240 STRUCTURAL BOTANY 



way as in a Dicotyledon, and needs no special 

 description. 



The cortical tissue is chiefly remarkable because it 

 contains a ring of resin canals (r in Fig. 97 ). The resin 

 canal is an intercellular space, surrounded by a circle 

 of thin-walled cells rich in protoplasm ; these cells 

 secrete the resin, which is poured into the canal. The 

 canal itself arises by the splitting apart of a group of cells, 

 so as to leave a space between them. The canals extend 

 for long distances through the stem. The secreting cells 

 lining the cavity are called the epithelium of the canal. 

 The epithelium is surrounded by another ring of cells, 

 which have rather thicker walls than those of the 

 cortex generally. The resin is of use to the plant by 

 rendering it distasteful to animals, which might other- 

 wise attack the young twigs ; it probably has other 

 functions also. 



The outline of the young stem, as shown in Fig. 97 

 in transverse section, is rendered irregular by the 

 projecting bases of the leaves. The older branches, 

 however, are cylindrical. This change is due to the 

 formation of periderm, which arises, as in the Wall- 

 flower, from a phellogen or cork cambium, but in 

 the Fir the phellogen is formed by the division of 

 a ring of cortical cells. Opposite the depressions 

 between the leaf -bases the phellogen arises quite 

 near the surface, but opposite the leaf-bases them- 

 <?elves it is deeply seated, and thus the whole layer 

 is nearly circular, as seen in transverse section 

 (see pd in Fig. 97). Cork is formed by the 

 phellogen towards the exterior, and everything out- 



