Many readers are undoubtedly aware of my appreciation for trees and all the wonders that they provide. There are just so many things to admire about trees, such as their endurance across many weather phenomena, their shape in response to their environment, the oxygen that they provide, the list goes on.
One interesting phenomenon of the tree is the leaf. As we see in this image, there is some variety in shape and color.
Morphology of the Leaf
A structurally complete leaf of an angiosperm consists of a petiole (leaf stalk), a lamina (leaf blade), and stipules (small structures located to either side of the base of the petiole). Not every species produces leaves with all of these structural components. In certain species, paired stipules are not obvious or are absent altogether. A petiole may be absent, or the blade may not be laminar (flattened). The tremendous variety shown in leaf structure (anatomy) from species to species is presented in detail below under morphology. The petiole mechanically links the leaf to the plant and provides the route for transfer of water and sugars to and from the leaf. The lamina is typically the location of the majority of photosynthesis. The upper (adaxial) angle between a leaf and a stem is known as the axil of the leaf. It is often the location of a bud. Structures located there are called “axillary”.
The type of leaf is usually characteristic of a species (monomorphic), although some species produce more than one type of leaf (dimorphic or polymorphic). The longest leaves are those of the Raffia palm, R. regalis which may be up to 25 m (82.38 ft) long and 3 m (9.84 ft) wide. The terminology associated with the description of leaf morphology is presented, in illustrated form, at Wikibooks.
Where leaves are basal, and lie on the ground, they are referred to as prostrate.
Basic leaf types
- Ferns have fronds
- Conifer leaves are typically needle- or awl-shaped or scale-like
- Angiosperm (flowering plant) leaves: the standard form includes stipules, a petiole, and a lamina
- Lycophytes have microphyll leaves.
- Sheath leaves (type found in most grasses and many other monocots)
- Other specialized leaves (such as those of Nepenthes, a pitcher plant)
Arrangement on the Stem
The leaves on this plant are arranged in pairs opposite one another, with successive pairs at right angles to each other (“decussate”) along the red stem. Note the developing buds in the axils of these leaves.
- Alternate – leaf attachments are singular at nodes, and leaves alternate direction, to a greater or lesser degree, along the stem.
- Basal – arising from the base of the stem.
- Cauline – arising from the aerial stem.
- Opposite – Two structures, one on each opposite side of the stem, typically leaves, branches, or flower parts. Leaf attachments are paired at each node and decussate if, as typical, each successive pair is rotated 90° progressing along the stem.
- Whorled (Verticillate) – three or more leaves 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.
- Rosulate – leaves form a rosette
- Rows – The term “distichous” literally means “two rows”. Leaves in this arrangement may be alternate or opposite in their attachment. The term “2-ranked” is equivalent. The terms tristichous and tetrastichous are sometimes encountered. For example, the “leaves” (actually microphylls) of most species of Selaginella are tetrastichous, but not decussate.
As a stem grows, leaves tend to appear arranged around the stem in a way that optimizes yield of light. In essence, leaves form a helix pattern centered around the stem, either clockwise or counterclockwise, with (depending upon the species) the same angle of divergence. There is a regularity in these angles and they follow the numbers in a Fibonacci sequence: 1/2, 2/3, 3/5, 5/8, 8/13, 13/21, 21/34, 34/55, 55/89. This series tends to a limit close to 360° × 34/89 = 137.52° or 137° 30′, an angle known in mathematics as the golden angle. In the series, the numerator indicates the number of complete turns or “gyres” until a leaf arrives at the initial position and the denominator indicates the number of leaves in the arrangement. This can be demonstrated by the following:
- alternate leaves have an angle of 180° (or 1/2)
- 120° (or 1/3) : three leaves in one circle
- 144° (or 2/5) : five leaves in two gyres
- 135° (or 3/8) : eight leaves in three gyres.
Hope you enjoyed this short overview of shape and arrangement of leaves.
This shot was taken with a Canon EOS 5D Mk III using an EF 24-105mm f/4L lens. The exposure settings were 1/60 second at f/6.3 and 400 ISO.