xxii OtJTLllSIES OF 30T^ANY. 



seveval 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 ?^*^°^°- 

 vasoular bundles forms a more continuous cylinder of network emitting lateral ofiaets into tne 

 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 ot *^e 

 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 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 colored, forming what is called heart-wood or duramen, the outer, younger, 

 and usually paler colored living layers oOQstituting the sap-wood 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 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 oast 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 stem, losing their 

 vessels, they retain only their bast or long wood-cells, usually curving outwards 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 ofi any outer layers. 



200. In the Xieaf, the structure of tbe petioles and principal ribs or veins is the same as that 

 of the young branches of which they are ramifications. In the expanded portion of the leaf the 

 fibro-vascular system becomes usually very much ramified, forming the smaller veins. These 

 are surrounded and the interstices filled up by a copious and very active cellular tissue. The 

 majority of leaves are horizontal, having a differently constructed upper and under surface. 

 The cellular stratum forming the upper surface consists of closely set cells, placed vertically, with 

 their smallest ends next the surface, and with few or no stomates in the epidermis. In the 

 stratum forming the uuder surface, the cells are more or less horizontal, more loosely placed, 

 and have generally empty spaces between them, with stomates in the epidermis communicating 

 with these intercellular spaces. In vertical leaves (as in a large number of Australian plants) 

 the two surfaces are nearly similar in structure. 



201. When leaves are.reduoed to scales, acting only as protectors of young buds, or without 

 taking any apparent part in the economy of vegetable life, their structure, though still on the 

 same pla.n, is more simple ; their fibro-vascular system is less ramified, their cellular system 

 more uniform, and there are few or no stomates. 



202. Bracts and floral envelopes, when green and much developed, resemble leaves in their 

 anatomical structure, but in proportion as tbey are reduced to scales or transformed into petals, 

 they lose their stomates, and their systems, both fibro-vascular and cellular, become more 

 simple and uniform, or more slender and delicate. 



203. In the stamens and pistils the structure is still nearly the same. The fibro-vascular 

 system, surrounded by and intermixed with the cellular tissue, is usually simple in the filaments 

 and style, more or less ramified in the flattened or expanded parts, such as the anther cases 

 the walls of the ovary, or carpellary leaves, etc. The pollen consists of granular cells variously 

 shaped, marked, or combined, peculiar forms being constant in the same species, or often in 

 large genera, or even Orders. The stigmatio portion of the pistil is a mass of loosely cellular 

 .substance, destitute of epidermis, and usually is in communication with the ovary by a channel 

 running down the centre of the style. 



