Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/122

Rh 112 BOTANY [NUTRITIVE ORGANS. down to the stalk, as in the Daisy ; subulate, narrow and tapering like an awl ; acuminate, or drawn out into a long point, as inFicus religiosa; mucronate, with a hard stiff point or mucro at the apex. When the parenchyma is deficient at the apex so as to form two rounded lobes, the leaf is obcordate or inversely heart-shaped ; when the deficiency is very slight the leaf is called emarginate (fig. 1 1 2), as having a portion taken out of the margin ; when the apex is merely Flattened or slightly depressed the leaf is retuse ; and when the apex ends abruptly in a straight margin, as in the Tulip-tree, the leaf is truncate. When the veins at the base of the lamina are prolonged downwards at an obtuse angle with the midrib, and rounded lobes are formed, as in TA Fig. 111. Fig. 112. Fig. 113. FIG. 111. Oblong leaf of a species of Senna. Compound leaves. Dog-violet, the leaf is cordate or heart-shaped ; it is kidney- shaped or reniform (fig. 113), when the apex is rounded as in Asarum. When the lobes are prolonged down wards and are acute, the leaf is sagittate (fig. 114); when they proceed at right angles, as in Rumex Aceto- sella, the leaf is hastate or halbert-shaped. When a simple leaf is divided at the base into two leaf-like appendages, it is called auriculate. When the veins spread out in various planes, and there is a large development of cel lular tissue, so as to produce a succulent leaf, such forms occur as conical, prismatical, ensiform or sword-like, acinaciform or scimitar-shaped, and dolabriform or axe- shaped. When the development of parenchyma is such that it more than fills up the spaces between the veins, the margins become u-avy, crisp, or undulated, as in Rumex crispus and Rheum undulatum. By cultivation the cellular tissue is often much in creased, giving rise to the curled leaves of Greens, Savoys, Cresses, Lettuce, &c. In Rushes the shoots which act as leaves are often terete. They are either barren or bear flowers. Their cellular tissue is often stellate, and the shoots sometimes exhibit a pe culiar spiral twisting. Amongst parallel veined leaves the margins are usually entire, espe cially when the veins converge. Compound leaves are those ill which the divi- Septenatc sions extend to the mid- &quot;&amp;gt; 115 of the Horse-chestnut (JStcu- &quot;*,, J/ vp ca!ianiJ i)- Such leaves, espe- .-,, . , Clall y when there are five leaflets, are called rib or petiole, and the digitate. separated portions become each articulated with it, and 11G receive the name of foliola or leafltts. The midrib, or petiole, has thus the appearance of a branch with separate leaves attached to it, but it is considered properly as one leaf, because in its earliest state it arises from the axis as a single piece, and its subsequent divisions in the form of leaflets are all in one plane. The leaflets are either sessile (fig. 115), or have stalks, called jjetiolules (fig. 116), accord ing as the vascular bundles of the veins spread out or divari cate at once, or re main united for a certain length. Com pound leaves have been classified ac cording to the na ture of the venation and the development of parenchyma. If we suppose that in a simple feather- veined unicostate leaf the divisions extend to the mid rib, and each of the primary veins spreads out or branches, and be comes surrounded with parenchyma, i AT. l a L i.1, Impari-pinnate (unequally pinnate) leaf of Rolsima. and the leaflets thus There are nine pairs of shortly-stalked leaflets fnrmprl liprnmr&amp;gt; ir- (foliola, pinnae), and an odd one at the extremity. ar At the base of the leaf stipules s are seen. ticulated to the pe tiole or midrib (fig. 116), the leaf becomes compound and pinnate. If the midrib and primary veins are not covered Plate X with parenchyma, while the secondary (or those coming off in a feather-like manner from the primary veins) are, and separate leaflets are thus formed which are articulated with the veins, the leaf is bipinnate. In this case the secondary veins form as it were partial petioles. A farther subdivision, in which the tertiary veins only are covered with parenchyma and have separate leaflets, gives tripinnate or decompound forms, in which case the tertiary veins form the partial petioles. A leaf divided still more is called supradecom- pound. When a pinnate leaf has one pair of leaflets it is unijuyate ; with two pairs, it is lijugate ; with many pairs, multijurjate (fig. 116). When a pinnate leaf ends in a pair of pinnae it is equally or abruptly pinnate (pari- pinnate) ; when there is a single terminal leaflet (fig. 116), the leaf is unequally pinnate (impari-pinnate) ; when the leaflets or pinnae are placed alternately on either side of the midrib, and not directly opposite to each other, the leaf is alternately pinnate ; and when the pinnae are of different sizes, the leaf is interruptedly pinnate. In the case of a simple multicostate leaf with radiating venation, if we suppose the ribs to be covered with parenchyma, so as to form separate leaflets, each of which is articulated to the petiole, the digitate form of compound leaf is produced ; if there are three leaflets the form is ternate or trifoliate ; Plate V, if four, quaternate ; if five, quinate ; if seven, septenate ; and so on. If the three ribs of a ternate leaf subdivide each into three primary veins, which become covered with parenchyma so as to be separate articulated leaflets, the leaf is liternate ; and if another threefold division takes place, it is triternate. The petiole or leaf-stalk is the part which unites the limb Petiole, or blade of the leaf to the stem (fig. 99, p}. It is absent in sessile leaves, and this is also frequently the case when the vagina is present, as in Grasses. It consists of a mass of fibro-vascular bundles with a varying amount of cellular