SECT. I 



MORPHOLOGY 



159 



collenchyma (Fig. 167, ph). The epidermis in this case covers the developing layer 

 of cork. 



True cork is wanting in Cryptogams, even in the Pteridophytes. When pro- 

 tection is required its place may be taken by the impregnation of the cell-walls 

 with a very resistant brown substance, or by the addition of suberised lamella to 

 the walls C^% 



Bark. — All tissues external to the pliellogen are cut off from food 

 supplies and consequently die. When the first cork layer has its 

 origin deep within the stem, a BARK of considerable thickness is formed 

 through the ensuing death of the j^eripheral tissues. If the-cork layer 

 formed by the phellogen be thin, the stem has a smooth surface, as 

 in the Beech ; if it produces thicker cork layers, the surface of the 





Fk;. 167. — Transverse section of a lenticel of Samhucus nigra, e, Epidermis ; ph, phellogen ; 

 I, complementary cells ; pi, phellogen of the lenticel ; pd, phelloderm. (x 90.) 



stem appears rough and full of fissures, as is the case in the Cork-oak. 

 The primary phellogen generally ceases its activity after a short time, 

 and another deeper-lying phellogen is formed. After a time this 

 new phellogen discontinues its functions, and another (Fig. 168) is 

 developed, as in the case of Quercus sessiliflora, until ultimately the 

 phellogen comes to be formed in secondary bast parenchyma instead 

 of in the primary tissue. That portion of the bast cut off by the 

 periderm loses its nutritive contents and only retains waste products. 

 If the layers of the secondary periderm constitute only areas of the 

 stem circumference, the bark will be thrown oi^' in scales, as in the 

 SCALY BARK of the Pine and Plane tree ; if, on the contrary, the 

 periderm layers form complete concentric rings, then hollow cylinders 

 of the cortical tissues are transformed into the so-called ringed bark, 

 such as is found in the Grape-vine, Cherry, Clematis, and Honeysuckle. 

 Bark which is not easily detached becomes cracked by the continued 



