Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/120

Rh 110 BOTANY [NUTRITIVE ORGANS. In a leaf having many ribs they may converge towards the apex, as in Cinnamon, or they may diverge, as in Sycamore and the Castor-oil plant (fig. 101). Thus the primary veins give off secondary veins, and these in their turn give off ter tiary veins, and so on until a complete network of vessels is produced, and those veins usxially project on the under surface of the leaf. To a distribution of veins such as this the name of reticulated or netted venation has been applied. In the leaves of some plants there exists a midrib with large veins running nearly parallel to it from the base to the apex of the lamina, as in Grasses (fig. 102); or with veins diverging from the base of the lamina in more or less Kio. 102. Stem of a Grass (Pod) with leaf. The sheathing petiole gv ending in a process 17?, called a ligule; the blade of the leaf,/. Fio. 103. Leaf of Fan Palm (Chamccropf), showing the veins running from the base to the margin, and not forming an angular net-work. parallel lines, as in Fan Palms (fig. 103), or with veins coming off from it throughout its whole course, and running parallel to each other in a straight or curved direction towards the margin of the leaf, as in Plantain and Banana. In these cases the veins are often united by cross veinlets, which do not, however, form an angular network. Such leaves are said to be parallel-veined. The leaves of Monocotyledons have generally this kind of venation, while reticulated venation most usually occurs amongst Dicotyle dons. Some plants, which in most points of their structure are Monocotyledonous, yet have reticulated venation. Such have been called Dictyogens. In vascular Acotyledonous plants there is frequently a tendency to fork exhibited by the fibro-vascular bundles in the leaf ; and when this is the case we have fork-veined leaves. This is well seen in many Ferns. The distribution of the system of vessels in the leaf is usually easily traced, but in the case of succulent plants, as Hoya, Agave, Stonecrop,andMesembryanthernum, the veins are obscure, and the leaves are said to be hidden-veined. In the cellular leaves of Mosses a median vein of several layers of cells is often visible, but as there are no fibro-vascular bundles present this is considered a false venation, and they are styled veinless (Avenia). TABULAR ARRANGEMENT OF VENATION. A. I eticulated Venation. I. Unicostate. A single rib or costa in the middle (midrib). 1. Primary veins coming off at different points of the midrib. a. Veins ending in curvatures within the margin, and forming what have been called true netted leaves (Lilac). t. Veins going directly to the margin and forming feather-reined leaves (Oak and Chestnut). 2. Primary veins coming off along with the midrib from the base of the leaf. II. Multicostate. More than one rib. Authors usually give to these leaves the general name of costate or ribbed. 1. Convergent. Bibs converging, running from base to apex in a curved manner, as in Cinnamon and Melastoma. There is occasionally an obscure rib running close to the edge of the leaf, and called intra-marginal, as in the Myrtle. 2. Divergent. Eibs diverging or proceeding in a radiating manner, as in Sycamore, Vine, Geranium, Castor-oil plant. 13. Parallel Venation. The term parallel is not strictly applic able, for the veins often proceed in a radiating manner, but it is difficult to find a comprehensive term. This venation may be characterised as not reticulated. I. Veins proceeding transversely from midrib to margin, usually with convexity towards the midrib, as in Musa and Canna. II. Veins proceeding longitudinally from base to apex. 1. Veins more or less convergent, as in Iris, Lilies, Grasses. 2. Veins more or less divergent, as in Fan Palms. C. Furcate Venation. Veins dividing in a forked manner, as in the case of many Ferns. In all plants, except Thallophytes, leaves are present Forms o: at some period of their existence. In Cuscuta (Dodder), l eaves - however, we have an exception. The forms assumed Plate X. by leaves vary much, not only in different plants but in the same plant. It is only amongst the lower classes of plants, Mosses, Characese, &amp;lt;fcc., that all the leaves on a plant are similar. As we pass up the scale of vegetable life we find them becoming more and more variable. The, structures in ordinary language designated as leaves are; considered so par excellence, and they are frequently spoken of as foliage leaves. In relation to their production on the stem we may observe that when they are small they are always produced in great number, and as they increase in size their number and rapidity of growth diminish correspondingly. The cellular process from the axis which develops into a leaf is simple and undivided ; it rarely remains so, but in progress of growth becomes segmented in various ways, either longitudinally or laterally, or in both ways. By longitudinal segmentation we have a leaf formed consisting of vagina, petiole, and lamina; or one or other of these may be absent, and thus stalked, sessile, sheathing, &c., leaves are produced. Lateral segmentation affects the lamina, producing indentations, lobings, or fissuring of its margins. In this way two marked forms of leaf are produced (1.) Simple form, in which the segmenta tion, however deeply it extends into the lamina, does not separate portions of the lamina which become articulated with the midrib or petiole ; and (2.) Compound form, where portions of the lamina are separated as detached leaflets (foliola), which become articulated with the midrib or petiole. In simple leaves, then, there is never more than one articulation, which is at the point of their insertion on the stem. In compound leaves there are one or more articu lations beyond the point of insertion on the stem. In both simple and compound leaves, according to the amount of segmentation and the mode of development of the parenchyma and direction of the fibro-vascular bundles, many forms are produced. Simple Leaves. When the parenchyma is developed Simple symmetrically on each side of the midrib or stalk, the leaf leaves, is equal; if otherwise, the leaf is unequal or oblique (fig. 100). If the margins are even and present no divisions, the leaf is entire (fig. 104); if there are slight projections of cellular or vascular tissue beyond the margin the leaf is not entire ; when the projections are irregular and more or less pointed, the leaf is dentate or toothed ; when they lie regularly over each other, like the teeth of a saw, the leaf is serrate (fig. 100); when they are rounded the leaf is crenate. If the divisions extend more deeply into the lamina than the margin, the leaf receives different names according to the nature of the segments ; thus, when the divisions extend about half-way down (fig. 105), it is cleft (fissits), and its lines of separation are called fissures : when