Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/113

Rh STEM IN DICOTYLEDONS. BOTANY 103 The medullary rays are in some cases, as in Clematis and Aristolochia, large and broad, while the woody wedges are comparatively small. The stems of Dicotyledonous plants occasionally present anomalous appearances in the structure and arrangement of their wood, bark, and medullary rays. In place of con centric circles there are sometimes only a few rows of wedge-shaped vascular bundles produced during the life of the plant, additions being made by the annual interposi tion of bundles of a similar kind, resembling in this respect the formation of woody bundles in the early growth of herbaceous plants. In Piperacese, Aristolochiaceae, and Menispermacese, these anomalous stems occur. In Gnetum (fig. 80), the vascular bundles, b, b, b, b, b, form zones, which Fig. 80. FIG. 80. Horizontal section of stem of Gnetum. in, pith; em, medullary sheath ; 6, b, b, b, 6, woody bundles forming seven concentric zones, each of which is the produce of several years; t, 1, 1,1, 1, 1, fibres of liber, forming interposed circles equal in number to the woody zones. FIG. 81. Peculiar stern of a Malpighiaceous plant of South America. The plant is Dicotyledonous. are each the produce of several years growth, and are separated by layers, I, I, I, I, I, I, which may be considered as representing different bast-layers. In some of the Menispermacese the separating layers are of a cellular and not of a fibrous nature. Many of the Malpighiacese, Sapindacese, and Bignoniaceae of Brazil exhibit stems in which the woody layers are arranged in a very irregular manner. In some of them (fig. 81) there is a central woody mass with from three to ten small secondary ones round it. Each of the masses contains true pith, derived either from the cortical cellular tissue, or from the original medullary centre. Around these separate collections of pith there is a medullary sheath and spiral vessels. JSTo annual rings have been detected in the secondary masses, but medullary rays exist usually in their outer portion. In some anomalous Sapindaceae, the central and lateral woody masses are enclosed in a common bark, with a continuous layer of liber. Some have supposed that the lateral masses are undeveloped branches united together under the bark. In some Bignoniaceae (fig. 82) the layers of wood are divided Fig. 82. Fig. 83. Fin. 82. Horizontal section of the stem of Dignonia capreolata, showing the crucial division of the woody layers. FIG. 83. Fragment of a stem of a climbing sjecics of Banisteria (/?. scaiulens). bowing tho effects of compressioa in a crucial manner into four wedge-shaped portions by the intervention of plates differing in texture from the ordinary wood of the plant, and probably formed by the introversion, or growing inwards, of the liber. In Aspidosperma excelsum (Paddle-wood) of Guiana, and in Heteropterys anomala, the stem assumes a peculiar lobed and sinuous aspect ; and in some woody climbing plants pressure causes the stems to become flattened on the side next the tree on which they are supported, while from being twisted alternately in different directions, they present a remarkable zigzag form, having the woody layers developed only on one side (tig. 83). In Firs the wood is occasionally produced in an oblique in place of a perpendicular manner, thus injuring the timber and causing it to split in an unusual way. The young plants produced from the seed of such twisted-wooded firs are said to inherit the peculiarity of their parents. Occasionally the Dicotyledonous stem becomes swollen at certain places, especially near the root, and thus exhibits a tuberous appearance. This peculiarity is said to be liable to occur in Coniferous plants grown from cuttings. In some of the lower class of plants a cellular stalk is produced, which on a transverse section presents an appear ance like that of a Dicotyledonous stem. Thus Lessonia fuscescens, a species of sea-weed, has stems which are often 5 to 10 feet long, and as thick as the human thigh, and which show concentric elliptic cellular rings. Such is also the case with Usnea melaxantha, a tree-like lichen. In these plants, however, the structure is entirely cellular, and quite distinct from that of Dicotyledonous plants. In the young stem of a Monocotyledonous plant the fibro- Stem in vascular bundles appear scattered in an irregular manner Monocoty through the fundamental parenchyma, becoming more le&amp;lt; * ons numerous towards the periphery. There is thus no primary separation in the stem into pith, cortex, and medullary rays, although the central cellular mass may be considered as representing the pith. Thus, if a section be made across a young Monocotyledonous stem, an appearance is observed such as is represented in fig. 84, where the vessels are seen Fig. 84. Fig. 85. FIG. 84. Transverse section of the stem of a Palm, which is Monocotyledonous. m, the central loose cellular portion ; /, the outer fibrous port ion, showing numer ous vascular bundles. The whole being covered by a false bark or rind. FIG. 85. Transverse section of the stem of a Palm, more advanced. as points, scattered through a cellular matrix, the circum ference not having any marked cortex, and the whole covered by an epidermis. A similar section of a further advanced stem, as of a Palm (fig. 85), shows numerous bundles of vessels dispersed irregularly in cellular tissue ; those near the centre, m, being scattered at a distance from each other, while those towards the outside are densely aggregated, forming a darkish zone b, and are succeeded at the circumference by a paler circle of less compact vessels I, with some compressed cells, covered by an epidermis e. The central cellular mass has no medullary sheath. In some cases its cells are ruptured, and disappear during the progress of growth, leaving a hollow cavity ; but in general it remains permanent, and is gradually encroached upon by the development of the vascular system. The peripheral portion differs from true bark in not being separable from the rest of the tissue. It has received the name of false bark, and consists of the epidermal cells e, and what has been called the cortical integument, /. This