Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/114

Rh 104 BOTANY [NUTRITIVE ORGANS. portion of the stem is often very inconspicuous, but some times it is much developed, as in Testudinaria Elephantipes, in which it is rugged, and is formed of a substance resembling cork in many respects. The fibro-vascular bundles of the stem very soon become closed, and thus all growth in them ceases, the cambium cells being converted completely into permanent tissue. Fig. 57 represents a transverse section of a closed nbro-vascular bundle from the stem of the maize (Zea Mays). In it the several elements seen in the section of the nbro-vascular bundle from a Dicotyledonous stem may be recognized, but there is no cambium, the cells marked v, v representing those cambiform cells which have last become permanent tissue. This absence of cambium necessarily curtails the growth of the bundle, and hence the limited diameter characteristic of the stems of Monocotyledons. In fig. 80 a diagram Fig. 86. Diagram of a Monocotyledonous stem. A, transverse section ; B, longitudinal section ; a, a, cellular tissue (Parenchyma) ; b, b, dotted vessels (Bothrenchyina); c, c, woody fibres (Pluurenchyma) j d, d, spiral vessels (Trachenchyma). (After Carpenter.) represents a transverse and longitudinal section of a Monocotyledonous stem. All the fibre-vascular bundles of the stem are common bundles, that is, they pass out into the leaf, and in these stems each bundle has a definite arrangement. At the point where the bundle curves out into the leaf it has its greatest thickness, gradually becoming attenuated as it passes upwards into the leaf and downwards into the stem. In some instances, however, the bundles as they descend increase at different parts of their course, probably by interstitial growth, and give rise to irregular swellings of the stem, as in Ceroxylon Andicola. The distension takes place occasionally at the base of the stem, as iu Euterpe montana. This downward prolonged portion does not, however, run a straight course, but first passing inwards towards the centre of the stem, it then gradually arches outwards towards the periphery. This passing inwards at first of the fibro-vascular bundle gave origin to the idea that the first formed fibres were gradually pushed towards the circumference by those which succeeded them, and that the wood portion of these stems was increased by additions to the centre ; hence the term endogenous applied to them, meaning internal growth. But, as has been shown, the fibro- vascular bundles really become external at the base, although internal above. On making a vertical section of the endogenous stem, as of a Palm (fig. 87), there is observed an interlacing of fibres, the fibro-vascular bundles are first directed towards the centre, and then curve outwards towards the circumference, so that those last formed ultimately become external. The term endogenous will, therefore, only apply strictly to the fibres at the early part of their course. The true distinction between exogenous and endogenous stems is, that in the former the fibro- vascular bundles remain open, a cambium ring being eventually formed from which the stem increases indefinitely in diameter. In the latter the fibro-vascular bundles soon become closed, and being scattered irregularly through the cellular tissue, and not in a circle, no cambium ring can be formed, and thus growth in a transverse direction is soon arrested, and the stem is of a comparatively uniform diameter throughout. The investing bark of the former permits an unlimited extension of woody growth beneath it ; the fibrous cortical layer of the latter, by maintaining an intimate union with the subjacent tissue, prevents unlimited increase in diameter. Hence we find that the stem does not attain the enormous diameter exhibited by some Dicotyledonous trees, such as Sequoia (Wellingtonia) gigantea and the Baobab, the former of which has been measured 116 feet in circumference. In consequence of this mode of formation, the outer part of a Palm-stein is the hardest and densest, and after acquiring a certain degree of firmness it resists all further distension, and frequently becomes so hard as to withstand the blow of a hatchet, and therefore a woody twining plant does less injury to it than to trees of exogenous growth (fig. 88). The growth Fig. 87. Fig. 88. FIG. 87. Vertical section of a Palm-stem, showing the vascular bundles, fa, curving downwards and interlacing. FIG. 88. Monocotyledonous stem, surrounded by a twining woody plant, and remaining uninjured. of endogenous stems may be said to resemble an upward growth of an exogen by the terminal bud only, for there is no cambium layer, and no peripherical increase. The terminal central bud is called a phyllopJior or phyllogen. As growth only proceeds in this manner, no annual rings of wood are formed. From the absence of concentric circles the age of a Palm cannot be estimated in the same way as that of an exogenous tree. The elongation, however, of each species of Palm is pretty regular, and by this some idea may be formed of its age. There are many herbaceous plants in Britain, as Lilies, Grasses, &c., having Mono cotyledonous stems, but there are no British Monocotyle donous plants with permanent aerial woody stems. All the British trees are Dicotyledonous. Illustrations cf Monocotyledonous stems must be taken from trees of other countries, and of these Palms furnish the best examples. Although this limited growth in a transverse direc tion is characteristic of most Monocotyledons, we find in stances where a true thickening ring is formed and an indefinite increase in diameter takes place, as in Dracaena, Yucca, &c. This thickening ring, however, originates in a manner different from that in Dicotyledonous plants. A layer of the fundamental cellular tissue parallel to the surface of the stem becomes inerismatic, and produces new- closed fibro-vascular bundles and new cellular tissue