Leaf

Deciduous plants in frigid or cold temperate regions typically shed their leaves in autumn, whereas in areas with a severe dry season, some plants may shed their leaves until the dry season ends. In either case, the shed leaves may be expected to contribute their retained nutrients to the soil where they fall.

Where leaves are basal, and lie on the ground, they are referred to as prostrate.

One leaf, branch, or flower part attaches at each point or node on the stem, and leaves alternate direction, to a greater or lesser degree, along the stem.
Two leaves, branches, or flower parts attach at each point or node on the stem. Leaf attachments are paired at each node.
An opposite arrangement in which each successive pair is rotated 90° from the previous.
Three or more leaves, branches, or flower parts attach at each point or node on the stem. As with opposite leaves, successive whorls may or may not be decussate, rotated by half the angle between the leaves in the whorl (i.e., successive whorls of three rotated 60°, whorls of four rotated 45°, etc.). Opposite leaves may appear whorled near the tip of the stem. Pseudoverticillate describes an arrangement only appearing whorled, but not actually so.

Sessile (epetiolate) leaves have no petiole and the blade attaches directly to the stem. Subpetiolate leaves are nearly petiolate or have an extremely short petiole and may appear to be sessile.

In peltate leaves, the petiole attaches to the blade inside the blade margin.

Characteristic in which a plant has small changes in leaf size, shape, and growth habit between juvenile and adult stages, in contrast to;
Characteristic in which a plant has marked changes in leaf size, shape, and growth habit between juvenile and adult stages.

Chloroplasts are generally absent in epidermal cells, the exception being the guard cells of the stomata. The stomatal pores perforate the epidermis and are surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts, forming a specialized cell group known as the stomatal complex. The opening and closing of the stomatal aperture is controlled by the stomatal complex and regulates the exchange of gases and water vapor between the outside air and the interior of the leaf. Stomata therefore play the important role in allowing photosynthesis without letting the leaf dry out. In a typical leaf, the stomata are more numerous over the abaxial (lower) epidermis than the adaxial (upper) epidermis and are more numerous in plants from cooler climates.

In ferns and most flowering plants, the mesophyll is divided into two layers:

Leaves are normally green, due to chlorophyll in chloroplasts in the mesophyll cells. Plants that lack chlorophyll cannot photosynthesize.

A vein is made up of a vascular bundle. At the core of each bundle are clusters of two distinct types of conducting cells:

Cells that usually move sap, with dissolved sucrose (glucose to sucrose) produced by photosynthesis in the leaf, out of the leaf.

The xylem typically lies on the adaxial side of the vascular bundle and the phloem typically lies on the abaxial side. Both are embedded in a dense parenchyma tissue, called the sheath, which usually includes some structural collenchyma tissue.

Leaves showing various morphologies (clockwise from upper left): tripartite lobation, elliptic with serrulate margin, palmate venation, acuminate odd-pinnate (center), pinnatisect, lobed, elliptic with entire margin
Shaped like an arrowhead and with the acute basal lobes pointing downward.

"Hairs" on plants are properly called trichomes. Leaves can show several degrees of hairiness. The meaning of several of the following terms can overlap.

Silky appearance through fine, straight and appressed (lying close and flat) hairs.
1. Pinnate (feather-veined, reticulate, pinnate-netted, penniribbed, penninerved, or penniveined)
Major veins extend close to the margin, but bend before they intersect with the margin.
2. Parallelodromous (parallel-veined, parallel-ribbed, parallel-nerved, penniparallel, striate)

Types 4–6 may similarly be subclassified as basal (primaries joined at the base of the blade) or suprabasal (diverging above the blade base), and perfect or imperfect, but also flabellate.

Branching repeatedly by regular dichotomy to give rise to a three dimensional bush-like structure consisting of linear segment (2 subclasses)
Primary veins straight or only slightly curved, diverging from the base in a fan-like manner (4 subclasses)
Single primary vein, the midrib, along which straight or arching secondary veins are arranged at more or less regular intervals (6 subclasses)
Numerous longitudinally parallel primary veins arising from a transverse meristem (5 subclasses)

A modified form of the Hickey system was later incorporated into the Smithsonian classification (1999) which proposed seven main types of venation, based on the architecture of the primary veins, adding Flabellate as an additional main type. Further classification was then made on the basis of secondary veins, with 12 further types, such as;

Veins partly prominent, the crest above the leaf lamina surface, but with channels running along each side, like gutters
Vein forming raised line or ridge which lies below the plane of the surface which bears it, as if pressed into it, and are often exposed on the lower surface. Tissue near the veins often appears to pucker, giving them a sunken or embossed appearance
Veins arranged like the rungs of a ladder, particularly higher order veins