148 BOTANY PART i 



They ultimately become rounded off, so that intercellular spaces are 

 formed between them and serve to increase the thickness of the living 

 cortex. The term periderm includes both cork and phelloderm. 



The cork of the Cork-oak ' t >//or//.v .Sti/n :/) is formed of broad layers of .soft 

 large cells, alternating with narrow layers of thinner cells, which mark the limit 

 of the annual growth. This may be seen in bottle-corks. The first, spon- 

 taneously developed cork of the Cork-oak is stripped off, whereupon a new phellogen 

 is formed in the deeper-lying tissue. The cork thus produced is removed every six 

 or eight years, and furnishes the cork of economic value. 



In many cases the phellogen takes its origin in the epidermis (Fig. 157 py). 

 This is the case in the Willow, in all Pomaceae, and in a number of other woody 

 plants. The epidermal cells become divided into outer and inner cells, the 

 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. 158, ph). The epidermis in this case covers the developing 

 layer of cork. 



True cork is confined to Ophioylossiuu among the Cryptogams. The fleshy 

 stipules of the Marattiaceae are covered with a brown layer arising by the irregular 

 division of sub-epidermal cells; the walls of this tissue are humified and it may 

 be termed pseudo-periderm ( 134 ). 



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

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

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

 through the ensuing death of the 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 discontinues its functions, and another (Fig. 159) 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 circumference, 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, Cherry, 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 appear- 

 ance 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 



