OUTLINES OF BOTANY. T!T 



very apparent the first season, especially towards its close. The fibro- 

 vascnlar bundles in Endogens usually anastomose but little, passing con- 

 tinuously into the branches and leaves. In Exogens the circle of fibro- 

 rascular 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, 

 like the wood, arranged in annual concentric circles (211), of which the 

 oater 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 theii 

 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 



