CHAP. II. 
EXOGENOUS STEMS. 
89 
mically as composed of four separate parts : — 1. The Epidermis, 
which is continuous with that of the leaves, resembling what 
is found upon their veins, like it composed of cells a little 
lengthened, and rarely furnished with stomates ; it often bears 
hairs. 2. The Epiphloeurn of Link, Phloeum or Peridermis of 
Mohl, consisting of several layers of thin-sided tubular cells, 
rarely coloured green. 3. The Mesopliloeum of Link, or cel- 
lular integument of others, composed of cells usually green, 
and placed in a different direction from those of the epi- 
phlceum; sometimes, as in Quercus Suber, containing cellular 
concretions. 4. The Endophloeum or Liber, of which a part is 
cellular and a part composed of woody tubes. These are mo- 
dified differently in different trees ; and the appearances of 
Cork in many plants, of thin white lamellae or hard plates in 
others, are so produced. Usually each stratum has a separate 
growth, which takes place by the addition' of new matter to its 
interior ; thus the endophloeum, or liber, grows next the al- 
burnum, the mesophloeum next the endophloeum, and the epi- 
phloeum next the mesophloeum ; the epidermis does not grow 
at all. Such growth is often indicated by concentric circles, 
which correspond in each layer with the zones of wood.* 
When the substance called Cork is formed, the epiphloeurn . 
consists of polyedral cells, which multiply with unusual 
rapidity and in great quantities. It does not appear to 
have any communication by lateral passages with the interior 
of the plant; although Dutrochet represents them to exist 
in Ulmus suberosa, where I cannot find them. After a certain 
age, it exfoliates in the Cork Tree, but in such plants as 
Acer campestre, Ulmus, &c., it is simply rent and thrown 
off' piecemeal. In the Birch, the Cherry, and similar trees, it 
forms annually only a few layers of tabular cellular tissue, 
arranged in transverse rows, which separate at a certain age 
into thin silvery lamellae : these have been improperly con- 
founded with the epidermis. The cause of the separation of 
the lamellae of the epiphloeurn of the Birch is found in the 
developement, between the lamellae, of a layer of thin-sided 
* But, according to Decaisne {Comptes rendus,Y. 393.), in Menispermaceae 
the liber is only formed for the first year, and is afterwards covered over by 
new wood ; and consequently is found near the centre round the pith, and 
not at the circumference. 
