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Its figure is generally half cylindrical, frequently channelled on the surface presented to the heavens; but in some monocotyledonous plants it is perfectly cylindrical, and in others it is a thin leafy expansion, called the sheath, or vagina, surrounding the stem (fig. 57. a). If the petiole is entirely absent, which is often the case, the leaf is then said to be sessile. Generally the petiole is simple, and continuous with the axis of the leaf; sometimes it is divided into several parts, each bearing a separate leaf or leaflet (foliolum): in such cases it is said to be compound; each of the stalks of the leaflets being called petiolules or stalklets (ramastra, Jungius). In some leaves the petiole is continuous with the axis of the lamina, from which it never separates; in others the petiole is articulated with each stalklet; so that, when the leaf perishes, it separates into as many portions as there are leaflets, as in the Sensitive Plant. When an apparently simple leaf is found to be articulated with its petiole, as in the Orange, such a leaf is not to be considered simple, but as the terminal leaflet of a pinnated leaf, of which the lateral leaflets are not developed. This is an important difference, and must be borne constantly in mind by those engaged in the investigation of natural affinities.
At the base of the petiole, where it joins the stem, and upon its lower surface, the cellular tissue increases in quantity, and produces a protuberance or gibbosity, which Ruellius, and after him Link, called the pulvinus, and De Candolle coussinet (fig. 57. a). At the opposite extremity of the petiole, where it is connected with the lamina, a similar swelling is often remarkable, as in Sterculia, Mimosa sensitiva, and others this is called the struma, or, by the French, bourrelet, (fig. 57. b).
Occasionally the petiole embraces the branch from which it springs, and in such case is said to be sheathing; and is even called a sheath, or vagina, as in grasses (fig. 57. a). When the lower part only of the petiole is sheathing, as in Apiaceæ, that part is sometimes called the pericladium. In grasses there is a peculiar membranous process at the top of the sheath, between it and the blade, which has received the name of ligula (fig. 57. b); for the nature of
this process see page 145. In the Asparagus, the petiole has the form of a small sheath, is destitute of blade, and surrounds the base of certain small branches having the appearance of leaves; such a petiole has been named hypophyllium by Link. In Trapa natans, Pontedera crassipes, and other plants, the petiole is excessively dilated by air, and acts as a bladder to float the leaves; except being thus in a state of dilatation, it does not differ from common petioles: it has, nevertheless, received the name of vesicula from De Candolle, who considers it the same as the bladdery expansions of Fuci. The petiole is generally straight: occasionally it becomes rigid and twisted, so that the plant can climb by it. In Combretum it hardens, curves backwards, loses its blade, and by degrees becomes an exceedingly hard, durable hook, by means of which that plant is able to raise itself upon the branches of the trees in its vicinity.
When the petiole grows upon the angles of the stem it is called by Link p. synedrus; when between them, p. cathedrus.
It has been said that the figure of the petiole usually approaches more or less closely to the cylindrical: this, however, is not always the case. In many plants, especially of an herbaceous habit, it is very thin, with foliaceous margins; it is then called winged. There are, moreover, certain leafless plants, as the greater number of species of Acacia, in which the petiole becomes so much developed as to assume the appearance of a leaf, all the functions of which it performs. Petioles of this nature have received the name of Phyllodia (fig. 57. c). They may always be distinguished from true leaves by the following characters: 1. If observed when the plant is very young, they will be found to bear leaflets. 2. Both their surfaces are alike. 3. They very generally present their margins to the earth and heavens, not their surfaces. 4. They are always straight-veined; and, as they only occur among dicotyledonous plants which have reticulated leaves, this peculiarity alone will characterise them.
But, besides the curious transformation undergone by the petiole when it becomes a phyllodium, there are several others still more remarkable: among these the first to be noticed is the cirrhus or tendril (Capreolus and Clavicula of the old
botanists). It is one of the contrivances employed by nature to support plants by aid of others stronger than themselves. It was included by Linnæus among what he called fulcra; and has generally, even by very recent writers, been spoken of as a peculiar organ. But, as it is manifestly in most cases a particular form of the petiole, I see no reason for regarding it in any other light. It may, indeed, be a modification of the inflorescence, as in the Vine; but this is an exception, showing, not that the cirrhus is not a modification of the petiole, but that any part may become cirrhose.
In some cases the petiole of a compound leaf is lengthened, branched, and endowed with the power of twisting round any small body that is near it, as in the Pea: it then becomes what is called a cirrhus petiolaris. At other times, it branches off on each side at its base below the lamina into a twisting ramification, as in Smilax horrida; when it is called a cirrhus peduncularis. Or it passes, in the form of midrib, beyond the apex of a single leaf, twisting and carrying with it a portion of the parenchyma, as in Gloriosa superba; when it is said to be a cirrhus foliaris. De Candolle also refers to tendrils the acuminate, or rather caudate, divisions of the corolla of Strophanthus, under the name of cirrhus corollaris.
As another modification of the petiole, I am disposed to consider with Link (Elem. 202.) the singular form of leaf in Sarracenia and Nepenthes (fig. 58.), which has been called a pitcher (Ascidium, Vasculum). This consists of a fistular
green body, occupying the place and performing the functions of a leaf, and closed at its extremity by a lid, termed the operculum. The pitcher, or fistular part, is the petiole, and the operculum the blade of a leaf in an extraordinary state of transformation. This is found, by a comparison of Nepenthes and Sarracenia with Dionaea muscipula; in that plant the leaf consists of a broad-winged petiole, articulated with a collapsing blade, the margins of which are pectinate and inflexed. If we suppose the broad-winged petiole to collapse, and that its margins, when they meet, cohere, there would then be formed a fistular body like the pitcher of Sarracenia (fig. 58. B), and there would be no difficulty in identifying the acknowledged blade of the one with the operculum of the other. From Sarracenia the transition to Nepenthes (fig. 58. A) is obvious.
The student must not, however, suppose that all pitchers are petioles, because those of Nepenthes and Sarracenia are Those of the curious Dischidia Rafflesiana (fig. 59.), figured by Wallich in his Planta Asiatica Rariores, are leaves, the margins of which are united. The pitchers of Marcgraavia and Norantea (fig. 60.) are bracts in the same state.
Spines of the leaves are formed either by a lengthening of the woody tissue of the veins, or by a contraction of the
parenchyma of the leaves: in the former case they project beyond the surface or margin of the leaf, as in many Solana and the Holly (Ilex aquifolium): in the latter they are the veins themselves become hardened, as in the palmated spines of the Barberry; the spiny petiole of many Leguminous plants is of the same nature as the latter. So strong is the tendency in some plants to assume a spiny state, that in a species of Prosopis from Chili, of which I have a living specimen now before me, half the leaflets of its bipinnate leaves have the upper half converted into spines.
2. Of Stipules.
At the base of the petiole, on each side, is frequently seated a small appendage, most commonly of a texture less firm than the petiole, and having a tapering termination. These two appendages are called stipules. They either adhere to the base of the petiole or are separate; - they either endure as long as the leaf, or fall off before it; they are membranous, leathery, or spiny;- finally, they are entire or laciniated.