Structure and Parts of a Leaf

No seriously! It’s more complex than you might imagine, and having a common language really helps when talking about the parts of a leaf.

For the avoidance of doubt, leaf refers to both the flat type that you imagine, and the needles and scales found on some evergreens. This is not a complete reference, just the parts that I find most useful.

You’d better get comfortable, this is a long post…

Simple Parts of a Leaf

This is a simplified, generalisation. There are, of course, exceptions and variations. The leaf Blade, attaches to the stem of the plant with it’s Petiole. Quite often you will find a Stipule at the base of the petiole, which is like a miniature leaf arrangement. Within the leaf, there will be at least one strengthening midrib, and veins running from it. The leaf receives water and returns sugars through these.

Identification Categories

When it comes to identifying plants by their leaves, there are a number of areas to consider:

1. The arrangement of the leaves on the stem.
2. Simple vs Compound Leaves.
3. Characteristics of the petiole.
4. Veins.

1. Arrangement

The arrangement of the leaves on the stem can provide vital clues to the identification of a plant. Indeed, for some plants, it’s even in the name. For example, Opposite-leaved, Golden Saxifrage.

These are the main arrangements you may encounter:

• Opposite – Two leaves from the same point at each point or node on the stem, growing in opposite directions.
• Alternate – One leaf attached at each point or node on the stem, each successive leaf growing in opposite directions.
• Basal – Arising from the base of the stem.
• Cauline – Arising from the Aerial stem.
• Whorled or Verticillate – Three or more leaves from the same point or node.
• Rosulate – The leaves form a rosette.
• Distichous – Leaves are attached in two rows. They can be either opposite or alternate in arrangement.

As a stem grows, leaves tend to grow in the optimum position for collecting light. This can result in leaves forming a helical pattern around the stem.

2. Simple vs Compound Leaves

So, a leaf is a leaf, right? Well, not exactly. There are simple leaves, and leaves which are made up of leaflets. In deciduous trees, for example, the part which detaches itself from the tree in Autumn (Fall) is a leaf. So in Oak trees, that’s a simple Oak leaf, whereas in Ash trees, it’s a compound leaf with multiple leaflets (see images# below).

A simple leaf may be deeply lobed, but as long as gaps do not reach the midrib, it is still a simple leaf. Each leaflet of a compound leaf may have it’s own Petiolule (equivalent of a Petiole) and Stipule (Stipel).

Types of Compound Leaf

• Palmately Compound – Leaflets radiate from the end of the Petiole, like the fingers of a hand e.g. Horse Chestnut
• Pinnately Compound – Leaflets are arranged along the main or mid-vein.
  • Odd Pinnate – With a terminal leaflet e.g. Ash.
  • Even Pinnate – Without a terminal leaflet e.g. Mahogany.
• Bipinnately Compound – The leaves are twice divided. The leaf has a main vein, and further secondary veins on which the leaflets are attached e.g. Silk Tree.
• Trifoliate – A pinnate leaf with just three leaflets e.g. Clover.

3. Characteristics of the Petiole

Leaves with a stalk (petiole) are said to be petiolate. Those without a stalk, which join straight to the branch are said to be sessile.

Where the blade of a leaf partially surrounds the stem, it is said to be clasping or decurrent. Where the blade completely surrounds the stem they are called perfoliate.

The stipule, where present, is a leaf-like appendage on each side at the base of the petiole. Stipules may remain (such as on roses) or be shed as the leaf expands, leaving scars; Known as stipulations.

4. Veins

Veins, occasionally referred to as nerves, extend into the leaf via the petiole and transport nutrients and water between the leaf and the stem. They also play a mechanical role in supporting the leaf structure. Branching from the main vein are secondary veins, and there can be many more branchings, sometimes leading to a net-like structure.

Leaf Shape Terminology

Image	Name	Description
	Auriculate	Having ear-shaped appendages near the petiole e.g. Arum Maculatum
	Cordate	Heart-shaped, with the petiole or stem attached to the notch.
	Deltoid	Shaped like Greek letter Delta, triangular, stem attaches to side.
	Digitate	With finger-like lobes, similar to palmate.
	Elliptic	Oval, with a short or no point.
	Hastate	Spear-shaped: Pointed, with barbs, shaped like a spear point, with flaring pointed lobes at the base.
	Lanceolate	Long, wider in the middle, shaped like a lance tip.
	Linear	Long and very narrow like a blade of grass.
	Lobed	Being divided by clefts, may be pinnately lobed or palmately lobed.
	Obcordate	Heart-shaped, stem attaches at the tapering end.
	Oblique	Asymmetrical leaf base, with one side lower than the other.
	Oblong	Having an elongated form with slightly parallel sides, roughly rectangular.
	Obovate	Teardrop-shaped, stem attaches to the tapering end; reversed ovate.
	Ovate	Oval, egg-shaped, with a tapering point and the widest portion near the petiole.
	Palmate	Palm-shaped, i.e. with lobes or leaflets stemming from the leaf base.
	Perfiolate	With the leaf blade surrounding the stem such that the stem appears to pass through the leaf.

Leaf Edge Terminology

	Entire	Even; with a smooth margin; without toothing.
	Ciliate	Fringed with hairs.
	Crenate	Wavy-toothed; dentate with rounded teeth.
	Lobate	Indented, with the indentations not reaching the center.
	Serrate	Saw-toothed; with asymmetrical teeth pointing forward.
	Doubly-Serrate	Each tooth bearing smaller teeth.

Source: Wikipedia

More information…

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