XXVi _ INTRODUCTION TO BOTANY. 
branches, consists of cellular tissue, occupying the centre or 
longitudinal axis of the stem. 
(2) the medullary sheath, which surrounds the pith, abounds in 
spiral vessels (153, ce), and is in direct connection, through its 
ramifications, with the leaf-buds, and the veins and nerves of 
leaves. 
(ce) the wood, which lies directly on the medullary sheath, is formed 
of woody tissue (153, 6), through which, in most cases, ducts 
(158, ¢), variously disposed, are interspersed. A new circle of 
wood is annually formed, on the outside of the circle of the 
previous year; the age of a stem therefore may be ascer- 
tained, in a large number of cases, by counting the numbers of 
its rings of wood: in some cases of tropical trees and ever- 
green trees of temperate climates, several rings of wood are 
formed in a year. The older and denser, comparatively sap- 
less wood, is called heartwood or duramen, and is often 
coloured ; the younger, living and incompletely formed, is the 
sapwood or alburnum, and is usually white. 
(d) the medullary rays, which originate in the pith, traverse the 
wood, and terminate 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 heartwood 
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, e); and the diber or inner 
bark, formed of long, tough, woody tissue, called dast-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 
envelope. 
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 
sup, it is returned into the stem, and either used up in the processes of 
further growth, or deposited either in the wood, in the stock, in the peren- 
nial part of the stem or root, or in any other part of the plant where matter 
is stored up for future use. 
§ 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 cellular system 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. 
(0) the endogenous, in which the principal merves usually extend 
