Shape of the leaves


The leaf performs functions of vital importance for a plant, it is the main seat of gaseous exchanges with the outside and of chlorophyll photosynthesis, the leaf is the lung and the energy center of a plant. Even without being botanical experts we know that nature has indulged in creating leaves of different shapes. The leaves can be large or small, simple or compound, in some plants that live in extreme environments, such as deserts, they have turned into thorns. A leaf is essentially composed of three parts; starting from the point of insertion in the branch, we find: the sheath, the petiole and the lamina in which we distinguish an upper and a lower page. This is the general scheme but in many cases up to two of these parts may be missing. The sheath may be missing, in this case the leaf is said to be petiolate, if the petiole is also missing, the leaf is sessile. Let’s try to analyze the shapes that the leaves can take, also trying to clarify some terms used for their description.

The leaves in which the length is much greater than the width can be: needle-like, long and narrow, not flat but cylindrical as in firs and pines; lesiniform if awl-shaped, a small awl, as in juniper; linear when it is long, narrow and flat; lanceolate if spear-shaped as in willow.If the length is not much greater than the width, the leaves can be: ovate when the widest area is towards the base, as in the birch; obovate when the widest zone is towards the apex as in oak; elliptical when the widest area is in the center as in the elm; oblong when their margins are more or less parallel; the leaves can also be spatulated or round. Most often the shape of the leaves it does not exactly correspond to the types described but to intermediate forms between one type and the other, we can have linear-lanceolate, ovate-round leaves and so on.

The shape of the simple leaves

The margin of the leaf, or more properly of the lamina, can be smooth as in the olive tree or have incisions. If the incisions are shallow the margin can be: wavy if the incisions are similar to waves as in the beech; crenate if the protrusions have a wavy outline, as in oak; dentate if there are protrusions directed outwards, as in the strawberry tree; notched if the teeth are perpendicular to the margin, as in cherry; serrated if the teeth are inclined towards the apex, as in hornbeam. If the incisions are deep, the leaves are said to be incised, we can have lobed leaves, if the incisions form lobes that do not reach the middle of the leaf, as in maple. If the projections, which arise from the marginal incisions, are arranged to the right and left of the median axis of the leaf, like the beards of a feather the leaf is pinnatolobata, if the lobes diverge like the fingers of an open hand the leaf is palmatolobata. Leaves with entire margins, slightly damaged or with incisions that do not reach the central vein, such as those we have seen, are called simple leaves.

Leaf Shape: The shape of compound leaves

If the marginal incisions reach the central rib, making the lobes become independent, so that the leaf resembles a twig, the leaf is said to be composed. The parts of the lamina are called leaflets, the central axis is called the rachis. The compound leaves are pinnate, when the leaflets are aligned to the right and left like the barbs of a feather, and palmate when they are arranged in a fan shape. The pinnate leaves are divided into imparipinnate and paripinnate, depending on whether or not there is a terminal leaflet, which makes the number of leaflets odd if present, even if missing. A leaf composed of only three leaflets, as in strawberry or clover, is called trifoliate. It can be bipinnata (twice pannata), as in the mimosa, and tripennata (three times pinnata).

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