Botany for Gardeners - The Basics of Leaves III

In The Basics of Leaves II, I introduced you to some of the most common types of leaf bases. Moving to the other end of the leaf, we find the leaf tip. Far from being just the end of the leaf, the tips can have important functions that may even help the plant survive. One type of leaf tip that you may have learned about through painful experience is the tip that is actually a spine or thorn. As always, botanists have special terms for such tips. Some leaves have a thorny tip that is clearly an extension of the midvein of the leaf. Such a tip is called mucronate. Other leaves have thorny tips that also include part of the leaf blade, or lamina, in the tip. These tips are called apiculate. If the tip is more like a bristle than a thorn, that tip is called aristate. Sharp or irritating tips can help to deter leaf-eating animals from snacking on them. To complete this defensive strategy, some leaves also have thorns on the margin (like holly), and even on the leaf blade itself (like thistle)!

A leaf tip that is frequently encountered in the garden is the acuminate type. This tip comes to a point, but is not hard like a thorn. The sides of the leaf join at the tip at less than a 90 degree angle, and they are concave. If the leaf sides are straight to convex instead of concave, that type of tip is said to be acute (see thumbnail picture above, right). Acuminate leaves come in other types, depending upon the overall leaf shape. Leaves that have a long and thin tip are said to be caudate, while leaves in which the tip tapers gradually to a point are called cuspidate. In rainforests, many plants and trees have leaves of familiar shapes, but which have thin extended tips known as "drip tips". These enable the leaf to shed water more efficiently. To conclude this introduction to leaf tips, a leaf with a rounded tip is called obtuse and one that looks rather squared off at the end is truncate.

The Proper Margin

Many leaves have smooth margins; these are known as entire. However, a variety of leaves have various configurations of marginal indentations that can be rounded, toothlike, sawlike, wavy, or hairy. Leaves with hairy margins are known as ciliate, while those with rounded, unpointed teeth are crenate. Margin teeth can be found as a single row, with the teeth pointing towards the leaf tip (serrate), or pointed outward from the margin (dentate). Some teeth are compound, meaning the teeth have little teeth on them as well. These are known as double serrate margins. A smooth wavy margin is undulate.

In the picture above, left are shown, from left to right: serrate, dentate, crenate, wavy, sinuate and incised leaf margins.

Now, a discussion of margins is not complete without introducing lobes. The pin Oak, for example, has dentate lobes, or what look like large teeth with smaller teeth on the ends. The difference between lobes and the smaller indentations I've just introduced you to is difficult at best, but a guide to go by is that if the indentation goes more than half way towards the main leaf vein, it is better off being referred to as a lobe. Lobes are referred to with terms similar to those used in describing types of compound leaves. For example, a leaf with comb-like lobes is referred to as pinnatifid, or pinnately lobed. If those lobes are narrow and numerous, the lobing is called pectinate. A roundish leaf with lobes arranged so the leaf looks somewhat like a hand is palmately lobed. If those lobe have lobes of their own, the leaf is pedate.

What are the arrangements?

Finally, I need to touch on the matter of how leaves are arranged on a stem. Common arrangements are opposite, in which each leaf has a leaf directly opposite to it on the stem, alternate, in which leaves appear one at a time in an alternating arrangement, and whorled, in which the leaves are grouped in a circle together with the stem at the center of the circle. Joe Pye weed is an example of a plant with whorled leaves. If the leaves are bundled together in clusters along the stem, as in pines, they are fascicled. Irises, with flattened leaves that overlap one another in two vertical rows or ranks, have an equitant arrangement. Leaves grouped into a basal rosette, with little or no stem, are rosulate, as in some biennials and Echeveria (hen and chicks).