in the 

 course of the summer are 

 not arranged closely to- 

 gether, but are separated 

 by intercellular spaces. 

 In winter the lenticels are 

 closed by ordinary peri- 

 derm. They are most easily detected in branches of one year's 

 growth, where they are to be seen in the summer in the form of 



FIG. 156. Formation of Bark in a Larch, as seen in a 

 piece of the outer portion of the stem cut both trans- 

 versely and longitudinally (nat. size) : r the secondary 

 cortex ; 7c plates of cork ; b the scales of bark cut off by 

 the cork. 



FIG. 167. Lenticel in the transverse section of a 

 twig of Elder (x 300): e epidermis; q pheUogen; I 

 cells, and pi the phellogen of the lenticel ; Ic cortical 

 parenchyma containing chlorophyll. 



