Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/112

 102 FERNS a large rosette with the growing point in the centre. Between the ordinary herbaceous ferns with short erect stems and the tree-ferns there is no hard and sharp line of demarcation. Most of the ferns of decidedly arborescent habit belong to the sub-order Cyatheacece, which has a sporangium and receptacle a little different from that of the typical Poly pod iacece, but in many of the genera of Poly- 2)odiace&amp;lt;e, especially Dicksonia, there are species that assume a more or less decidedly arborescent habit. Nearly all Filices are perennial, but we have an instance of annual duration in Gymnogramme leptopliylla. Growing Point. The apex of the stem, whatever the dimensions of the mature plant, is always occupied by a si .igle apical cell, and the whole growth is entirely depend ent upon the repeated segmentation of this cell. In form it is either wedge-shaped with a transverse section like that of a bi-convex lens, or it has the form of a three-sided pyramid with a convex base forming the outer face. In the former case the segments are produced from the cell alter nately right and left, and the stem, which is gradually built up by the repeated subdivision of the segments so formed, is always distinctly bilateral, and produces its fronds in two rectilinear series. In the latter case the segments are derived in spiral succession from the three interior faces of the apical cell, and the leaves, which are developed from the masses of tissue to which these segments give rise, are gradually displaced from the simple triseriate arrangement due to the segmentation of the apical cell into more com plicated forms of phyllotaxis. Brandling is effected by the partition of the growing point ; the single stem therefore bifurcates. This scarcely ever happens in ferns with erect stems, such as NepJirodiinn Fi ix-mas, or in tree-ferns. New shoots are, however, some times formed from the bases of the leaf-stalks, just as in some species they are normally produced from the fronds themselves. When the terminal point is injured, lateral shoots are often formed in this way. A specimen of Dick- sonia antarctica has been described with as many as twenty- nine crowns, due doubtless to such a cause. Structure of the Stem. The repeated subdivision of the cells of the growing point gives rise to a mass of tissue, at first uniform in character, but which is subsequently differentiated. Particular tracts of cells are modified in the process of growth to form the fibro-vascular bundles. These are more or less flattened plates irregularly elliptical in transverse section. As in all the Pteridophyta they are closed, and incapable, when once formed, of further increase in diameter. They consist of a mass of &quot;xylem&quot; surrounded by &quot;phloem.&quot; The xylem consists, in addition to a few spiral vessels at the foci of the elliptical tranverse section, of scalariform vessels cells of comparatively large diameter, with oblique ends, and with their walls intermittently thickened by transverse projecting ridg es (fig. 2). Amongst the vessels are oblong cells containing starch in winter (wcod- parenchyma). The phloem contains sieve-tubes, and ex ternal to these thick-walled bast fibres. The whole fibro- vascular bundle is bounded first by one or two layers of oblong starch-containing cells (phloem-sheath), and external to these by a single layer of narrower cells, often thickened externally and darker in colour the vascular bundle sheath. In very slender stems the fibro-vascular bundles form a central axis (Ilymenopliyllaceai), but when the stems attain any size they are arranged in an anastomosing cylindrical net work (fig. 3), in which a mesh corresponds to the base of a frond. This cylinderseparates the remaining or fundamental tissues of the stem into two portions, an internal central medulla and an external cortex. This is shown in fig. 4; the dark lines, however, do not belong to the bundles which correspond to the light spaces enclosed by them ; the dark lines themselves consist of a mass of thickened elongated cells termed &quot;sclerenchyma,&quot; which form protecting sheaths to the fascicular tissue. The medulla and cortex are in the main formed of pareuchymatous tissue, often loaded FIG. 2. Scalariform vessels. FIG. 3. Fibre-vascular network from the stem of the JlaiC Fern (Xcplirodium Filix-mas). The spaces contained by the meshes correspond to the attach ments of the bases of the leaves. FIG. 4. Transverse section of the stem of a Tree-Fcrn (Cyatliea). m, medullary ground-tissue; p, cortical ground-tissue; e, hypodermal sclerenchyma; r, libro-vascular bundles, with sheathing sclerenchyma (/). with starch. In Cyathea medullaris this starchy pith was used by the Maoris as an article of food. The medulla is not, however, entirely parenchymatous ; it is usually per meated by a number of anastomosing filiform bundles (usually also invested with a sclerenchymatous sheath), which eventually pass into the leaves through the meshes in the fibro-vascular cylinder. The greater part of the fibro- vascular tissue of the leaves is, however, continuous with the lower portion of the margins of the meshes, which runs out into bundles arranged round the outer circumference of the leaf stalk, and surrounding therefore the bundles which pass through the meshes from the medulla of the stem. In some cases, however, these medullary bundles contract an adhesion to the margins of the meshes as they pass through them. The surface of the stem, and also of the leaf-stalks, i.3 usually strengthened by the sclerenchymatous develop ment of the superficial portions of the fundamental tissue. This coats the surface with a layer which would be imper vious to air were it not interrupted by spaces filled with soft parenchyma, by which the internal tissues of the stem are brought into relation with the atmosphere. In Pteris aquilina, the rhizome or horizontal subterranean stem is marked laterally by two lines where the colourless cortical parenchyma is not covered in, and in many tree-ferns the bases of the petioles have their sclerenchymatous invest ment perforated by pits filled in the living plant by a soft- celled tissue which speedily becomes pulverulent. The roots of ferns are always slender and wiry. They are continually formed during the growth of the stem in acropetal succession. In tree-ferns they consequently ac cumulate on the lower portion as they grow downward, giving it a large apparent diameter. In creeping species they at once attach themselves. They are cylindrical in all cases, scarcely exceeding one-eighth of an inch in diameter, dark-brown approaching black in colour, and clothed with root-hairs. In ferns the occurrence of a special form of &quot; trichomes &quot; is noticeable in the fiat multicellular membranous scales, to which the name of &quot;paleai&quot; is given. They are found in the greatest abundance on the stein and lower part of the stipes, but they often extend to the rachides of the frond, and sometimes to the foliar surfaces, especially in yoang fronds. There is no definite correlation in the character of these paleae with other points of structure. The most character istic are the long thread-like bright-brown paleai of Eudicksonia, which are so abundant that those of Dicksonia Culdta are exported in quantity from Madeira to stuff