OUTLINES OF BOTANY. Xli 



organs of plants. In the Root, although it is constructed generally on the 

 same plan as the stem, yet the regular organization, and the difference between 

 Exogens and Endogens, is often disguised or obliterated by irregularities of 

 growth, or by the production of large quantities of cellular tissue filled with 

 starch or other substances (192). There is seldom, if ever, any distinct pith, 

 the concentric circles of fibro- vascular tissue in Exogens are often very indistinct 

 or have no relation to seasons of growth, and the epidermis has no stomates. 



197. In the Stem or branches, during the first year or season of their growth, 

 the difference between Exogens and Endogens is not always very conspicuous. 

 In both there is a tendency to a circular arrangement of the fibro-vascular 

 system, leaving the centre either vacant or filled with cellular tissue (pith) only, 

 and a more or less distinct outer rind is observable even in several Endogens. 

 More frequently, however, the distinction is already very apparent the first 

 season, especially towards its close. The fibro-vascular bundles in Endogens 

 usually anastomose but little, passing continuously 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 longitudi- 

 nal 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 ramifi- 

 cations of the system. Like the pith, it gradually disappears in old wood. 



3, the ivood, 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 concentric circles 

 (211), which usually remain active during several years, but in older stems the 

 central and older layers become hard, dense, comparatively 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 wdiat carpenters call the silver 

 grain. 



5, the baric, which lies outside the wood, within the epidermis. It is, like 

 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 con- 

 taining chlorophyll (192) ; and which is usually the layer of the preceding 

 season. The innermost and youngest circle, next the young w T ood, 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 concentric 

 circles of Exogens. The wood consists of a matrix of cellular tissue irregu- 

 larly 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 their vessels, they retain only 

 their bast- or long wood-cells, usually curving outwards towards the rind. The 



