It may be remarked that the hairs should be taken from flowers which have only just opened; for this curious and inexplicable rotation is connected with the growth of the cell; and when this has attained maturity, it no longer occurs. The phenomenon may be observed in many other hairs of plants, as those of common groundsel (Senécio vulgáris) ([Pl. I.] fig. 12 a, b), and in the cells of the leaves of some water-plants; but I must refer to the article “Rotation” in the Dictionary for further information.
The most important variety of hair is that derived from the Cotton-plant (a kind of Mallow), and forming the cotton of commerce. These hairs spring from the epidermis of the seeds. The cells composing it are very long and soft, becoming flaccid and easily bent when dry ([Pl. IX.] fig. 13).
Stings.—Stinging hairs or stings may be well illustrated by reference to the common large nettle (Urtíca dioíca). In this plant they consist of a thick-walled cell, bulbous at the base, which is imbedded in the epidermis ([Pl. I.] fig. 21), the pointed end being terminated by a very minute dilatation or knob. The sting contains an acrid liquid, which escapes when the little knob is broken off in wounding the skin, and produces the well-known irritation. By the side of the figure of the sting is represented the point of a fine needle (fig. 20), showing that the expression “sharp as a needle” has no force when microscopic bodies are in question.
Stom´ata (στὁμα, mouth).—On viewing a strip of epidermis, the observer will be sure to notice certain oval or roundish bodies ([Pl. I.] fig. 13 a), composed of mostly two kidney-shaped cells in apposition but leaving a chink between them; these are the stomata. They communicate beneath with the intercellular passages, of which they may be considered the mouths; and by their agency a direct communication is established between these passages and the air. The two cells which guard the orifice are termed the “guard cells.”
Stomata are most numerous on the under surface of leaves; they are entirely absent in plants growing under water, and in most of the lower plants. In many of the stomata, viewed in the ordinary way, the air situated between the guard cells is indicated by the black spot or dot present; but after a time, or by the application of a gentle heat to the slide, the air becomes displaced by the water, and their structure becomes very distinct.
In certain plants, the epidermis is imbued with flint or sil´ica; so that even when burnt to an ash the stomata are still quite distinct. Examples of this may be found in the stalk or culm of grasses, as in straw, the shining epidermis of which is siliceous; or the epidermis of canes. Among the lower plants, this peculiarity is especially curious in the species of Equisétum, or mares’-tails.
The manner in which the veins of leaves are arranged is worthy of special attention, as it forms one of the characters by which the two leading divisions of the Vegetable Kingdom are characterized. Thus in one of these divisions the veins are branched, so as to form a network throughout the leaf; the plants with these netted veins, to which belong our trees, shrubs, and most herbs, are the Dicotylédons, or Ex´ogens; while in the second division, the veins run parallel to each other, being little or not at all branched, and not forming a network. The plants with parallel veins, among which are our grasses, lilies, &c., are the Monocotylédons or En´dogens.
Stems.—In the stems of plants, the tissues are arranged round a centre; otherwise, in the simpler and lower plants, they agree in structure with leaves, the centre being occupied by some element of fibro-vascular tissue, as simple wood-cells, a few vessels or ducts.
In the higher or flowering plants, the stem exists in two distinct forms, corresponding to the differences above noticed in the arrangement of the veins of the leaves; these must be considered separately.
In the Dicotyledons or Exogens ([Pl. I.] fig. 36), the centre of the stem, in a transverse section, is seen to be occupied by the pith or medulla, which is represented in the figure by the innermost circle. Immediately outside and around this is a narrow ring, indicating the section of a sheath to the pith, and called the medullary sheath. Next comes a broad ring of wood of the first year’s growth (fig. 36 a), traversed, from the pith to the bark, by wedge-shaped paler rays, termed the medullary rays. Outside the first year’s wood is the newer and paler wood of the second year (b); and so on, a new ring of wood being added outside the preceding layer for each year of growth of the stem.