The green colour of the organs in question is easily proved to be almost entirely owing to the action of light. Plants which grow in the dark are of a sickly white, which is the case with any parts artificially or accidentally covered with earth, as in cultivated cellery or as paragus, whose stems and leaf-stalks are purposely managed in this way, to render their flavour and appearance more deli cate. Such blanched parts soon become green on exposure to light. Leaves are subject to a sort of disease, by which they become partially spotted or streaked with white or yellow. In this state they are termed variegated, and occasionally con tribute to the ornament of our gardens. The whiteness frequently extends to the leaf-stalk, and sometimes to the branch, as may be seen in the variegated alder. Such varieties are propagated by cut tings, layers, or roots, but not by seed. They appear to be somewhat more ten der than the plant in its natural state, One variety of the holly has, in addition to a yellow variegation, a beautiful tinge of purple, but this is a rare instance. In the amaranthus tricolor the leaves are naturally adorned with most beautiful and splendid colours, and some in other spe cies of the same genus with more uniform and less vivid tints.
The irritable nature of some leaves is remarkable, not but that all leaves may truly be said to possess irritability' with respect to light. The phenomena how ever to which we now allude are of the most striking kind. The sensitive plant, mimosa pudica, common in hot-houses, when touched by any extraneous body, folds up its leaves one after another, while their foot-stalks droop as if dying. After a while they recover themselves again. Each leaf of the dionma musci pula, or Venus's fly-trap, is furnished with a pair of toothed lobes, which, when touched near the base, fold themselves together, and imprison any insect that may be in their way. It is presumed that the air evolved by the body of the dead insect may be wholesome to the plant, for leaves are known to purify air im pregnated with carbonic acid gas, pro duced from the breathing of animals or the burning of a candle. The sarracenia, of which several species from America are now cultivated in our more curious gardens, bears tubular leaves which retain water in their hollows, and imprison in sects, whose putrefying bodies evidently produce a quantity of bad air, and analo gy leads us to suppose that air is destined to be serviceable to the constitution of the vegetable. See Dr. Smith's Intro duction, page 195.
Many important botanical distinctions are founded upon the situations and forms of leaves, These are explained by the following terms.
Fella radicalia, are radical leaves, as in the primsose.
F. caulina, stem, leaves, and ramea, branched leaves. The situation of the latter is either alternate, opposite, scat tered, or clustered. Several leaves stand ing round a stem or branch are termed Terticillata, whorled : such as either ter nate, quaternate, or quinate, &c.
F. insbricata, imbricated, lie one over the other like tiles upon a house.
F. decennia, cross each other in pairs alternately, as in many plants wilh oppo site leaves.
F. disticha, two-ranked, spread in two directions like the yew.
F. aecunda, unilateral, lean all towards one side. Some leaves are erect, other reflexed or recurved; but the greater part spread more or less horizontally. A few are obliquely twisted, and still fewer are reversed, resupinata, what should be the upper surface becoming the under, as in the beautiful alstrnmeria. Curt. Mag. t. 139.
F. petio/uta, are such as stand on foot stalks; seaside, sessile leaves, grow imme diately from the branch or root without any stalk.
P. peltata, peltate leaves, have the foot. stalk inserted into their centre, like the handle of a shield, to which the name al ludes, witness the common nasturtium trap xolum.
F. amplexicaldia, clasp the stem or branch with their base.
F. decurrentia, run down the same part in the form of a leafy border, as in many thistles.
F. connate are united at their base.
F. perfoliata have the stem running through them, as in hair's-ear, bupleurum rotundifolium.
F. vagina/ilia sheath the stem on each other, as in most grasses.
F. equitantia clasp each other in two op posite rows, being compressed at the base, as in many common species of iris.
The form of leaves is either simple, as in grasses, lilies, &c or compound, as in parsley, elder, roses, &c. Simple leaves are either integre, undivided, like those just mentioned, or lobed, like the vine, holly-hock, and many others.
The following forms of simple leaves respect their outline only.
Foliunt orbiculatum, as nearly circular as possible, which is very rare.
Subrotundum, roundish, is much more common.
Osmium, ovate, the shape of an egg, very frequent.
Obovatum, obovate, the same figure, with the broad end uppermost.
Ellipticutn or ovate, elliptical, or oval, being broa.lest in the middle.
°Mangum, oblong, several times longer than broad, without any very decided form.
Spatulatum, spatulate, of a roundish figure, tapering into an oblong base.
Cuumforme, wedge-shaped, broad at the summit, tapering down to the base.