XX INTRODUCTION TO BOTANY. 
Anatomists distinguish, &c. 
(d) the medullary-rays, which originate in the pith, traverse the wood, and ter- 
minate in the bark, and are formed of cellular-tissue : they occur as vertical 
plates, radiating from a centre, and keep up a communication between 
- the living portion of the interior of the stem, and its outer surface. As — 
the heart-wood is formed, the inner portions of the medullary-rays die. 
In wood they are what carpenters call the silver-grain. : 
(e) the bark, which lies outside the wood, and forms the outer layer of the — 
stem, It is coated by the epidermis (157), and like the wood, consists 
of concentric layers ; namely, the corky-layer, or dry, outer bark, formed 
of hard, compressed cells; the cellular or green or middle-bark, formed 
of loose, thin-walled, pulpy cells, containing chlorophyll (156, f) ; and 
the liber or inner bark, formed of long, tough, woody tissue, called bast-— 
cells, The liber, like the wood, is annually deposited ; the green layer — 
is a product of the first year only, being soon choked by the corky en- — 
velope. 
168. The mineral food of plants, absorbed by the roots, passes upwards through 
the younger wood of the stem, mixing with previously organized matter, but not — 
being essentially altered ; in this state it is called sap or crude-sap. The crude sap, 
as it ascends through the stem, is attracted into the leaves, where it is exposed to — 
the direct action of sunlight, under which influence alone can assimilation take place. — 
As assimilated or elaborated-sap, it is returned into the stem, and either used up in 
nial part of the stem or root, or in any other part of the plant where matter is 
— s § 5. The Leaf. 
169. Anatomically the leaf consists of a central fibro-vascular system or woody — 
skeleton, derived from the woody system and medullary sheath of the stem ; a cel- 
wae es surrounding the fibro-vascular, and interwoven with it, and derived — 
from the middle bark ; and an outer-skin or epidermis, pierced by stomates. 
. 170. The fibro-vascular system is arranged on two principal types : 
(a) the exogenous, in which the nerves and veins branch irregularly and 
usually anastomose into a sort of network. 
(b) the endogenous, in which the principal nerves usually extend unbranched — 
from the base to the apex, and are connected by cross-bars or unbranched — 
veins. . “ 
The first of these types is generally characteristic of Exogens, the latter of Hndo- 
gens ; but there are various intermediate conditions, and some Endogens have been — 
called Dictyogens, because they have netted-veined leaves ; several Exogens also — 
have straight-veined leaves. tee 
171. Leaves usually extend horizontally, and have usually an wpper and an under 
surface, differing in anatomical structure. In the cellular stratum of the upper sur- 
face the cells are closely set and placed vertically, with their smallest ends next the 
surface ; in the lower stratum the cells are more or less horizontal, more loosely 
\ 172. Leaves are functionally the most active of the vegetable organs. In them 
, ; ‘es 
ag 
lowing results; 
which he may decompose am: 
in nature except in the : 
