XXX INTRODUCTION. 



198. Tho 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 stein. 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 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 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 off any outer layers. 



200. In the Leaf, the structure of the 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, form- 

 ing 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 differ- 

 ently 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 

 under surface, the cells are more or less horizontal, more loosely placed, and have ge- 

 nerally 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 reduced 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 plan, is more simple ; their fibro-vascular system is less rami- 

 fied, 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 they are reduced to scales or trans- 



