Xylem

“ The space between the phloem and xylem is filled in with either parenchyma or fibres, or both. In some cases the vessels of the xylem meet in the centre of the root, while in other cases the centre of the stem is occupied by pith parenchyma. Each bundle is surrounded by parenchyma cells, and in iris, calamus, and veratrum, rhizomes, and endodermis, surrounds the bundles located in the centre of the stem, consisting of thin-walled (mechanical) cells. ”

“ Parenchymatous cells occur in contact with the outer edge of the xylem-cylinder some of which are distinguished by an irregular reticulate pitting. The tangential section represented in fig. 148, B, illustrates the appearance of a shoot of L. vasculare in which no secondary xylem is present: the central strand of tissue consists of the parenchyma abutting on the xylem with several leaf-traces (lt) passing upwards in an almost vertical course from the outer edge of the stele. ”

“ The increase in girth of the central cylinder was often accompanied by a considerable development of cortical periderm. The roots differed from the shoots in having no carinal canals, and in the possession of a solid pith and centripetally developed primary xylem groups alternating with strands of phloem. ”

“ Immediately beyond the primary xylem occur numerous thin-walled parenchymatous cells with spiral and reticulate pitting; beyond these is the broad zone of167 secondary xylem, x2, composed of scalariform tracheae and numerous medullary rays consisting of one, two, or several rows of radially elongated elements with spiral and reticulate pitting. In tangential sections the rays are seen to vary considerably in size, some being made up of a single row of cells while others are longer and broader; through the latter leaf-traces pass horizontally. Portions of medullary rays are seen at mr in fig. ”

“ The vascular axis of the main axis of the frond is characterised by the hour-glass shape of the xylem which consists entirely of tracheae, most of which are reticulately pitted. In a transverse section (fig. 310, A) the two ends of the stele are dissimilar; at one end of the long axis is a small bay of thin-walled tissue (phloem) enclosed by a narrow band of xylem, and 449at the other the bay is open and has two protoxylem groups. ”

“ This centripetal or root-like character of the primary xylem is an important feature in recent as in fossil Lycopods. The close agreement between the roots and stems of recent species in the disposition of the vascular elements also denotes a simpler type of anatomy 41than occurs in the majority of vascular plants in which stem and root have more pronounced structural peculiarities. A pericycle, 2–6 cells in breadth, encloses the xylem and phloem bands and this is succeeded by an endodermis, 2–3 cells broad, with vaguely defined limits. ”

“ The junction between the primary and secondary xylem is shown on a larger scale in fig. 186, B. The tissues abutting on the secondary xylem have not been preserved; the outer cortex, which consists chiefly of secondary elements, is divided superficially into unequal ridges corresponding to the leaf-cushions which have been more or less obliterated as the result of growth in thickness of the stem.
9. Lepidodendron Pedroanum (Carruthers) .
In 1869 Mr Carruthers described some specimens of vegetative stems and isolated sporangia, collected by Mr Plant in Brazil, as Flemingites Pedroanus [385] . ”

“ A longitudinal section of either the stem or leaf stalk shows that all the cells are decidedly elongated, especially those of the fibro-vascular bundle. The xylem (Fig. 68, C, x) is made up principally of large empty cells, with pointed ends, whose walls are marked with closely set, narrow, transverse pits, giving them the appearance of little ladders, whence they are called “scalariform,” or ladder-shaped markings. These empty cells are known as “tracheids,” and tissue composed of such empty cells, “tracheary tissue.” ”

“ In a bundle examined in the basal portion of a leaf the bulk of the xylem is found to be centrifugal in position, but internally to the protoxylem there is a group of centripetal tracheids; higher up in the petiole the xylem is mainly centripetal, the centrifugal wood being represented by a small arc of tracheids external to the protoxylem and separated from it by a few parenchymatous elements. Finally, in the pinnae of the frond the centrifugal xylem may disappear, the protoxylem being now exarch in position and abutting on the phloem. ”

“ The bundle-system is of course continuous with that of the petiole and stem. The leaf-bundles are always collateral (the phloem being turned downwards and the xylem upwards) , even in Ferns, where the petiolar strands are concentric, and they have the ordinary mesodesm and peridesm of the collateral bundle. The latter is often sclerized, especially opposite the phloem, and to a less extent opposite the xylem, as in the stem. ”

“ We may sum up the changes in structure towards the ends of the vascular bundles thus: The thickening of the walls is less, and the elements become narrower and shorter; the xylem becomes simplified by the loss of fibers and vessels, until finally only delicate tracheids are left (Fig. 21) , the thickenings of which are at length not spirals or nets for the most part, but irregular pittings. Moreover, they are nearly isolated. ”

“ In a single section of this species one often sees several strands of xylem enclosed in a common cortex with the main vascular axis; these are the xylem bundles of lateral pinnae. Metaclepsydropsis duplex shows the method of branching of the petiole vascular axis which has already been noticed in Diplolabis and Zygopteris. ”

“ The secondary xylem of Lepidodendron and Sigillaria affords a striking example of water-conducting tissue of homogeneous structure comparable with the wood of Conifers rather than with that of Angiosperms. It was presumably formed, for the most part, under uniform climatic conditions: the absence of rings of growth points to uninterrupted supply to evergreen shoots exposed to no alternation of activity and arrested growth. ”

“ These “bordered” pits are very characteristic of the wood of all conifers.
The structure of the root is best studied in the seedling plant, or in a rootlet of an older one. The general plan of the root is much like that of the pteridophytes. The fibro-vascular bundle (Fig. 75, M, fb.) is of the so-called radial type, there being three xylem masses (x) alternating with as many phloem masses (ph.) in the root of the seedling. This regularity becomes destroyed as the root grows older by the formation of a cambium ring, something like that in the stem. ”

“ A broad sheath of thicker-walled 215elements, s, surrounds numerous scattered transfusion tracheae, t, and below the two xylem strands, x, which are embedded in delicate parenchyma there is a crescentic band of dark tissue, a, resembling the smaller strand, a, in fig. 142, A′, and the secretory zone tissue of a Lepidodendron stem. ”

“ The assimilatory tissue is composed either of palisade cells or of chlorenchyma or is absent, in which latter case the primary cortex functions as an aqueous tissue. The pericycle is composed of groups of stone-cells and presents an isobilateral structure. Wood forms a composite hollow cylinder. Medullary rays occur only in F. cretica and are uniseriate. The size of the xylem bundles and the size and abundance of vessels vary on the upper and lower surface of the prostrate axis. Soft bast forms a continuous ring and follows the outline of the wood cylinder. ”

“ Fig. 47.—Transverse section of the stele portion of the root of Andropogon Sorghum. × 150
Co. Cortex; en. endodermis; pe. pericycle; ph. phloëm; pr. protoxylem; xy. xylem vessel.
Fig. 48.—A portion of the transverse section of the leaf of Panicum javanicum. × 100
Ep. c. An ordinary epidermal cell; st. stomata; sc. sclerenchyma; ph. phloëm; chl c. chlorenchyma; m. motor cells; xy. xylem.
For a detailed study of the structure of the leaves of grasses the leaf of the grass Panicum javanicum may be chosen. In a transverse section of this leaf, the vascular bundles are very conspicuous. ”

“ The outer cortex is limited by a well-defined layer of thick-walled cells, which may be spoken of as the epidermoidal [694] layer. Roots possessing this superficial layer of thicker cells have no doubt lost the original surface-layer which produced the absorptive root-hairs.
Fig. 91. Diagrammatic sketch of a transverse section of a young root of Calamites. x, xylem; l, lacuna. After Hick. Central cylinder (stele) of root, px, protoxylem; ph, phloem; x2, secondary xylem; l, phloeoterma. × 75. After Williamson and Scott. ”

“ The thick-walled outer cortex bears at its periphery numerous multicellular hairs. Some of the xylem strands given off from the stele no doubt supplied adventitious roots, but in most cases the outgoing branches are leaf-traces and the numerous sections of axes of different sizes seen in fig. A point to a repeated subdivision of the crowded fronds. The structure of a petiole is shown in figs. C and D. As seen in fig. C, the oval vascular strand has three protoxylem groups, px, on its flatter side; a well-defined epidermal layer is shown at e in fig. ”

“ The longitudinal section (fig. 169, D) shows the cambial cells, a, next the secretory zone, sc, passing internally into crushed and imperfectly preserved elongated elements which are presumably miniature tracheae, and these are succeeded by older and more completely lignified xylem elements, x. In larger shoots the amount of secondary tissue is considerably greater ”

“ The branched filamentous rhizoids which spring from the lower region of the stem also correspond to protonemal branches. The structure of both stem and leaf reaches a high grade of organization in some mosses. Not only are thick-walled sclerenchymatous cells developed to give rigidity to the periphery of the stem and the midrib of the leaf, but in many cases a special water-conducting tissue, consisting of elongated cells, the end walls of which are thin and oblique, forms a definite central strand in the stem. ”

“ Embryo thread-shaped, spirally coiled in the rather fleshy albumen, destitute of cotyledons, sometimes with a few alternate scales (belonging to the plumule) ; germination occurring in the soil.—Leafless annual herbs, with thread-like yellowish or reddish stems, bearing a few minute scales in place of leaves; on rising from the ground becoming entirely parasitic on the bark of herbs and shrubs on which they twine, and to which they adhere by means of suckers developed on the surface in contact. Flowers small, cymose-clustered, mostly white; usually produced late in summer and in autumn. ”

“ The roots of air-plants, such as certain orchids, draw their nourishment from the air.
The experiment of marking roots and stem has been already tried, but it should be repeated. Repetition of experiments is always desirable, as it fixes his conclusions in the pupil's mind. The stem grows by a succession of similar parts, phytomera, each part, or phyton, consisting of node, internode, and leaf. Thus it follows that stems must bear leaves. The marked stems of seedlings show greater growth towards the top of the growing phyton. It is only young stems that elongate throughout. ”

“ The flowering plants have several distinct and visible organs for the formation and fertilization of their seed, to each of which is assigned a special and necessary office. In the flowerless section, on the contrary, there are none of these visibly separate agencies in reproduction, and what are usually termed the seeds do not show any parts representative of the developed product. In the true seeds, which belong to flowering plants alone, are contained the rudiments of a stem, leaves, and root, but in the spores of the flowerless plants nothing of the kind is found. ”

“ The generic characters of the first three species may be stated as follows:
Stems dichotomous, with rudimentary subulate leaves, sometimes obsolete in terminal branchlets and fertile branches; and in decorticated specimens represented only by punctiform scars. Young branches circinate. Rhizomata cylindrical, with circular root-areoles. Internal structure of stem, an axis of scalariform vessels enclosed in a sheath of imperfect woody tissue and covered with a cellular bark more dense externally. Fruit, naked sac-like spore-cases, in pairs or clusters, terminal or lateral. ”

“ I am afraid it has been rather a difficult history to follow step by step, and yet it is perfectly clear when once we master the succession of growths. Starting from a spore, the thread-mass or protonema gives rise to the moss-stems forming the dense green carpet, then the green flowers on some of the leaf-stems give rise to a plant-egg, which roots itself in the stem, and grows into a perfect plant without leaves, bearing merely the urn in which fresh spores are formed, and so the round goes on from year to year. ”

“ They have hollow stems, with partitions at the nodes; the leaves consist only of a whorl of scales at each node, these coalescent into a sheath: from the axils of these leaf-scales, in many species, branches grow out, which are similar to the stem but on a much smaller scale, close-jointed, and with the tips of the leaves more apparent. At the apex of the stem appears the fructification, as it is called for lack of a better term, in the form of a short spike or head. This consists of a good number of stalked shields, bearing on their inner or under face several wedge-shaped spore-cases. ”

“ The tip of every twig is composed of minute cells endowed with a potentiality of development like that which characterises the embryonic tissues of a seedling just emerged from the seed. In the course of its growth, each branch, by means of its living and dividing cells, contributes to the several parts of the complex mechanism of the tree. While the greater number of cells acquire a permanent form and lose the « 9 » power of further development, there remains a cylinder of cells endowed with perpetual youth. ”