PARTS OF THE DICOTYLEDONOUS STEM. 93 
their life, which varies in different plants, generally become cracked 
in various directions in consequence of the pressure which is 
exerted upon them by the growth of the wood and liber be- 
neath, and thus assume a rugged appearance, as in the Eim 
and Cork-oak. In some trees, as the Beech, the bark, however, 
always retains its smoothness, which circumstance arises, partly 
from the small development of the parenchymatous layers, and 
partly from their great distensibility. Other smooth-barked 
stems, such as those of the Holly and Ivy, owe their peculi- 
arities in this respect to similar causes. When the bark has 
thus become cracked and rugged, it is commonly thrown off in 
large pieces, or in plates or layers of various sizes and appear- 
ance. The epidermis in all cases separates early, and is 
replaced by cork-cells. By this separation and peeling off of 
portions of the bark, its thickness is continually diminished. 
This decaying and falling away of the outer layers of the old 
bark does not in any way injure the tree ; hence, it is evident 
that the old cellular layers of the bark, like the pith and inner 
rings of the wood, have nothing to do with its life and growth after 
a certain period. The new rings of wood, the cambium-layer, 
and the recently formed liber, are the parts of an exogenous stem 
which are alone concerned in its active development and life. 
Having now described the different parts which enter into 
the structure of an Exogenous or Dicotyledonous stem, we will, 
in conclusion, recapitulate them, and place them in a tabular 
form :— 
1. Pith or Medulla, belonging to the parenchymatous system. 
2. Medullary Sheath, consisting of spiral , 
vessels. These belong to the 
fibro-vascular sys- 
tem, and together 
form the wood 
(xylem) properly 
so called. 
3. Wood, composed of interrupted rings, 
one of which is developed annually 
on the outside of the previous 
rings, and consisting ordinarily in 
perennial plants of wood-cells and 
pitted vessels. 
a 
4, Medullary Rays, composed of muriform parenchyma con- 
necting the pith and the parenchymatous layers of the bark. 
5. Cambium-layer, consisting of vitally active or generating cells 
containing protoplasm, &c., from which additions are made 
annually to the wood and liber. 
6. The Bark, composed of two systems— 
1. Inner Bark, Phloém, or Liber, formed essentially of liber- 
cells and sieve-tubes, and thus belonging to the fibro- 
vascular system ; and increasing by the annual addition 
of a new layer on its inner surface. 
