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XXVIU INTHODUCTIOK. 



branches, with the petioles and veins of leaves, and other raniifieations of the system. 

 Like tlie pith, it gradually disappears in old wood. 



3, the tcood^ which lies immediately ontside the medidlary 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), wliich usually 

 renuiin 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 caUed heart-ivood or (hframen, the outer, younger, and usually paler-coloured living 

 layers constituting the sapivood or alburnum. 



4, the medidlar(/ raifs, which form vei'tical plates, originating in tlic 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 medullaiy rays ceases to be active, but they usually may still be seen in old wood, 

 forming what carpenters call the sliver grain. 



5, the hark, wliich lies outside the wood, within the epidermis. It is, like the 

 Avood, arranged in annual concentric circles (211), of which the outer older ones become 

 dry and hard, forming the corky layer or oider hark\ which, as it is distended by the 

 thickening of the stem, either cracks or is cast off with the epidermis, which is no longer 

 dl.-stinguisliable. Within tlie corky layer is the cellular, or green, or middle hark^ formed 

 of loose thin-walled pnlpy 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 Uher or inner harTc^ formed of long tough woody tissue called bast-cells. 



199. Tlie Endogenous stem, as it grows old, is not marked by the concentric circles 

 of Exogrng. The wood consists of a matrix of cellular tissue irregularly traversed by 

 vertical cords or bimdles 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 rhid. The old wood becomes more compact and 

 harder towards the circumference than in the centre. The epidermis or rind cither 

 hardens so as to prevent any increase of diameter in the stem, or it distends, without 

 increasing in thickness or splitting or casting oif any outer layers. 



200. In the Iieaf^ the structure of the petioles and principal rihs 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 systcui becomes usually veiy much ramified, form- 

 ing the smaller veins. These are surroimded and tlie interstices filled up by a copious 

 and very active cellular tissue. The majority of leaves are horizontal, having a differ- 

 ently constructed npper 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 tliese 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 buas, or 

 without taking any apparent part in the economy of vegetable life, their structure, 

 though still on the same plan, is more simple ; their libro- vascular system is less rami- 

 fied, their cellular system more unifonn, and there are few or no stomates. 



202. Eracts and floral envelopes, when green and much developed, resemble leaves 

 in tlieir 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 fihro- 

 vascular system, surrounded by and uitermixcd with the cellular tissue, is usually sim- 

 ple in the filaments and style, more or less ramified in the flattened or expanded ptU'ts, 

 such as tlie anther-cases, the walls of the ovary, or carpellary leaves, etc. The pollen 

 consists of granvdar 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 



