Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/115

Rh STEM IN MONOCOTYLEDONS.] BOTANY 105 between them. The new woody tissue so formed thus corresponds to the secondary wood in Dicotyledonous stems, and the new fibro-vascular bundles are all cauline, that is, do not pass out into the leaves. By this means an enormous increase in diameter of the stem may take place. And not unfrequently the primary central portion of the stem gives way, and thus a hollow cylinder formed by this secondary wood remains. This was well seen in the famous Dracaena Draco, or Dragon tree of Orotava, in the Canary Islands, which had a hollow stem capable of holding several men. The composition of the vascular bundles in different parts of their course varies. Thus, at the upper part, tracing them from the leaves towards the centre, they contain spiral vessels, pitted vessels with some cellular tissue, a few laticiferous vessels, and woody fibres resem bling those of liber. As we descend to the older por tions of the bundle, where the tissues have become per manent throughout, the spiral vessels disappear, then the pitted vessels, and at the periphery nothing but pleurenchy- niatous tissue remains, forming a complicated anastomosis or network. Not unfrequently at the nodes of the stem a network of horizontal vessels occurs. They are well seen in many Grasses, where the central portion of the stem has given way, and there they serve to strengthen the stem. The branching of Monocotyledonous stems is originally the same as in Dicotyledons, namely, a monopodial form; fre quently however, the axillary buds do not unfold, and the form becomes much varied. In Palms this is well seen, and usually no lateral shoots are formed. In some Palms, however, as the Doum Palm of Egypt (Ilyphcene thelaica), th? lateral shoots are developed in such a way that the stem appears to fork. Other examples of the development of branches are seen in the case of the Screw-pine (Pandanus), in Grasses, as the Bamboo, in Asparagus, Cordyline, Dracaena, etc. In some instances the axillary shoots detach themselves from the parent stem and form an independent plant, as in Lilium bulbiferum (fig. 71). The bases of the leaves produced from the stem remain attached to the stem in many Palms, being surrounded by a fibrous sub stance, the mattulla or reticulum. When the internodes of the caudex of a Palm are not much elongated, the scars of the leaves are seen forming spirals on the stem, as in the Coco-nut and Date. In Xanthorrhcea Hastile the same arrangement is observed. In this plant also a curious internal structure of the stem occurs. On making a vertical section the structure appears to be that of a Dicotyledon. The woody part is formed of vertical loose fibres as in Palms, and there are other fibres, radiating from the centre, and cutting the preceding at right angles. These horizontal fibres resemble the medullary rays, but differ in their structure. They probably serve for the origin of leaves, which are numerous, and are dis posed throughout the whole length of the stem. In Palms, such as species of Chamsedorea, the internodes are much lengthened, and rings are seen on the stem at distant intervals, showing thickened node-like joints. Some Palm stems, as those of Calamus Puidentum, the common Cane, are very thin and slender. In many Monocotyledonous plants the stem remains below ground, developing shoots which are simple, as in Banana and Plantain, or branched, as in Asparagus. In the former the stem above ground is an herbaceous shoot, composed of the sheaths of the leaves. It dies after fruiting, and is succeeded by other shoots from the subterranean stem. The shoots or buds from such stems occasionally remain in part below ground in the form of bulbs, as in Lilies, Tulips, and Hyacinths ; or as corms, as in Colchicum, Crocus, Gladiolus, and Arum. In some instances the aerial stem has the usual Monocoty ledonous structure, while in the underground stem the fibro- vascular bundles are arranged in a circle, enclosing a central cellular mass or pith, and thus resembling in struc ture a Dicotyledonous stem. This structure has been remarked in the Smilax or Sarsaparilla family. Lindley calls these plants Dictyogens, from their netted leaves, by which they differ from most Monocotyledons. Amongst Acotyledonous plants there are some which Stem in possess stems consisting entirely of cellular tissue, whilst Acotyle in others the stems have a well-developed vascular system. tlons Of the former we have examples in Mosses, Hepaticae, and Characeae; the latter we find represented in ferns, Equisetaceae, and Lycopodiaceae. The term Acroyenous has been used as descriptive of the stems of Acotyledonous plants, as they were supposed to be formed by additions to the summit, and by the elongation of vessels already formed. They are also sometimes called Acrobrya. But as in the case of the terms exogenous and endogenous applied to the stems of Dicotyledons and Monocotyledons respectively, recent research has shown that the term acrogenous cannot have the significance formerly attached to it ; for in some Acotyledonous plants a true cambium ring is formed by which layers of tissue are successively added, and the stem increases in diameter, and also in many instances the fibro-vascular bundles develop from above downwards. The characters of the stem, however, enable us easily to distinguish it from that in Dicotyledons or Monocotyledons. With merely a slight exception there is no provision for lateral growth of the stem, and all increase in size takes place by an elongation of the terminal growing-point. No cambium ring is as a rule formed; where it does exist, it is not produced in the same way as in Dicotyledons, but rather resembles the formation of the cambium ring in Dracaena amongst Monocotyledons. When a permanent woody stem occurs amongst Acotyledonous plants it resembles in general aspect the stem of a Mono cotyledonous plant, having nearly a uniform height, and being usually unbranched and producing a tuft of leaves (fronds) at the summit. Tree-ferns furnish the best example of this kind of stem. In them it is denominated a stipe, and it often attains the height of 120 feet. The stem in Acotyledonous plants is distinguished from that in Dicotyledons by the absence of annual rings of wood, of (with only slight exception) a cambium ring, of a separable bark, and by the fact that the fibro-vascular bundles are all closed ; in this latter character they agree with Monocotyledons, but are distinguished from them by the arrangement of the vascular bundles. In Acotyledonous stems growth takes place by division of a single apical cell, situated at the extreme end of the punctum vegetationis. By divisions of this cell two portions of tissue are marked out in the stem, an inner portion, from which arise the fibro-vascular tissues when they exist, and an outer or cortical portion. The primary fibro-vascular bundles originate from the cellular tissue of the stem in a manner analogous to what occurs in Dicotyledons and Monocotyledons, a procambial bundle being first formed, which differentiates subsequently into xylem and phloem layers, but the bundles always become closed. The character of the stem varies very much in the several families of Acotyledonous plants. In Characeae the stem consists of a series of joints Characea?. (internodes), each composed of a single much elongated cell. Interposed between successive internodes are the nodes, each composed of a whorl of small cells, from which the leaves are developed, one leaf from every coll of the node. In the genus Chara a cortex is found completely investing the internodal cell. It is formed by the develop ment, from every cell, of the nodes of an ascending and descending lobe. The ascending lobes of one node and the descending lobes of the next higher node meet in the middle of the intervening internode, and there interlock in IV. 14