THE STEM 53 
30. Cork.—The tissue outside the cambium is known 
as Bark (fig. 35). It consists of the remains of the 
original cortex, present in the seedling, the primary and 
secondary phloem, and a new tissue called Cork. 
The original seedling possessed a strong external 
single layer of cells called the Eprpermis. The outside 
wall of these cells was very thick and consisted of a 
transformed kind of cellulose called Curry. In older 
stems, however, this epidermis has disappeared and its 
place taken by a new tissue called Cork. The cells 
immediately underneath the epidermis become rejuven- 
ated as some in the interfascicular tissue did, and this 
ring of meristematic tissue, like the cambium, gives rise 
to two kinds of tissue, fresh cortical cells on the in- 
side called, COLLENCHYMA, and cork cells on the outside. 
This meristematic tissue is called CorK CamBium or 
PHELLOGEN. The new cortical tissue is called Parnio- 
pERM. The three layers together form the PERipERM 
(fig. 36). 
The epidermis is stretched and cracked by the cork 
growing beneath it, and finally falls away in flakes. 
Also the green stem, green because of the chlorophyll 
present in the cortical tissue, becomes brown because 
of the formation of cork. The cork itself is a brown 
spongy tissue, the walls of the cells being made of 
another transformed kind of cellulose called SuBERIN. 
The cells contain no protoplasmic contents, and so render 
the tissue light and spongy like the pith. Also suberin 
is very impervious to liquids, and it is for this reason 
that the periderm of the cork oak is used as stoppers 
for bottles. 
