xxvm 
OUTLINES OP 
(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 tnedullary rays , which form vertical plates, orginating 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 dis- 
tended 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 
connexion 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 dis- 
tends, 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 ex- 
panded 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 hori- 
zontal, having a differently constructed upper and under surface. The cellular 
stratum for min g 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 epi- 
dermis. In the stratum forming the under surface, the cells are more or less hori- 
zontal, more loosely placed, and have generally empty spaces between them, with 
stomates in the epidermis communicating with these intercellular spaces. In ver- 
tical 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 struc- 
ture, though still on the same plan, 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 they 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 capillary 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 stigmatic portion of the pistil is a mass of loosely cellular substance, destitute 
of epidermis, and usually is in communication with the ovary by the channel run- 
ning down the centre of the style. 
204. Tubers, fleshy thickenings of the stem cr other parts of the plant, succulent 
