Part 11 (2/2)
Up to this point, all the cells of the embryo are much alike, and the embryo, like that of the bryophytes, is cooniu 55); but before the e cells a differentiation of the tissues begins In the axis of each of the four divisions the cells divide lengthwise so as to form a cylindrical mass of narrow cells, not unlike those in the steo a further change; the walls thicken in places, and the cells lose their contents, for tissue (tracheary tissue), found only in the two highest sub-kingdoms The whole central cylinder is called a ”fibro-vascular bundle,” and in its perfect form, at least, is found in no plants below the ferns, which are also the first to develop true roots
The young root and leaf now rapidly elongate, and burst through the overlying cells, the forround, the latter growing upward through the notch in the front of the prothalliu 67, _B_) The leaf is more or less deeply cleft, and traversed by veins which are continuations of the fibro-vascular bundle of the stalk, and themselves fork once or twice The surface of the leaf is covered with a well-developed epider the space between the veins contain numerous chloroplasts, so that the little plant is now quite independent of the prothallium, which has hitherto supported it As soon as the fern is fir this noith the developoniu fern is the equivalent of the sporogoniu, like it, the direct product of the fertilized egg cell; and the prothallium represents the ans In the fern, however, the sporogonium becomes entirely independent of the sexual plant, and does not produce spores until it has reached a large size, living e, on the other hand, is veryso small as to be ordinarily completely overlooked; but its resemblance to the lower liverworts, like _Riccia_, or the horned liverworts, is obvious The ter plant, or sporogoniuuish between the sexual plant and the spore-bearing one produced from it
The common maiden-hair fern (_Adiantu the structure of the full-grown sporophyte, but almost any other common fern will answer The maiden-hair fern is conized by the forround ste 67, _C_), which is covered with brownish scales, and each leaf consists of a slender stalk, reddish brown or nearly black in color, which divides into two equal branches at the top Each of these main branches bears a row of smaller ones on the outside, and these have a row of delicate leaflets on each side (Fig 67, _E_) The steround by est of these, near the growing point of the stem, are unbranched, but the older ones branch extensively (_C_)
On breaking the stem across, it is seen to be dark-colored, except in the centre, which is traversed by a woody cylinder (fibro-vascular bundle) of a lighter color This is sometimes circular in sections, sometimes horse-shoe shaped Where the stem branches, the bundle of the branch may be traced back to where it joins that of the main stenified, three regions First, an outer row of cells, often absent in the older portions; this is the epidermis Second, within the epidermis are several rows of cells sier, and like theer cells, with thicker golden broalls (Fig 67, _I_)
The latter, if sufficiently nified, show distinct striation of the walls, which are often penetrated by deep narrow depressions or ”pits” This thick-walled tissue is called ”stony tissue”
(schlerenchyranules, which the iodine test shows to be starch All of this second region lying between the epiderround tissue The third region (fibro-vascular) is, as we have seen without the microscope, circular or horse-shoe shaped It is sharply separated froround tissue by a row of small cells, called the ”bundle sheath” The cross-section of the bundle of the leaf stalk resembles, almost exactly, that of the stem; and, as it isthe arrange 67, _G_) Within the bundle sheath (_sh_) there are tell-e eular outlines, and distinctly separated walls; and an outer portion (_y_) filling up the space between these central cells and the bundle sheath The central tissue (_x_) is called the woody tissue (xylem); the outer, the bast (phloem) The latter is composed of smaller cells of variable foritudinal section of either the steated, especially those of the fibro-vascular bundle The xylee empty cells, with pointed ends, whose walls arethem the appearance of little ladders, whence they are called ”scalarifors These empty cells are known as ”tracheids,” and tissue composed of such empty cells, ”tracheary tissue” Besides the tracheids, there are a few sranular contents
The phloem is composed of cells similar to the latter, but there er ones (Fig 67, _J_), whose walls are marked with shallow depressions, whose bottoms are finely pitted These are the so-called ”sieve tubes”
For microscopical examination, either fresh or alcoholicopaque sections transparent
The leaves, when young, are coiled up (Fig 67, _C_), owing to growth in the earlier stages being greater on the lower than on the upper side As the leaf unfolds, the stalk straightens, and the upper portion (blade) becoeneral structure of the leaf stalka series of cross-sections at different heights, and exaement is essentially the same as in the steround tissue are dark-colored, but the inner ground tissue is light-colored, andpart of the stereenish color, due to the presence of chlorophyll
The section of the fibro-vascular bundle differs at different heights
Near the base of the stalk (Fig _D_ i) it is horseshoe-shaped; but, if exaoing to each of the main branches of the leaf These secondary bundles divide further, for the veins of the leaflets
The leaflets (_E_, _F_) are one-sided, the principal vein running close to the lower edge, and the others branching froin, which is deeply lobed, the lobes being again divided into teeth The leaflets are very thin and delicate, with extremely smooth surface, which sheds water perfectly
If the plant is a large one, so leaves are at once distinguished by having the middle of each lobe of the leaflets folded over upon the lower side (_F_) On lifting one of these flaps, numerous little rounded bodies (spore cases) are seen, whitish when young, but beco brown as they ripen If a leaf with ripe spore cases is placed upon a piece of paper, as it dries the spores are discharged, covering the paper with the spores, which look like fine broder
[Illustration: FIG 68--_A_, vertical section of the leaf of the maiden-hair fern, which has cut across a vein (_fb_), 150 _B_, surface view of the epidermis fro pore, 150 _C_, longitudinal section of the fibro-vascular bundle of the leaf stalk, showing tracheids with ladder-shaped h the tip of a root, 150 _a_, apical cell _Pl_ young fibro-vascular bundle _Pb_ young ground tissue _E_, cross-section of the root, through the region of the apical cell (_a_), 150 _F_, cross-section through a full-grown root, 25 _r_, root hairs _G_, the fibro-vascular bundle of the same, 150]
A microscopical examination of the leaf stalk shows the tissues to be alround tissue, whose cells are thin-walled and colorless (soft tissue or ”parenchyma”) instead of stony tissue The structure of the blade of the leaf, however, shows a nu off a little of the epider off a thin slice with a razor, itthe air if necessary with alcohol It is coular outline, except where it overlies a vein (Fig 68, _B_, _f_) Here the cells are long and narroith heavy walls The epidermal cells contain numerous chloroplasts, and on the under surface of the leaf breathing pores (_sto _stoma_), not unlike those on the capsules of so pore consists of two special crescent-shaped epider or pore co with an air space below They arise froitudinal wall, that separates in thethe space between
[Illustration: FIG 69--_A_, iuium, surface view, 150: i, froes in the developiuiu _st_ point of opening _G_, group of four spores, 150
_H_, a single spore, 300]
By holding a leaflet between two pieces of pith, and using a very sharp razor, cross-sections can be68, _A_ The epidermis (_e_) bounds the upper and lower surfaces, and if a vein (_fb_) is cut across its structure is found to be like that of the fibro-vascular bundle of the leaf stalk, but round tissue of the leaf is co nue and numerous intercellular spaces, filled with air, and co pores These are the principal assi cells of the plant; _ie_ they are principally concerned in the absorption and decomposition of carbonic acid from the atmosphere, and the ia (Fig 69), are at first little papillae (_A_), arising from the epidermal cells, from which they are early cut off by a cross-wall In the upper cell several walls next arise, for a short stalk, composed of three rows of cells, and an upper nearly spherical cell--the sporangium proper The latter now divides by four walls (_B_, _C_, i-iv), into a central tetrahedral cell, and four outer ones The central cell, whose contents are ain by walls parallel to those first forium now consists of a central cell, surrounded by two outer layers of cells Froroup of cells is formed by further divisions (_D_), which finally become entirely separated from each other The outer cells of the spore case divide only by walls, at right angles to their outer surface, so that the wall is never more than two cells thick Later, the inner of these two layers becoanized, so that the central iule layer of cells (_E_)
Each of the central cells divides into four spores, precisely as in the bryophytes The young spores (_G_, _H_) are nearly colorless and are tetrahedral (like a three-sided pyramid) in form As they ripen, chlorophyll is formed in them, and some oil The wall becomes differentiated into three layers, the outer opaque and brown, the two inneraround the outside of the ripe spore case is a single row of cells (_an_), differing fro their inner walls thickened Near the bottom, two (sometimes four) of these cells are wider than the others, and their walls are ly thickened It is at this place (_st_) that the spore case opens When the ripe sporangiu of thickened cells (_an_) contractsthe sporangiuerminate readily under favorable conditions, and form after a time the sexual plants (prothallia)
The roots of the sporophyte arise in large nu point of the ste roots are pointed at the end which is also light-colored, the older parts beco dark brown A cross-section of the older portions shows a dark-brown ground tissue with a central, light-colored, circular, fibro-vascular bundle (Fig 68, _F_) Growing from its outer surface are nunified the walls of all the outer cells (epiderround tissue) are found to be dark-colored but not very thick, and the cells are usually filled with starch There is a bundle sheath ofthe fibro-vascular bundle fro 68, _G_) shows a band of tracheary tissue in the centre surrounded by colorless cells, all about alike