XXV 111 
OUTLINES OE BOTANY. 
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, 
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 lark, 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 hark, formed 
of loose tliin-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 oil' 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- 
formed 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 sim- 
ple in the filaments and style, more or le3s 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 stigmatic 
portion of the pistil is a mass of loosely cellular substance, destitute of epidermis, and 
