Leaf

Submerged leaves, or leaves which are developed under water, differ in structure from aerial leaves. They have usually no fibro vascular system, but consist of a congeries of cells, which some times become elongated and compressed so as to resemble veins. They have a layer of compact cells on their surface, but no true epidermis, and no stomata. Their internal structure consists of cells, disposed irregularly, and sometimes having spaces which are filled with air for the purpose of floating the leaf. When exposed to the air these leaves easily part with their moisture, and become shrivelled and dry. In some cases there is only a network of filament-like cells, the spaces between which are not filled with parenchyma, giving a skeleton appearance to the leaf, as in Ouvirandra fenestralis (Lattice plant).

A leaf, whether aerial or submerged, generally consists of a flat expanded portion, called the blade, or lamina, of a narrower portion called the petiole or stalk, and sometimes of a portion at the base of the petiole, which forms a sheath or vagina, or is de veloped in the form of outgrowths, called stipules. All these por tions are not always present. The sheathing or stipulary portion is frequently wanting. When a leaf has a distinct stalk it is petiolate; when it has none, it is sessile, and if in this case it embraces the stem it is said to be amplexicaul. The part of the leaf next the petiole or the axis is the base, while the opposite extremity is the apex. The leaf is usually flattened and expanded horizontally, i.e., at right angles to the longitudinal axis of the shoot, so that the upper face is directed towards the heavens, or the lower towards the earth. In some cases leaves, as in Iris, or leaf-like petioles, as in Australian species of Acacia and Eucalyptus, have their plane of expansion parallel to the axis of the shoot, there is then no distinction into an upper and a lower face, but the two sides are developed alike; or the leaf may have a cylindrical or polyhedral form, as in mesembryanthemum. The upper angle formed between the leaf and the stem is called its axil ; it is there that leaf-buds are normally developed. The leaf is sometimes articulated with the stem, and when it falls off a scar remains; at other times it is continuous with it, and then decays, while still attached to the axis. In their early state all leaves are continuous with the stem, and it is only in their after growth that articulations are formed. When leaves fall off annually they are called deciduous; when they remain for two or more years they are persistent, and the plant is evergreen. The laminar portion of

a leaf is occasionally articulated with the petiole, as in the orange, and a joint at times exists between the vaginal or stipulary portion and the petiole.

Venation.

The arrangement of the fibro-vascular system in the lamina constitutes the venation. In an ordinary leaf, as that of the elm, there is observed a large central vein running from the base to the apex of the leaf, this is the midrib (fig. 1) ; it gives off veins laterally (primary veins). The venation is described as pin nate or feather-veined. In some cases, as sycamore or castor oil (fig. 2), in place of there being only a single midrib there are several large veins (ribs) of nearly equal size, which diverge from the point where the blade joins the petiole or stem, giving off lat eral veins. The venation is then palmate. The primary veins give off secondary veins, and these in their turn give off tertiary veins, and so on until a complete network of vessels is produced, and those veins usually 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 3) ; or with veins diverging from the base of the lamina in more or less paral lel lines, as in fan palms (fig. 7), 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 Dicotyledons.

Some plants, which in most points of their structure are mono cotyledonous, yet have reticulated venation ; as in Smilax and Dioscorea. 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 and mesembryanthemum, the veins are obscure. The function of the veins, which consist of vessels and fibres, is to form a rigid framework for the leaf and to conduct liquids.