. The Botanical Class-Book and Flora of Pennsylvania. but these diversities arc reducible to two general plants, whichlay the foundation of the first grand distinction of Floweringplants into Exogens and Exdogens. 24 EXOGENOUS STEMS. 70. The difference between the two, as to the structureof their stems, is briefly this. Thosfi of Exogenous plantsincrease in diameter by the annual formation of a new layerof wood, which is deposited between the preceding layer andthe bark; i^othtr words the wood increases by annual addi-tions to the outside. Flowering plants whose stems grow inthis way are calle
. The Botanical Class-Book and Flora of Pennsylvania. but these diversities arc reducible to two general plants, whichlay the foundation of the first grand distinction of Floweringplants into Exogens and Exdogens. 24 EXOGENOUS STEMS. 70. The difference between the two, as to the structureof their stems, is briefly this. Thosfi of Exogenous plantsincrease in diameter by the annual formation of a new layerof wood, which is deposited between the preceding layer andthe bark; i^othtr words the wood increases by annual addi-tions to the outside. Flowering plants whose stems grow inthis way are called Exogenous Plants orExogens (outsidegrowers.) 71. In stems of the Endogenous structure the new woodymatter is deposited within the old, and towards the centre,which becoming more and more occupied with woody fibresas the stem grows older, causes a gradual distention of thewhole, thereby increasing its diameter, the new wood pushingthe old outward,. Accordingly the plants of this division arecalled Endogons (inside growers.) EXOGENOUS STEMS. 19. i. 72. The stem of an Exogen, Fig. 19, is composed of threeseparate parts, the pith, wood, and bark. The pith (medulla.)occupies the centre of the plant, consisting of spongy cellulartissue, at first abounding with nutritive matter for the nour-ishment of the terminating buds, but as the plant advancesin age it becomes dry and much reduced in volume. It issurrounded by a narrow Eone of vascular tissue, called the EXOGENOUS STEMh). lh medullarv sriEATinNO, which is composed of woody fibreand spiral vessels and is the only part of the stem in whichthese latter occur. 73. The Wood consists of ligneous fibre, vasiform tissueand ducts, arranged in concentric row3 or laycr0 pervaded bythin, firm, plates of condensed cellular tissue passing fromthe pith through the medullary sheath to the bark, denomi-nated medullary rays, Fig. 19, which aie epiite conspicuousin vertical sections of the oak, or the maple, where they aresometimes called Si
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