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 (z. 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 
Fig. 90.— Formation of cork in a branch oi Ribes iiigriiut 
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- 
losjen c; k the cork-cells arranged radially in rows formed from 
c in centrifug-al order, pd phelloderm (parenchyma containing 
chlorophyll formed centripetally from c) (X 5So). 
