636. Revelations of the microscope. We have now completed a brief survey of the phenomena of visible vegetation. We commenced with the root and now the consideration of the seed with its embryo completes the circle and brings us around to the root again. We have studied hitherto superficially, as best we were able by the unassisted eye. But the microscope opens to us a new world in botany, more wonderful and fair, if possible, than that which we have already surveyed. No just appreciation of microscopic botany can be obtained from drawings or descriptions. Here the microscope itself is the only adequate teacher.
637. Next inquiries. We have seen and studied the general organs of vegetation and their metamorphoses; but of what are these organs made? What their structure within? What their office and use in the life and growth of the plant? These inquiries must next bo answered.
638. Structure of plants cellular. All forms of vegetable structure, however numerous and diverse, are alike compos3d of little bladders, called vescicles or cells. We can often discern the cells in some structures with the naked eye, as in the pith of elder, pulp of snowberry, and especially plain in the pulp of orange. Other structures, which appear as a solid mass to the eye, are seen at once, under the lens, to consist of cells also - even the most solid wood or the stony substance of the peach. A thin cutting (shaving) from the rhizome of the blood-root, magnified 100 diameters, appears in outline (to say nothing of its brilliant coloring) as here sketched (557). Therefore
639. The cell is the elementary organism which by its repetitions makes up the mass of all vegetation. It is defined as a closed sac composed of membrane containing a fluid.
640. The primary form of the cell is spheroidal. In some cases it retains this form during its existence, but generally, in growing, it takes new and various forms, which, on account of the two causes which control them, may be classed as inherent and casual.
641. The inherent forms of the cell, or those which depend upon its own laws of growth, may be referred to three general types;
(1) spheroidal, like pollen grains, the red snow-plant, the cells of leaf-tissue, etc., varying to oblong, or lobed, or stellate; (2) cylindrical, or tube-form, as most wood-cells are; (3) tabular or flattened, as the cells of the epidermis.
642. The casual forms result from external pressure, as of cells crowding against cells, in stems or pith. In this way spheroidal cells may become cubical, 8-sided, 12-sided, etc; tubiform cells prismatic, and tabular cells 4-angled, hexagonal, etc., in outline according to the original pattern.
643. In magnitude the plant cell varies from 1/100 to 1/300 of an inch in diameter; the more common size is about 1/300 inch. The cells of elder pith measure about 1/200 inch; those of parenchyma (leaf-tissue) about 1/400; consequently, 64,000,000 of them would occupy only one cubic inch. The cells of cork are computed to be 1/1000 inch in diameter - 1000 millions to a cubic inch.
644. But the length of some cells is much more considerable. Wood-cells measure 1/50 inch; bark cells, as flax, hemp, nearly 1/2 inch; the cells of some plant-hairs an inch or more.
645. The wall of the new cell consists of two layers; the outer one a firm, colorless membrane, made of cellulose, the inner a plastic, gelatinous layer applied to the outer, and chiefly concerned in cell-life and multiplication. This is called the primordial utricle.
646. It is best seen when treated with a weak solution of nitric acid, iodine, or alcohol. It thus becomes colored, contracts, and lies loose in the cell.
647. The cell wall is easily permeated by fluids flowing in and out It must, therefore, be regarded as porous; although it appears perfectly entire even under the highest magnifier.
648. A secondary layer is subsequently added to the outer layer, between it and the primordial utricle, as if to strengthen it. This new layer is seldom entire, but perforated and cleft in a great variety of patterns, leaving certain points or parts of the cell-wall still bare and discernible by their transparency. Hence the following varieties of cells:-
557, Section of the rhizome of Blood-root. a, a. A bundle of wood-cells. The shaded cells contain the color.
649. Wood cells, which are finally filled up by the repetitions of the secondary layers, leaving only minute points of the original cell-wall bare and transparent.
Forms of cells. 560, Wood-cells. 561, Cellular tissue of a rootlet, etc.
650. Pitted cells, a variety where larger transparent points appear, surrounded by 2 or 3 rings (pine and the Coniferae in general).
651. Spiral cells, where the secondary layer consists of spiral fibers or bands. There may be a single fiber, or several (2 to 20) united into a band. It is usually elastic and may be drawn out and uncoiled.
562, Polyhedral cells of parenchyma in pith of Elder. 563, Stellate cells in pith of Rush. 565, Spherical cells in Houseleek. 566, Wood-cells and ducts of Oak. 564, Wood-cells of the Flax-fiber.
These beautiful cells may be well seen in a shoot of elder, in the petiole of rhubarb, Geranium, strawberry. In the two latter, if gently pulled asunder, the coiled fibers appear to the naked eye.
652. Annular cells, when there are numerous rings within, instead of a spiral coil, as in the stems of balsam and some Cryptogamia.
653. Scalariform cells, when the rings seem conjoined by bars crossing between them, giving an appearance compared to a ladder (scala), as in the vine and ferns. Porous cells with the secondary layers full of perforations, reticulated cells, as if a net work; and many other forms.
654. Cellulose, the material of which the outer cell-walls and other secondary layers are made, is proved by a chemical analysis to consist of three simple elements, carbon, hydrogen, oxygen, in the proportions of C24 H20 O20, - carbon and the exact elements of water. In the material of the primordial utricle nitrogen is added. Out of these four simple elements (C H O N) with slight additions of lime, silex, and a few other earthy matters, God is able to produce all the countless varieties of plants which clothe and beautify the earth.
655. Contents of the cell. Some cells contain air only. Others arc filled with solid matter; but the greater part contain both fluids and solids. There is the cytoblast, a globular atom, earnest of new cells; and protoplasm, the nourishing semi-fluid, both of the same material as the primordial utricle, and with it, and the fluid cell-sap, ever flowing, acting, combining, transforming, and producing either new cells or products like the following.
656. The coloring matter, which gives to fruits and flowers their bright and varying tints of yellow, red, and blue, is generally dissolved in the cell-sap which is otherwise colorless; but
567, Cells, a, of the pulp of Snow-berry, showing the nucleus; b, of the parenchyma of the leaf of Pink, showing the granules of chlorophyll?. 569, Cell of a Cactus, soaked in Alcohol, the primordial utricle separated and contracted. 569, Coll of pleurenchyma of Pine, dotted. 570, Sketch to illustrate the nature of those dots; a. dot seen in front; b, a side view of the same. 571, Traehenchyma, a spiral cell from the sporange of Equisetum. 572, Spiral vessel of the Melon, single thread; 573, of the Elder, 4 threads. 574, Annular duct, distended by rings instead of a coil. 575, Scalariform vessels, from Osmunda (Fern). 576, A dotted duct from Gym-nocladus (Coffee-tree). 578. Spiral vessels apparently branched. 577, Branching spirals in the Gourd.
657. Chlorophylls, the green coloring matter of leaves, consists of green corpuscles floating in the colorless sap or attached to the colorless wall. In the indigo plant these corpuscles are blue and constitute that poisonous drug.
658. The starch of the plant also originates here, in the form of little striated granules of the same composition as cellulose (C24 H20 O20). Some 20 such granules appear in the same cell, either loosely or compactly filling it. Starch is nutritive matter, sealed up for preservation and future use.
Contents of cells. 579, Cells of Potato containing starch grains. 580, Starch grains from the potato; 5S1, from the E. Indian Arrow-root. 582, Raphides, acicular crystals, in a cell of Poly-anthes tuberosa. 583, Crystals in a cell of Cactus. 584, Cells from the palp of Pear, coated internally; a longitudinal section; 585, Transverse section. 586, Starch granules from W. Indian Arrow-root.
659. Gum, sugar, salts, acids, alkalies, poisons, medicines, whatever is peculiar in the properties of each vegetable substance, may also be held in solution in the cell-sap and invisible, unless forming
660. Raphides, little bundles of crystals, acicular or of some other form, seen in the cells of rhubarb, Cactus, Hyacinth.
661. The development of new cells in the plant is the process of its growth. This is accomplished within the pre-existing cells and by the agency of their contents. The primordial utricle divides itself into two or more utricles, by septa growing from its sides until they meet. These then acquire the cellulose layer outside, the cytoblast inside, at the expense of the old cell, which shortly gives place to its new progeny. Thus cells multiply, and by millions on millions build up the fabric of the plant.