STRUCTURAL BOTANY. THE CELL. The simplest form of vegetable life and the most elementary phase from which all others are developed is that of the minute vesicle or cell. Cells are very small transparent bladders or cysts, with walls or boundaries of cellulose, and generally contain fluids-an egg may be accepted as an illustration of a cell, of which the shell represents the cell-wall. The normal form of the cell is that of a sphere or spheroid, having a length not much exceeding its breadth. An aggregation of these cells or vesicles constitutes the cellular tissue of plants. CELL MODIFICATION. Were the cell walls to retain their original form when compacted together, there would be numerous spaces left unoccupied between points of attachment; these might be termed intercellular spaces. Slight pressure causes the cells to accommodate themselves to the space they have to occupy, and they assume that form which shall best fill up the interstices and occupy the least room--namely, a modification of the dodecahedral with an hexagonal section; hence this is the most common form in the cellular portions of plants. TISSUES. PARENCHYMA (Gr. for the substance of the lungs, liver, &c.,) is the name generally given to cellular tissue, which exhibits hexagonal cells when cut across. In fact it is the distinctive name for all ordinary membranous cellular tissues in general. As all forms of tissue are derived from the simple cell, of which they are but subsequent modifications-intermediate forms are met with of all the varieties, and the line of demarcation can scarcely be fixed-therefore the terms cellular and vascular tissues are employed with limitation of meaning and value attached to them; adjectives are employed which seem to mark phases of the same tissue. The simplest plants (Algae and Fungi) consist entirely of Parenchyma. The pith of trees and rushes are cellular, as are also all plants in their earliest stages. VASCULAR TISSUE. Under the general name of vascular is included the form of tissue, which, with greater propriety, is termed woody tissuePleurenchyma or Prosenchyma. WOODY TISSUE consists of elongated tubes of a fusiform or spindle-shape, tapering at each end. It is cylindrical, long, fine and tough-characteristics which distinguish it from cellular tissue. It is found in the wood inner bark (bast) and veins of leaves. The walls of this kind of tissue become thickened by successive layers of lignine, and when cut in section the concentric lines or markings of the successive deposits may often be traced. BAST (or woody tissue of the liber) consists of or contains much longer, very thick sided and tougher, but more flexible cells than those of the wood itself-their position demands such qualities, being situated near the circumference of stems and branches where expansion is indispensable. The strength of this kind of tissue renders it extremely valuable in the arts-divested of extraneous matter it furnishes hemp, flax, china grass, &c. The distinct forms of woody tissue in linen, and cellular tissue in cotton, are easily detected under the microscope. GLANDULAR WOODY TISSUE is a peculiar variety of disk-bearing prosenchyma, occuring in coniferous and some few other plants; the sides of the tubes are furnished with disks, sometimes in single, and at others, in double or triple rows. These disks or depressions have small orifices in their centre. FIBRO-VASCULAR TISSUE consists of thin cylindrical tubes, with fibre spirally coiled up within them; it has also been called Trachenchyma (trachea Lat. windpipe). Spiral vessels are the type of this kind of tissue. These vessels which are fusiform (spindle shape) overlap at the ends, where, the intervening membrane being absorbed, they communicate freely with each other. They are found in the medullary sheath, veins of leaves, &c. When the fibre occurs in rings, it is called annular. When the tubes are prismatic, the fibre broken and arranged at equal distances above each other like the steps of a ladder, it is termed Scalariform (scalae Lat. a ladder, as in ferns). Sometimes the arrangement is very irregular, branching, making a complete net work; the branches, however, unite and run into each other in a very irregular manner, forming tubular ducts, which seem to convey fluids, called Laticiferous vessels or Cienchyma (kines Gr. I move). The walls are not marked. It commonly occurs in exogens, found in many sorts of cinchona, scattered through the tissue intervening between the middle cortical layer (mesophloum) and the liber. The fluid conveyed (called latex) is either colored or transparent. It has of late been suspected to be of a formative character. The origin of cells has been referred to the formation of a central point amongst the formative fluid, around which a congregation of atoms take place till a firm layer is formed, this becomes ultimately the cell-wall, which enlarges, assimilates fluids and becomes a cell, containing within it the nucleus, or, as it is termed, cytoblast. (katos, Gr. a cell, blastis, a germ.) In growing, the walls of young cells enlarge much faster than the nucleus; the latter soon ceases to grow and is left in the centre or remains adherent to the wall on one side, or more commonly it dissolves and disappears. In older cells the liquid contents and the protoplasmic lining also disappear, and only walls of cellulose remain as the permanent vegetable fabric. CELL MULTIPLICATION.--A living cell formed in whatever manner, has the power of multiplying itself by dividing into two; then again into two more, and so on. Vegetable growth consists, first, of the growth or expansion of each cell up to its full size, which, ordinarily, is very soon attained; second, of what is called their merismatic multiplication, namely: the successive division of cells into two. This takes place only when they are young and active, and mostly before they are full grown. It is effected by the formation of a partition across the cavity of the cell, dividing it into two. In this way a single cell gives rise to a row of connected cells, when the division takes place in one direction only; or to a plane or solid mass of such cells, when it takes place in two or more directions, thus producing a tissue. In this multiplication of cells by division, as in the original formation of a cell, the contents and the protoplasmic lining play the most important part. The nucleus, when present, as it commonly is, first divides into two, then the lining membrane or primordial utricle, is gradually constricted or infolded at the line of division, which, soon meeting in the centre, separates the whole contents into two parts by a delicate partition, upon this a layer of cellulose is deposited as a permanent wall, which completes the transformation of one cell into two. New cells may be formed from old cells by fissuration or internal division, a division or septum separates the old cell into two partitions, each of which becomes a complete cell. New cells are also produced on the outside of old ones as branches. This process has been termed gemmation or budding. This fact has been proved in the growth of sea weeds. CONTENTS OF CELLS are— The primordial utricle and protoplasm, the former being the deposited lining of the cell-wall. Deposits of sclerogen (to make hard) in layers, giving great solidity to some cells. (ex. vegetable ivory.) Water, generally containing other matters in solution. Jelly, which is, perhaps, the same as pectin, the mucilage which abounds in some algæ. Starch-Chlorophyll, the green, coloring matter. Chromule, coloring matter other than green, (wax, oil, camphor, resin and raphides. The presence of the latter gives the grittiness to China Rhubarb.) Other compounds are found deposited in the vascular system. Plants built up with Cellular or Vascular Tissue may be divided as follows: THALLOPHYTES-Plants with a flattened expansion, as Algæ. CORMOPHYTES-Plants with a stem. CRYPTOGAMIA-Flowerless plants..... PHENOGAMIA-Flowering plants......... { Acrogens or summit growers. PARTS OF THE PLANT. If we take a living plant (out of the winter season) we always find, with a few immaterial exceptions, a root, a stem or trunk, and leaves. These three organs may therefore be considered as essential to the existence of the plant. They are called Organs of Nutrition because they contribute to its actual life. But the death of the individual is a necessity in vital life; provision must therefore be made for the bringing into existence of a new plant similar to itself. The organs with this object in view are Organs of Reproduction. These consist of the so-called flower (with its parts) containing the ovary and ovules, which develop into the fruit with the seed, the latter producing the future indi vidual. The various organs of a plant in its most perfect form may be classified as follows: 1. Root. ORGANS OF NUTRITION-Necessary for life. 2. Stem. |