In angiosperm anatomy, a leaf can be identified by where it emerges from the node. In a node, a leaf emerges below the axillary bud.
Leaf Parts
Leaves are generally composed of a few main parts: the
blade
and the
petiole
.
Figure \(\PageIndex{2}\): A leaf is usually composed of a blade and a petiole. The blade is most frequently the flat, photosynthetic part. The petiole is a stem that attaches the leaf blade to the main stem of the plant. As plants have radiated, diversified, and adapted to different environments, you'll see that there are many variations on this theme. The photo on the left is a palmate leaf, the diagram on the right is a pinnate leaf. Photo by Maria Morrow,
CC-BY 4.0
. Diagram on the right from Chapter 3.4.2, Botany, by Algiers, Ha, and Morrow.
The Leaf Blade and Vascular Arrangement
The leaf blade is (usually) the flat, photosynthetic part of the blade. In eudicots, the leaf will have a central midvein (also called the midrib), with smaller veins branching off from there. This type of vein organization is called
netted venation
. The edge of the blade is the
margin
.
In monocots, the veins may be all approximately the same size or there may be a larger midvein, but the veins run parallel to each other. This is called
parallel venation
.
The Petiole
Most leaves have a stem that attaches the blade of the leaf to the rest of the plant. This is the petiole. However, in some plants, the leaves do not have a petiole and the blade is directly attached to the plant stem. These leaves are
sessile
(lacking a petiole).
Stipules
Some plants will have paired appendages found at the base of the leaf. These are called
stipules
. Stipules can look leaf-like or take on other forms (e.g. spines or tendrils).
Compound Leaves
A
compound leaf
looks like a branch with leaves emerging from it. However, these leaves are not accompanied by axillary buds (i.e. they do not emerge from a node). This branch is in fact a compound leaf composed of
leaflets
. Though there are no buds at the base of the leaflets, there will be a bud at the base of the compound leaf's petiole.
A compound leaf can be
palmate
or
pinnate
. In a pinnate compound leaf, the leaflets are attached to a central stem called a
rachis
. The leaflets can be sessile or may be attached to the rachis by
petiolules
. Compound leaves can be even more complex by being compounded multiple times.
Leaf Arrangement
In angiosperms, leaf arrangement is determined by how many leaves emerge per node.
Alternate Leaf Arrangement
In
alternate leaf arrangement
, one leaf emerges per node, giving the appearance of alternating leaves.
Opposite Leaf Arrangement
In
opposite leaf arrangement
, two leaves emerge per node. Think of this as how your arms are opposite each other on your torso.
Whorled Leaf Arrangement
In
whorled leaf arrangement
, three or more leaves emerge per node. I try to remember this as "around the whorled (world)", because the leaves often encircle the stem.