« PreviousContinue »
9. Feather-veined (pennivenium), when the venæ primariæ of a reticulated leaf pass in a right line from the midrib to the margin, as in Castanea. This has the same relation to the radiating leaf that the curve-veined bears to the straightveined; it is the folium penninervium of De Candolle.
10. Hidden-veined (introvenium). To this I refer all leaves the veins of which are hidden from view by the parenchyma being in excess, as in Hoya, and many other plants. Such a leaf is often inaccurately called veinless. De Candolle calls a leaf of this nature, in which the veins are dispersed through a large mass of parenchyma, as in Mesembryanthemum, vaginervium.
In case it should be necessary to explain the direction that the primary veins take when they diverge from the midrib, this can be denoted by measuring the angle which is formed by the midrib and the diverging vein, and can either be stated in distinct words, or by applying the following terms: — thus, if the angle formed by the divergence is between 10° and 20°, the vein may be said to be nearly parallel (subparallela); if between 20° and 40°, diverging ; between 40° and 60°, spreading; between 60° and 80°, divaricating; between 80° and 90°, right-angled; between 90° and 120°, oblique ; beyond 120°, reflexed (retroflexa).
With regard to the forms of leaves, this subject properly enters into Glossology; because the terms applied by Botanists to differences in the outline of those organs are, in fact, applicable to any varieties in the figure of any other flat body. Nevertheless, as it may be a matter of convenience to the student to know upon what principles the most remarkable forms of leaves, or of other divided parts, are thought to be connected with each other, I here translate the observations upon the subject made by Alphonse de Candolle, whose Introduction to Botany may be supposed to embody the latest opinions of his father.
“ Leaves put on a multitude of forms, depending upon the manner in which they are severally organised, especially with regard to their division and the direction of their veins. These veins being in general symmetrical on the two sides of the midrib, leaves themselves are almost always of some
regular figure, as, for instance, oval, rounded, elliptical, &c. Their regularity, however, is never mathematical; and there are certain leaves, like those of the Begonia, the two sides of which are most remarkably unequal.
“ Leaves are either entire, that is, without toothings of any kind; or toothed in various ways upon their edges; or divided more or less deeply into lobes, which leave void spaces between them, which we call recesses (sinus).
“ Differences of this kind only become intelligible when one starts from the idea that a leaf is a mere expansion of tissue, in which the parenchyma is more or less extended, according to the divergence of the vessels that compose the veins, and to the degree of vegetating vigour of every species upon all points of its surface. In this expansion, which constitutes vegetation, it may be understood that a cellular tissue, mingled with firm parts like veins, ought to assume, especially at the edges, very different appearances. Each vein is to be considered as surrounded with parenchyma as well as the ligneous fibres of the stem. When this parenchyma stretches a great deal between the principal veins, and unites them completely up to their extremities, the leaf is entire ; but when the separation of the principal veins is greater, and the cellular tissue is comparatively less extended, the union of parenchyma takes place in only an imperfect manner, and thus lobes and openings are produced in the middle of the leaf, or various kinds of toothings in its circumference.
In support of this theory, which has originated with M. De Candolle, it must be remarked that the bladders of cellular tissue have a great tendency to grow together when they come in contact in a young state. The fluids which tissue secretes are more or less viscid; the growth of the bladders in diameter causes them to press against each other; they are extremely homogeneous in different parts of the same organ; all these may be supposed to concur in producing the pheno: menon of which the grafting of one plant upon another is the most striking example. The structure of flowers depends upon the existence of this tendency, as will be shown hereafter. With regard to leaves, Dracontium pertusum affords a verification of this theory in the irregular holes pierced through the middle of its blade between the veins. The more weak the developement of this leaf has been, the larger are the holes, which, in some instances, even extend to the margin, when the leaf beomes lobed. In this case it is difficult to deny that the parenchyma developes and combines more towards the edge of the leaf than in the centre; while, on the other hand, by a different direction and another mode of developement of the parenchyma, the contrary takes place in the greater part of leaves. The fact, that divisions are the deepest in those individuals of the same species whose vegetation has been least favoured by humidity and the nature of the soil, is a confirmation of this theory.
“ Palm trees seemed to offer an exception to this mode of accounting for the formation of lobes; but the recent observations of Mohl have demonstrated that those plants also are conformable to the theory. The leaves of Palm trees begin by being apparently simple, they then gradually divide from the extremity to the base of the blade, and there are on the edges of the divisions some ragged remnants, which look as if they indicated an actual rending asunder. But Mohl, by observing these leaves microscopically, when first developing, ascertained that these divisions never are intimately united at their edges, and that they are merely held together by a net of down. This may possibly depend upon the dry and leathery texture of their leaves, which causes the bladders to be converted into hairs instead of uniting in consequence of their great approximation. If the adhesion is incomplete, it is no wonder that the leaves should separate in proportion as the veins diverge by the enlargement of the leaf. Palm leaves, then, are not, as has been supposed, simple leaves which divide into lobes contrary to what happens in other plants ; they are divisions bordered by a parenchyma which has never been united to that of the division next it, and which, in consequence, does not tear, but only separates.
“ The unequal degrees of union of the parenchyma that surrounds the veins, combined with the arrangement of the latter, form the principles on which the nomenclature of divided leaves has been contrived.
“When the parenchyma between the primary veins is not united, so that the blade is composed of several distinct parts combined by the midrib only, the distinct portions or lobes are called segments. They differ from the leaflets of more compound leaves merely by the circumstance of not being jointed with their support, nor deciduous. A leaf having such segments is called dissected.
“ If the lobes are united near the base around the origin of these veins, we name them partitions, and the leaf is said to be parted.
“ Supposing the lobes to be united as far as the middle, they become divisions, their recesses are fissures, and the adjectives formed from these are made to end in fid, as multifid, quinquefid, &c.; this should not be applied to any cases in which the divisions extend below the middle of the veins; it is, however, frequently applied to cases of a division as deep as the midrib.
“ Finally, if the adhesion of the lobes is complete, and if the parenchyma which separates the extremity only of the veins is not extended to the extremity of the principal veins, or beyond them, the leaf is merely toothed (dentate); the salient parts are toothings. When the toothings, or teeth, are rounded, they become crenels, and the leaf is crenelled (or crenate). This form of leaf is not very important, because it is not connected with the arrangement of the primary veins, while that of the lobes, already mentioned, always is.
“ The terms that express precisely the important subdivisions of the leaf are combined with those which indicate venation. Thus a feather-veined leaf (pennivenium) may be either pennatisected, or pennatiparted, or pennatifid, according as it has segments, partitions, or fissures. In like manner a palm-veined leaf (this is what I call radiating, p. 133.) may be palmatisected, palmatiparted, or palmatifid ; and so on.
“ In like manner we say that a leaf is trisected, trifid, or triparted, when we would draw attention to the number and depth of the lobes of a leaf, rather than to the relation they bear to the veins. And, on the other hand, we may, by neglecting the number of the lobes, simply indicate their presence by saying that a leaf is pennatilobed, palmatilobed, and so on.
“ The lobes themselves are sometimes subdivided upon the same principle as the leaf itself. We then say that a leaf is bipennatisected, bipennatiparted, &c.; if the subdivisions of the lobes are themselves lobed, we may say tripennatisected, tripennatiparted, &c. Finally, in cases where leaves are extremely divided, and the parenchyma of the ultimate ramifications of the veins does not unite and form lobes, we say, in general terms, that the leaf is multifid, laciniated, decomposed, or slashed; terms which express the appearance of a leaf, without any very precise signification.”
With regard to compound leaves, their leaflets always have the primary veins running at an angle more or less acute towards the margin. « This is perfectly intelligible if we reflect that their lateral veins represent not the primary, but the secondary and tertiary veins of simple leaves, which latter are always pennated.
“ The leaflets of pennated leaves are usually placed opposite each other in pairs along a common petiole. These pairs of leaves are called in Latin juga : thus a leaf with one pair is unijugum ; with two pairs, bijugum, &c.
« Usually one of the leaflets terminates the petiole; the leaf is then unequally pinnated (imparipinnatum); but sometimes there is no odd leaflet, and the petiole ends abruptly, or in a point or tendril: this is equally pinnated, pari-pinnatum.
“ Sometimes the leaflets themselves are subdivided into other leaflets (folium bipinnatum, tripinnatum). In this case, the lateral petioles which bear the leaflets are called partial ; and the small supports of the leaflets themselves, stalklets ( petiolules).”
Such are De Candolle's ideas of the typical formation of leaves. They offer a convenient mode of studying the modifications in structure of these organs, and are, to all appearance, founded upon a
57 correct view of the subject.
The PETIOLE, or leafstalk (fig. 57., a—b), is what connects the blade with the stem, of which it was considered by Linnæus as a part. It consists of one or more bundles of fibro-vascular tissue surrounded by cellular substance.