SECT. I 



MORPHOLOGY 



141 



latter of which assume the function of a phellogen. More frequently the phello- 

 gen develops from the layer of cells next adjoining the epidermis, as, for example, 

 in the Elder (Sambucus nigra), where it takes its origin from the outermost layer 

 of collenchyma ( Fig. 156, ph). 



At the same time that the cork is forming from the outer side of the 

 phellogen, a so-called cork cortex or phelloderm is also frequently 

 developed from its inner side. The cells of the 

 phelloderm retain their living protoplasm, and 

 usually contain chloroplasts. They ultimately be- 

 come rounded off, so that intercellular spaces are 

 formed between them. The term periderm in- 

 cludes both cork and phelloderm. All secondary 

 tissues given off by the cambium ring towards the 

 periphery, together with all the secondary tissues 

 formed by the phellogen from both its inner and 

 outer sides, are designated collectively secondary 



CORTEX. 



All tissues external to the phellogen are cut 

 off from food supplies, and consequently die. 

 When the first cork layer has its origin deep fig. 155.— Transverse sec- 

 within the stem, a BARK is formed through the tion of the peripheral 

 ensuing death of the excluded peripheral 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 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 dis- 

 continues its functions, and another (Fig. 157) 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 arcs of the stem circum- 

 ference, the bark will be thrown off 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, Clematis, and Honeysuckle. Bark which is not easily 

 detached becomes cracked by the continued growth in thickness of 

 the stem, and has then the furrowed appearance so characteristic of the 

 majority of old tree-trunks. The usual brown or red colour of bark, 

 just as in similarly coloured heart-wood, is occasioned by the presence 

 of tannins, to the preservative qualities of which is due the great 

 resistance of bark to the action of destructive agencies. The peculiar 



tissues of a one-year- 

 old twig of Pirns com- 

 munis al the beginning 

 of the formation of 

 periderm, ph, Phel- 

 logen. (xSOO.) 



