THE EPIDERMAL TISSUE. 



107 



take place, by which the number of the rows of cells is increased. Of the two newly 

 formed cells of each radial row (i. e. vertical to the surface of the organ) one remains 

 thin-walled, rich in protoplasm, and capable of division ; the other becomes suberised and 

 permanent. Thus arises, usually parallel to the surface of the organ, a layer of cells 

 capable of division, which continues to form new cork-cells, the Cork-cambium or layer 

 of Phellogen. In general this is the innermost layer of the whole cork-tissue, so 

 that the cork increases centrifugally, and new layers of cork are constantly formed 

 out of the phellogen inside those already in existence. But, according to Sanio, it 

 also happens, when cork is, beginning to be produced, that the formation of per- 

 manent cells proceeds centripetally, or an alternation of centripetal and centrifugal cell- 

 formation takes place in the young cork-tissue. But sooner or later the centrifugal 

 formation of cork with phellogen on the inner side always commences, a result of 

 the circumstance that the tissues lying on the outside of completely suberised layers 

 of cells die sooner or later. Usually the 

 formation of cork begins first at single 

 places of the periphery of lignified branches; 

 but the phellogen gradually forms a con- 

 tinuous layer, from which new layers of 

 cork are developed centrifugally. When 

 in this manner a layer of cork arises, 

 increasing progressively from the inside, 

 it is termed Periderm. The develop- 

 ment and configuration of the cork- 

 cells may change periodically during the 

 formation of periderm; alternate layers of 

 narrow thick-walled and broad thin-walled 

 cork-cells are formed; the periderm then 

 appears stratified, like wood with annual 

 rings, as in the periderm of the cork-oak, 

 birch, &c. In some cases the phellogen 

 gives rise not only to cork-cells, by which 

 the periderm increases in thickness, but 

 parenchymatous cells are also formed con- 

 taining chlorophyll ; but it is only daughter- 

 cells of the phellogen lying on the inner side 

 (facing the wood) that undergo this meta- 

 morphosis. In this manner the green cor- 

 tical tissue of some dicotyledonous plants 

 becomes thickened by the layers of tissue 

 proceeding from the phellogen, which 

 Sanio terms the suberous cortical layer or 

 Phelloderm. This occurs, for example, in 



branches two years old or more of Salix purpurea and alba^ the beech, &c. In 

 such cases the phellogen lies between the periderm and the phelloderm, the outer 

 daughter-cells producing cork-cells, the inner phelloderm (Fig. 90). The layers of 

 periderm which first undergo suberisation sometimes bear a very close resemblance to 

 true epidermis, as, for instance, in branches one year old (August) of the Scotch fir, 

 where, while the epidermis still remains, the cork-cambium is formed in the cortical 

 parenchyma, and at first presents the appearance as if a second epidermis were formed 

 with cells greatly thickened on the outside. 



As the epidermis is at first replaced by the periderm, so this again is afterwards 

 replaced by the formation of bark when the increase in thickness continues long and 

 vigorous. In large trees, as oaks and poplars, the boughs are covered with epidermis in 

 their first year, when several years old with periderm, the older branches and the stem 



sdi=S' ' 



Fig. 90. — Formation of cork in a branch oi Ribes nigmtn. 

 one year old ; part of a transverse section; e epidermis, h hair, 

 b bast-cells, pr cortical parenchyma distorted by the increase 

 in thickness of the branch ; K the total product of the phel- 

 logen c ; k the cork-cells arranged radially in rows formed from 

 c in centrifugal order, pd phelloderm (parenchyma containing 

 chlorophyll formed centripetally from c) (X 550). 



