xxviii OUTLINES OF 



pith (198, 1), and an external separable bark (198, 5). In Endogens (Monocotyle- 

 dons) the woody system is in separate small bundles or fibres running through the 

 cellular system without apparent order, and there is usually no distinct central pith, 

 nor outer separable bark. 



196. The anatomical structure is also somewhat different in the different 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 sel- 

 dom, 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 

 lias 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 sytem, 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, how- 

 ever, 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 longitudinal axis 

 of the stem. It is active only in young stems or branches, becomes dried up and com- 

 pressed 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 dis- 

 posed 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 be- 

 come 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, ra- 

 diating 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 outer older ones be- 

 come 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 ca3t 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 concentric 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 stern, losing their vessels, they retain only their bast- or long wood-cells, 



