An introduction to physiological and systematical botany/Chapter 17
OF THE SEVERAL KINDS OF FULCRA, OR APPENDAGES TO A PLANT.
The word Fulcrum, whose proper meaning is a prop or support, has been applied by Linnæus not only to those organs of vegetables correctly so denominated, such as tendrils, but also to various other appendages to the herbage of a plant, none of which are universal, or essential, nor is there any one plant furnished with them all. I prefer the English term Appendages for these organs in general, to Props, because the latter applies only to one of them. Seven kinds of these are distinguished by Linnaeus, nor do I find it necessary to enlarge that number.
1. Stipula. The Stipula, a leafy appendage to the proper leaves or to their footstalks. It is commonly situated at the base of the latter, in pairs, and is extremely different in shape in different plants.
The most natural and usual situation of the Stipulas is in pairs, one stipula on each side of the base of the footstalk, as in Lathyrus latifolius, Engl. Bot. t. 1108, whose stipulas are half arrow-shaped; also in Willows, as Salix stipularis, t. 1214, and S. aurita, t. 1487. In Rosa, Potentilla, and many genera allied to them, the stipulas are united laterally to the footstalk. See Potentilla alba, t. 1384. In all these cases they are extrafoliaceæ, external with respect to the leaf or footstalk; in others they are intrafoliaceæ, internal, and are then generally simple, as those of Polygonum, t. 1382, 756, &c. In a large natural order, called Rubiaceæ, these internal stipulas in some cases embrace the stem in an undivided tube above the insertion of the footstalks, like those of Polygonum just mentioned; in others, as the Coffee, Coffea arabica, and the Hamellia patens, Exot. Bot. t. 24, they are separate leaves between the footstalks, but meeting just above their insertion. The European Rubiaceæ have whorled leaves, as Asperula, Galium, Rubia, &c.; but Asperula cynanchica, Engl. Bot. t. 33, has sometimes two of its four leaves so small as to look like stipulas, seeming to form an intermediate link between such as have whorled leaves and such as have opposite ones with stipulas. The next step from Asperula is Diodia, and then Spermacoce. In the two last the bases of the stipulas and footstalks are united into a common tube.
Some stipulas fall off almost as soon as the leaves are expanded, which is the case with the Tulip-tree, Liriodendron tulipifera; in general they last as long as the leaves.
The absence or presence of these organs, though generally an indication that plants belong to the same natural order and even genus, is not invariably so. Some species of Cistus have stipulas, others none, which is nearly the case with grasses. The stipula in this, one of the most distinct of all natural orders, is peculiar, consisting of an internal white membrane crowning the sheath of their leaf, and clasping the culm. See Phalaris canariensis, Engl. Bot. t. 1310, and Lagurus ovatus, t. 1334. In Aria cærulea, t. 750, a few minute hairs supply its place, while Sesleria cærulea, t. 1613, and some maritime grasses, have scarcely more than the rudiment of a stipula; Old writers call this organ in grasses by a peculiar name ligula, and others denominate it membrana foliorum, but both terms are superfluous. A curious instance of stipulas supplying the place of leaves is observable in Lathyrus Aphaca, t. 1167, which has only one or two pair of real leaves on the seedling plants, and those soon disappear, serving chiefly to prove, if any proof were wanted, that the rest are true stipulas.
Remarkably scariose, Or dry membranous stipulas are seen in Illecebrum Paronychia, Fl. Græc. t 246, and in the genus Pinus.
2. Bractea. The floral leaf, a leafy appendage to the flower or its stalk. It is of a variety of forms, and sometimes green. sometimes coloured. The Lime-trees, Tilia enropæa, t. 610, and parvifolia, t. 1705, have a very peculiar oblong pale floral leaf, attached to the flower-stalk. The Lavenders, see Curt. Mag., t. 400 and 401, have coloured bracteas, and the Purple-topped Clary, Salvia Horminum, Fl. Græc. t. 20, exhibits a gradation from the proper leaves to green bracteas, and from them to coloured ones, which last are barren, or unaccompanied by flowers. Hence I am induced to believe this plant a mere variety of S. viridis, t. 19, all whose bracteas are green and fertile. Bartsia alpina, Engl. Bot. t. 361, and Melampyrum arvense, t. 53, display an elegant transition from leaves to coloured bracteas. The Orchis tribe have green leafy bracteas, different in size in different species. A most beautiful large and coloured bractea is produced in Mussænda frondosa, Hort. Mal. v. 2. t. 18, from one of the teeth of the calyx, also in M. glabra of Willdenow, and two new species brought from America by Mr. John Fraser. Spinous bracteas of a curious construction guard the calyx in Atractylis cancellata. Linnæus observes that no bracteas are to be found in the class Tetradynamia.
3. Spina. A Thorn. This proceeds from the wood itself, and is either terminal like Hippophae rhamnoides, Engl. Bot. t. 425, Rhamnus catharticus t. 1629; or lateral as Cratægus (or Mespilus) Crus-galli, tomentosa, parvifolia, &c.
Linnæus observes that this sometimes disappears by culture, as in the Pear-tree, Pyrus sativa, which when wild has strong thorns; hence he denominates such cultivated plants tamed, or deprived of their natural ferocity. Professor Willdenow, Principles of Bot. 270, considers thorns as abortive buds, and thence very ingeniously and satisfactorily accounts for their disappearance whenever the tree receives more nourishment. The permanent footstalks of the Gum Tragacanth shrub, Astragalus Tragacantha, are hardened into real Spines, as are the flowerstalks in Pisonia, as well as the stipulas of Xanthium spinosum and the Mimosæ.—Linn. Mss.
4. Aculeus, a Prickle, arises from the bark only, and comes off with it, having no connection with the wood, as in Rosa, Rubus (the Bramble Raspberry, &c.), and Zizyphus, Willd. Sp. Pl. v. 1. 1102.
This is not liable to disappear by culture, being very distinct in nature from the last.
5. Cirrus. A Tendril. This is indeed properly called a fulcrum or support, being intended solely to sustain weak and climbing stems upon more firm and sturdy ones. By its means such climbers often reach, in tropical forests, to the summits of lofty trees, which they crown with adventitious blossoms. Tendrils or claspers when young are usually put forth in a straight direction; but they presently become spiral, making several circumvolutions, by which they take hold of any thing in their way, and then assume a firmer texture. After accomplishing a certain number of turns in one direction, some tendrils have a power of twining subsequently the contrary way; many of them moreover are branched or compound, so that the chances of their meeting with a support are multiplied. The Vine, Vitis vinifera, the various species of Passion-flower, and the Pea or Vetch tribe afford good examples of spiral tendrils. The Virginian Creeper Hedera, or, as it ought to be called, Vitis quinquefolia, has branched tendrils, whose extremities adhere to the smoothest flint, like the fibres of Ivy. Gloriosa superba, Andr. Repos. t. 129, and Flagellaria indica, have a simple spiral tendril at the end of each leaf; for they belong to the Monocotyledones, the structure of whose whole herbage is generally of the most simple and compendious kind. The flower-stalks of Cardiospermum Halicacabum bear tendrils; but a most singular kind of tendril if it may so be called, which certainly has a right to the name of fulcrum, is found in the Annona hexapetala, Linn. Suppl. 270. The flower-stalk of this tree forms a hook, and grasps the neighbouring branch, serving to suspend the fruit, which is very heavy, resembling a bunch of grapes, and indicates the plant in question to be either a Michelia or an Uvaria.
6. Glandula, a Gland, is defined by Linnæus as a little tumour discharging a fluid. Such are abundant on the stalk and calyx of a Moss Rose, Curt. Mag. t. 69, and between the serratures of the leaf of Salix pentandra, Bay-leaved Willow; also on the footstalks of Viburnum Opulus, Engl. Bot. t. 332, and various species of Passion-flower. The liquor discharged is in the first mentioned instances resinous and fragran, in the latter a sort of honey.
7. Pilus. A Hair. This, according to the Linnæan definition, is an excretory duct of a bristle-like form. Such it undoubtedly is i t the Nettle, Urtica, Engl. Bot. t. 148, and t. 1236, whose bristles are tubular and pervious, having each a bag of poison at its base, like the fang of a serpent; as well as in numerous plants whose hairy coats exude a viscid moisture. But the hairs which clothe many plants are merely a protection against cold, heat, or insects. Sometimes they are hooked, sometimes branched and entangled, as in Mullein, Verbascum, t. 549, &c. In Croton, Sulanum, and Lavatera they have often a starry figure. Very generally they are found, under a microscope, to be curiously jointed. Some Begoniæ bear on their leaves flat little straps called by authors ramenta, shavings, instead of cylindrical hairs; but I know not that they at all differ in nature from the usual pubescence, nor do they merit to be particularly distinguished. Some of the natural order of asperifoliæ, as Echium, t. 181, and Lycopsis, t. 938, especially some exotic species of this order, are clothed with curious white hard tubercles from which their bristles proceed. Echium pyrenaicum, Desfont. Atlant. v. 1. 164, is an instance of this.
The pubescence of plants varies greatly in degree according to differences of soil or exposure; several kinds, as Mentha hirsuta, t. 447, 448, naturally hairy, being occasionally found smooth, but if transplanted they soon resume their proper habit. Yet the direction of the hairs or bristles proves a very sure means of distinguishing species, especially in the genus Mentha, the hairs about whose calyx and flower-stalk point differently in different species, and I have found it the only infallible distinction between one Mint and another. See Trans. of Linn. Soc. v. 5. 171. The accurate Dr. Roth has lately applied the same test to the species of Myosotis, which all botanists before him had either confounded under M. scorpioides, Engl. Bot. t. 480, or else separated upon vague principles. Some species of Galium are admirably characterized by the bristles of their leaves, or of parts of their leaves, being hooked backward or forward. We therefore accept the 272d maxim of Linnæus's Philosophia Botanica with that limitation which he himself has allowed in his commentary upon it. "The Pubescence," says he, "is a ridiculous distinction, being for the most part effaced by culture." After quoting examples, he concludes: "We are therefore not to have recourse to the hairiness or spines of plants but in case of absolute necessity." Such necessity every botanist will allow to have existed in the Menthæ and in Myosotis scorpioides; and though the degree of pubescence varies from culture, and even its structure be changeable, as in Hedypnois hispida, Engl. Bot. t. 554, and hirta, t. 555, its direction is I believe as little liable to exception as any character that vegetables present.