OUTLINES OF BOTANY. xlv 
very apparent the first season, especially towards its close. The fibro- 
vascular bundles in Endogens usually anastomose but little, passing con- 
tinuously into the branches and leaves. In Exogens the circle of fibro- 
vascular bundles forms a more continuous cylinder of network emitting 
lateral offsets into the branches and leaves. 
198. The Exogenous stem, after the first year of its growth, consists of 
1, the pith, a cylinder of cellular tissue, occupying the centre or longi- 
tudinal axis of the stem. It is active only in young stems or branches, 
becomes dried up and compressed as the wood hardens, and often finally 
disappears, or is scarcely distinguishable in old trees. 
2, the medullary sheath, which surrounds and encases the pith. It 
abounds in spiral vessels (188, 3), and is in direct connection, when young, 
with the leaf-buds and branches, with the petioles and veins of leaves, and 
other ramifications of the system. Like the pith, it gradually disappears 
in old wood. 
3, the wood, which lies immediately outside the medullary sheath. It 
is formed of woody tissue (188, 2), through which, in most cases, vessels 
(188, 3) variously disposed are interspersed. It is arranged in annual con- 
centric circles (211), which usually remain active during several years, but 
in older stems the central and older layers become hard, dense, compara- 
tively inactive, and usually deeper coloured, forming what is called heart- 
wood or duramen, the outer, younger, and usually paler-coloured living 
layers constituting the sapwood or alburnum. 
4, the medullary rays, which form vertical plates, originating in the 
pith, and, radiating from thence, traverse the wood and terminate in the 
bark. They are formed of cellular tissue, keeping up a communication 
between the living portion of the centre of the stem and its outer surface. 
As the heart-wood is formed, the inner portion of the medullary rays ceases 
to be active, but they usually may still be seen in old wood, forming what 
carpenters call the silver grain. 
5, the bark, which lies outside the wood, within the epidermis. It is, 
hke the wood, arranged in annual concentric circles (211), of which the 
outer older ones become dry and hard, forming the corky layer or outer 
bark, which, as it is distended by the thickening of the stem, either cracks 
or is cast off with the epidermis, which is no longer distinguishable. 
Within the corky layer is the cellular, or green, or middle bark, formed of 
loose thin-walled pulpy cells containing chlorophyll (192); and which is 
usually the layer of the preceding season. The innermost and youngest 
circle, next the young wood, is the liber or inner bark, formed of long 
tough woody tissue called bast-cells. 
199. The Endogenous stem, as it grows old, is not marked by the con- 
centric circles of Exogens. The wood consists of a matrix of cellular tissue 
irregularly traversed by vertical cords or bundles of woody and vascular 
tissue, which are in connection with the leaves. These vascular bundles 
change in structure and direction as they pass down the stem, losing theiz 
vessels, they retain only-their bast- or long wood-cells, usually curving out- 
wards towards the rind. The old wood becomes more compact and harder 
towards the circumference than in the centre. The epidermis or rind 
either hardens so as to prevent any increase of diameter in the stem, or it 
distends, without increasing in thickness or splitting or casting off any 
outer layers. 
200. In the Leaf, the structure of the petioles and principal ribs or 
