« PreviousContinue »
They are particularly numerous, and highly developed, upon the petioles and the backs of the leaves of Ferns. They consist of cellular tissue alone, without any vascular cords, and are known from leaves not only by their anatomical structure, but also by their irregular position, and by the absence of buds from their axils. The student must particularly remark this, or he will confound with them leaves having a ramentaceous appearance, such as are produced upon the young shoots of Pinus. Link remarks, that they are very similar in structure to the leaves of mosses. The term striga has occasionally been applied to them (Dec. Théor. Elém. ed. 2. 376. Link, Elém. 240.); but that word was employed by Linnæus to designate any stiff bristle-like process, as the spines of the Cactus, the divaricating hairs of Malpighia, and the stiff stellated hairs of Hibiscus. So vague an application of the term is very properly avoided at the present day, and the substantive is rejected from modern glossology; the adjective term strigose is, however, occasionally still employed to express a surface covered with stiff hairs.
GLANDS are small collections of firm cellular tissue, which is often much harder and more coloured than that which surrounds it. They are of several kinds.
Stalked glands (fig. 16. a, b, c, d, f, h, i, l, m, n,) are elevated on a stalk which is either simple or branched: they secrete some peculiar matter at their extremities, and are often confounded with the glandular hairs above described, from which they have been well distinguished by Link. According to that botanist, they are either simple (fig. 16. a, b, d, g, h, i,) or compound (c, f, k, l, m ); the former consisting of a single cell, and placed upon a hair acting as a simple conduit, occasionally interrupted by divisions; the latter consisting of several cells, and seated upon a stalk containing one or more conduits, formed by rows of cellular tissue. They are common upon the rose and the bramble, in which they become. very rigid, and assume the nature of aculei. For the sake of distinguishing them from the latter, they have been called sete by Woods and myself, but improperly; they are also the aiguillons of the French. In Hypericum they abound on the calyx and corolla of some species, but do not give out any exudation; they contain, however, a deep red juice within their cells. In some Jatrophas they are much branched; in many Rutacea they form a curious humid appendage at the apex of the stamens. Lately the glandular apparatus of plants has received the attention of Meyen, who has published on the subject an elaborate paper, from which the foregoing figures are taken. He admits the distinction of simple and compound glands, and regards them both as unquestionable organs of secretion. To some of the former he assigns occasionally more cells than one in the gland that terminates them; but the hair to which they belong is always simple. Of the compound glands some are hollow, others solid. They both vary exceedingly in form. Of the above figures the following brief descriptions will serve to illustrate the subject sufficiently for an introductory work :
Simple Glands:-a, a simple stalked gland, from the outside of the flower of Sinningia barbata; it consists of a cylindrical cell springing from the epidermis, then of two smaller cells, and finally of a fourth, which is the gland; b is a gland composed of six cells, of which the two lower are small, cylindrical, and colourless, forming a stalk, the four upper spherical, larger, and filled with secreted matter; d, simple pestle-shaped
glands of Sisymbrium chilense; in the pestle there is a yellow volatile oil; at the base are three cells of the epidermis; g, from the inside of the under-lip of Antirrhinum majus: the cell containing the glandular matter is at first cylindrical; it then forms a head, from which another cylindrical joint is emitted, to which a second head is afterwards formed, and upon which another joint with its head is eventually developed ; h is a stalked gland from the stem of the same plant; ¿, a double-headed gland from the flower stalk of Lysimachia vulgaris.
Compound Glands:-c, a compound sessile gland of Dictamnus albus, consisting of a skin which is colourless, and a centre which is filled with a thick green etherial oil; f, compound glands from the flower stalk of Sanguisorba carnea; k, a side view of the compound gland of the hop, which chemists call Lupulin (Meyen entirely denies the accuracy of Raspail's description of this body); l, a compound red gland from Ailanthus glandulosa; m, oblong stalked glands from Begonia platanifolia they resemble drops of resin, or something of that sort.
Other modifications of glandular apparatus are what some botanists call papulæ, or papillæ (fig. 16. e and n.)
Glandulæ utriculares of Guettard; these are transparent elevated points of the epidermis, filled with fluid, and covering closely the whole surface upon which they appear. In other words, they are elevated, distended bladders of the epidermis. The presence of papillæ upon the leaves of the ice plant gives rise to the peculiar crystalline nature of its surface.
There are, moreover, in many plants internal glands, that is to say, collections of cells densely compacted, and filled with secreted matter which hardens them, or renders them transparent. They are in some cases nearly of the nature of cysts, already described. In Dictamnus alba they form spherical nuclei, lying just below the cuticle, and filled with an etherial oil, rich in resin and camphor (fig. 10.) In Nepenthes they occur in two different states; the one as angular nuclei below one of the forms of stomate found in that plant; the other as hard, deep brown disks, lining the cavity of the pitcher, sunk below the epidermis, through which there are
openings corresponding with them, and no doubt forming the apparatus by which the water contained in the pitcher is secreted. They have been noticed and figured by Meyen, but were long before mentioned in this work (1835), and have been figured by me in the second volume of Lady's Botany (t. 47.). The opening through the cuticle immediately above them shows that they are internal organs; nevertheless, Meyen considers them external glands. Internal glands are very common in Labiatæ.
Sessile glands, verruce, or warts, are produced upon various parts, and are extremely variable in figure. In Cassias, they are seated upon the upper edge of the petiole, and are usually cylindrical or conical; in Cruciferous plants they are little roundish shining bodies, arising from just below the base of the ovary; in the leafless Acacias they are depressed, with a thickened rim, and placed on the upper edge of the phyllodium; they are little kidney-shaped bodies upon the petiole of the Peach and other drupaceous plants; and they assume many more appearances. They are common upon the petiole, as in Passiflora; they are also found upon the calyx, as in some species of Campanula, and at the serratures of the leaves, when they are considered by Röper (De Floribus Balsaminearum, p. 15.) to be abortive ovules; and they appear upon the pericarp and the skin of the seed; in the latter case they are called spongiola seminales by De Candolle. In figure they are round, oblong, or reniform, and occasionally cupulate, when they receive the name of glandes à godet (glandulæ urceolares) from some French writers. Warts are the glandes cellulaires of Mirbel; but they must not be confounded with the glandes vasculaires of the same writer, which are not mere excrescences of the epidermis, but modifications of well known organs. (See Discus, further on.) Of this nature are the hypogynous glands of Cruciferous plants aleady referred to.
Lenticular glands (Lenticelles of De Candolle; Glandes lenticulaires of Guettard;) are brown oval spots found upon the bark of many plants, especially willows: they have been thought to indicate the points from which roots will appear if the branch be placed in circumstances favourable to their production, and are considered by De Candolle to bear the
same relation to the roots that buds bear to young branches. (Premier mém. sur les Lentic., in the Ann. des Sciences Naturelles.) In Tree ferns it is hardly posssble to doubt that the tubercles so common on the surface of the trunk, are the points of roots either prepared for developement, or arrested in their growth by the dryness of the air that surrounds them; for we find (in Dicksonia arborescens, for instance) that the part of the stem which is next the ground is covered with roots, and the part above it, surrounded by drier air, is covered with tubercles. But it is not at all improbable that the lenticular glands of the stems of ordinary trees, and the tubercles of tree ferns are different bodies, although confounded under one name.
It is extremely doubtful whether true lenticular glands are any thing more than portions of the epiphloum, disorganised by some unknown power. Mohl states that they are found in the epiphloum, that is, between the epidermis and the mesophloum, and consist of greenish or colourless (or in Berberis yellow, and Sambucus red) cells which lie in rows perpendicular to the axis of the branch, and united towards the interior with the mesophlæum. He considers them a partial formation of cork.* Unger compares the true lenticular glands to the Soredia of Lichens, and the reproductive granulations of Jungermanniaceae; and he considers them in some way connected with the respiratory process: even as obliterated respiratory organs. Meyen regards them, not as obliterated respiratory organs, but as formations intended to maintain an air communication between the exterior rind and the new green bark of trees; for he says that the tissue of old bark is so compactly combined as to cut off all direct communication between the air and the cavernous parenchyma of the green bark.
6. Of Prickles.
PRICKLES (aculei) are rigid, opaque, conical processes, formed of masses of cellular tissue, and terminating in an
* I take this from Taylor's Magazine, xii. 58, where there is an (imperfect?) translation of Meyen's report on this and other subjects. Mohl's original paper I have not seen, and the translation is in part unintelligible.