Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/116

Rh 106 BOTANY [NUTRITIVE ORGANS. a prosenchymatous manner. Subsequently by cell-division a complicated structure is developed in this investing mass. No fibro-vascular bundles are found in Characese. Muscineaj. In Mosses and many Hepaticae we find no fibro-vascular system, but a slender leafy cellular stem. In it there is sometimes a differentiation into an inner and a peripheral mass of tissue. In the former the cells have usually thin walls, and are not coloured, though in some cases a thickening of the walls of the central cells is observed, and a sort of rudimentary bundle is formed. The circum ferential cells have usually thickened walls, and are not unfrequently coloured. Ferns. In Ferns we have a familiar example of an Acoty- ledonous stem. Ordinary ferns of Britain seldom attain any height, but usually creep along the ground, form ing rhizomes. But in the Tree-ferns of warm climates the caudex frequently attains a great height. A transverse section of the stem of a fern (fig. 89) exhibits an irregular circle of fibro-vascular bundles, composed of masses/, v, of various forms and sizes, situated near the circumference. the centre m being formed of cellular tissue, frequently with solitary fibro-vascular bundles scattered through it, and often becoming hollow. On the outside of the vascular circle cells exist, p, covered by an epidermal layer or cellular integument e, often of hard and dense consistence, and marked with the scars of the fronds. The vascular system is of greater density than the rest of the tissue, and is distinguished by the dark colour of the outer portion (fig. 89, /), which surrounds the paler Fig. 89. Fit;. 90. FIG. 89. Transverse section of the stem of a Tree-fern (Cyathea). Cellular tissue &amp;gt;n the centre, m ; that of the circumference, p; vascular bundles,/, ?, consisting of dark-coloured woody fibres/, and paler vessels, chiefly scalariform and dotted, v; the outer cortical portion formed by the bases of the leaves e Fio. 90. Fibro-vascular network from stem of Aspidium Filix-mas. (Sachs.) vessels in the centre. In a very young stem only a single fibro-vascular bundle is found in the axis of the stem, surrounded by a mass of cellular tissue ; but as growth proceeds a network of anastomosing bundles is formed in place of the central bundle, constituting a wide meshed hollow cylinder (fig. 90), and separating the fundamental ellular tissue of the stem into an outer cortical and an inner medullary portion. In addition, however, scattered through the pith, other bundles occur, as in Pteris aquilina where two large cauline bundles are found and in Tree- ferns numerous small bundles are found which pass out through the meshes of the cylinder into the leaves The primary bundles are ribbon-shaped and broad, and curve outwards at their margins, from which the bundles for the eaves pass off a leaf arising always from an opening of the network. The xylem portion of the fibro-vascular mndles is characterized by the great abundance of variform _ vessels, spiral vessels also being present, and intermixed with them thin-walled prosenchymatous Us, containing in winter a largo, amount of starch. This constitutes the inner, lighter - coloured portion of the bundles. The phloem portion, which completely surrounds the bundle, has also starch-cells, and, in addition, sieve- tubes and elongated bast-like thick-walled fibres are found at the periphery of the bundle. The whole bundle is usually enclosed by a distinct sheath of elongated cells ; these cells are frequently much thickened and of a dark brown colour, hence the appearance presented on transverse section of a dark layer enclosing a lighter-coloured part. The central cellular portion of the stem frequently becomes ruptured and absorbed in old stems, which are thus hollow. The bases of the leaves remain long attached, but ultimately fall off, leaving marked scars (fig. 91, c), which are at first Fig. 92. Ficr. 91. FIG. 91. -Rhizome of Mfile Fern, Lastrea (Atpidiuni) Filix-mas, showing the scars (cicatrices) of the leaves (fronds), with the markings of the vascular bundles, c. FIG. 92. Longitudinal section of rhizome of Kquisetum, showing septum, f-, between cavities ft, h ; g, g, fibro-vascular bundles ; /, vallecular canal; s, leaf- sheath. (Sachs.) close together, but often separate afterwards by interstitial growth. On these scars or cicatrices the markings of the vessels are easily seen, arranged in the same manner as those of the stem. From all points of the woody cylinder stem-roots may be given off, and frequently they are formed so abundantly as completely to invest the stem and conceal it. In EquisetaceaB or Horsetails we have another example In Equis of an Acotyledonous stem. The true stem in those plants tacese - is a rhizome underground, from which aerial shoots are sent up annually. Every aerial stem consists of a series of joints (internodes), which are usually hollow and closed at their base by a thin septum (fig. 92). Each internode passes up into a leaf-sheath embracing the next internode, and this sheath is usually split into teeth at its upper margin. In transverse section a circle of fibro-vascular bundles is seen (fig. 9 3 ) marking off an inner medullary from an outer cortical portion, and separated one from the other by a mass of parenchyma. These bundles are all common bundles. From each tooth of the leaf-sheath the bundle may be traced, passing vertically clown into the internodes Fig. 94. FIG. 9&quot;. Transverse section of a rhizome of Eqtiiscrnm. y, fibre-vascular bundle; I, vallecular canal ; ft, central cavity. (Sachs.) FIG. 94. To show union of the fibro-vascular bundle of an upper with a lower internode ; i, i, internode ; *, node. Sachs.) as far as the node beneath, and at the lower end dividing into two, uniting by each branch with a bundle of the next lowest internode (fig. 94). In transverse section the bundle