LEAF-CHARACTERS. 75 air, by which the specific gravity of the leaf is diminished, and it is thus enabled to float in the water. Submersed leaves have no true epidermal layer, and no stomata, both of which would be useless from their being always exposed to similar hygrometric conditions. (3) The External Features of Leaves.*-The point by which a leaf is attached to the stem or branch is called its insertion. The first leaves are called cotyledons, and those that next succeed them are termed primordial. Leaves are called radical when they arise at or below the surface of the ground, from a shortened stem, or crown of the root, as it is commonly called. Leaves which arise. from the main stem are called cauline, those from the branches ramal, and the modified leaves of flower stalks are termed bracts. Other forms of insertion are denoted by the terms petiolate, sessile, peltate, amplexicaul or embracing, sheathing, decurrent, perfoliate, and connate. As to arrangement, leaves are alternate, opposite, whorled, decussate, or fascicled. They may be either inflexed, conduplicate, plaited, circinate, convolute, involute or revolute, in the bud; and in relation to each other they may be valvate, twisted, induplicate, equitant, half-equitant or obvolute, and supervolute. The leaf-blade may be hidden-veined, netveined, parallel-veined, or fork-veined, while net-veined leaves may be radiate or pinnate or ribbed veined. The blades of simple leaves differ in margin, incision, apex, general outline, and form. Margins are entire, serrate, retroserrate, biser * As the external features of plants can only be learned by direct observation, and as this portion of the science of botany should be made familiar first of all, we assume that it has been already learned, and give here only a brief enumeration of the botanical characters of leaves. Any pupil who has begun the study of this book, and has found that the terms above employed do not call to mind the forms and structures they denote, should at once set himself to the study of leaves. The "Descriptive Botany," in Appletons' series of "Science Text-Books," will be found a practical and sufficient guide to the scientific observation of plants. 76 ANOMALOUS LEAF-FORMS. rate, serrulate, dentate, twice dentate, crenate, bicrenate, crenulate, wavy, and curled. Incisions take the form of lobes or of segments, according to the depth of the fissures, which is indicated by the terms fid and partite following their number, as trifid, tripartite. Obtuse, blunt, retuse, emarginate, and truncate apexes have to be noted. In general outline leaves are equal or oblique. They may be linear, needleshaped, awl-shaped, lanceolate, oval, oblong, ovate, obovate, wedge-shaped, spatulate, cordate, obcordate, kidney-shaped, arrow-shaped, hastate, auricled, rotund or subrotund, orbicular, or combinations of these, as cordate-ovate. In form— that is, solid configuration-they may be cylindrical, pyramidal, conical, etc., or, when hollowed out, are said to be tubular, hooded, and the like. Compound leaves are pinnate or digitate, and are described by the number and arrangement of their leaflets, and by their petioles, petiolules, rachis, joints, and pulvinus. The forms of petioles and the ligule, stipules, and stipels have to be studied. Stipules are adnate, leafy, opposite, axillary, ochreous, or interpetiolar. They are very rare in Monocotyledons, unless we regard the ligule of grasses as analogous to them. They are wholly absent in Acotyledons. (4) Anomalous Forms of Leaves.-As the branches of a stem sometimes take the form of spines or tendrils (see pages 56, 57), so the parts of a leaf may assume similar modifications, as well as others still more remarkable. Spines of Leaves.-The veins of leaves may project beyond the blade, and become hard and spiny, or the whole lamina may become spinous. The petiole may assume a spiny character, either at its apex or at its base, or the stipules may become transformed into spines. Tendrils of Leaves.-Any part of the leaf may also become transformed into a tendril. The midrib of the blade of a simple leaf may project beyond the apex, and form a tendril, or some of the leaflets of a compound leaf may become transformed into branched tendrils. The petiole may become cirrhose, and the stipules may assume the form of tendrils. Phyllodes. In some leaves the parts forming the fibro-vascular tissue of the petiole, instead of remaining till they reach the blade before separating, begin to diverge as soon as they leave the stem or branch, and become connected by parenchyma as in the ordinary blade of a leaf. To such petiole the name of phyllode has been applied. The phyllode is sometimes terminated by a true compound blade, and its nature is thus clearly ascertained, but in most instances no such blade is produced. These phyllodes may be distinguished from true blades, not only by the occasional production of a compound lamina, as just mentioned, but also by other circumstances. Thus: Ist. By their venation, which is more or less parallel instead of reticulated, as is the case generally in Dicotyledons, in which class of plants they alone occur. 2d. By their being placed nearly or quite in a vertical direction--that is, turning their margins upward and downward instead of their surfaces. And 3d. By their two surfaces resembling each oth FIG. 119.-A portion of the stem with some leaves of Venus's Flytrap (Dionea muscipula). 1. Lamina fringed with hairs, and hence said to be ciliated. p. Winged petiole. er, whereas in true blades a manifest difference is commonly observable between their upper and lower surfaces. (See also Leaves of Dicotyledonous Plants, page 78.) Frequently the petiole presents at its two edges a leaflike border, called a wing, when it is said to be winged, as in Venus's Flytrap (Fig. 119, þ) and many other plants. Ascidia or Pitchers.-These are the most remark 78 ASCIDIA OR PITCHERS. able of all the anomalous forms presented by leaves. They may be seen in the species of Nepenthes or Pitcher Plants (Fig. 120), in the species of Sarracenia (Fig. 121), and in many other plants. These curious organs may be either formed from the petiole, or the blade of the leaf. Thus, in the species of Sarracenia (Fig. 121), the pitcher appears to be produced by the folding inward of the two margins of a phyllode, which unite below, and form a hollow body or pitcher; but they are still separate above, and thus indicate its origin. In the Nepenthes (Fig. 120), the petiole first expands into a phyllode, then assumes the appearance of a tendril, and penthes distillatoria). ultimately forms a pitcher, p; this is 2. Pitcher closed by the closed above by a lid, l, called the operculum, which is united to it by an articulation, and is commonly regarded as a remarkable transformation of the blade. FIG. 120.-Pitcher of a spe lid, l. (5) General View of the Leaves in the Three Classes of Plants.The leaves of plants present certain marked differences, which may be summed up as follows: I. Leaves of Dicotyledonous Plants. In these the venation is commonly reticulated; but in a few plants, as Ranunculus lingua, the so-called blades have parallel veins, and have been therefore considered by some botanists as presenting exceptions to the ordinary reticulated venation of Dicotyledons; but these are not usually regarded as true blades, but as varieties of phyllodes or transformed petioles, FIG. 121.-Pitcher of rea. GENERAL VIEW. 79 from which they only differ in being placed horizontally. (See page 77.) The leaves of Dicotyledons are very commonly articulated to the stem or branch, often compound, and variously indented at their margins, or incised in different ways. 2. Leaves of Monocotyledonous Plants.-In these the venation is commonly more or less parallel; but the leaves generally of plants of the order Smilaceæ and of four other allied orders, as well as some in the Araceæ, present exceptions to this character, for in them the veins branch in various directions and form a network, as in the leaves of Dicotyledons. Some of these plants, as the Smilaceæ and the four allied orders, were therefore separated from other Monocotyledons by Lindley, and placed in a class by themselves, called Dictyogens, from the Greek word signifying a net. But this class has not been accepted by botanists. In Monocotyledonous plants the leaves are also usually not articulated; and the margins of their blades are generally entire. They are also commonly simple; often sheathing at the base; and seldom have stipules, unless the ligule is to be considered as the analogue of those organs. 3. Leaves of Acotyledonous Plants.-In plants of this class, when the leaves or fronds have veins, these are commonly arranged in a more or less forked manner. Such leaves are usually not articulated; either sessile or stalked; frequently toothed or incised in various ways; and often highly compound. |