How Can You Decide Whether
Something is a Leaf or Not?
ince roots and stems possess so many shapes, sizes,
functions and manners of being, it's easy to guess that leaves do the same. In fact, the
world of leaves is so varied, even downright weird sometimes, that it can actually be hard
to figure out where a plant's leaves are. If you look at a maple tree's branch, it's easy
to recognize that the leaf is the flat, green thing attached to the branch with a slender
little stem-like item -- the leaf is the thing that turns colorful in the fall, and falls
off. No problem there. On the other hand, are you sure where the actual leaves are on the
tomato sprig in the picture at the right? Is what you see one or more leaves, or a
division of just one leaf, or what
Here are some hints that sometimes
can help you decide whether the thing you're seeing is a leaf:
- Leaves of most woody trees and bushes consist of a flat blade connected to the
woody stem or twig via a slender, stick-like petiole. The blade and the petiole
make up the leaf. In the tomato-plant picture above, at the very bottom notice that the
stem sort of splits. The pencil-like thing going off toward the right is the petiole of a
compound leaf. Therefore, the picture above shows two full-sized compound leaves and a
younger one developing. Each leaf has about seven main leaflets, with some smaller ones as
- On most
woody trees and bushes, inside the narrow angle formed where the petiole connects with the
stem, there arise one or more buds. At the right the more or less horizontal streak
at the top is a twig, the large black area is the leafblade of a Hophornbeam tree, Ostrya
virginiana, (with three insect-eaten holes in the blade margins), the more or less
vertical line connecting the blade and the twig is the petiole, and the fingerlike thing
pointing downward at about 7 o'clock is a bud. Buds are usually conical or globe-shaped,
and typically covered with a few or several scales. Buds contain embryonic leaves, stems,
or flowers, depending on the buds' type. Buds never occur on petioles or leaf
blades. Therefore, everything on one side of the petiole's attachment point with the
stem (where buds are) is leaf, while everything on the other side of the attachment point
is stem. Tomato plants, like many herbs, don't produce easily recognizable buds.
- Finding buds is especially helpful if you're confronted with leaves divided into
subdivisions, or leaflets. Such leaves are called compound leaves, while
undivided leaves are known as simple leaves. Hundreds, even thousands, of leaflets
can occur in a single compound leaf. The picture of the Poison Ivy leaf at the left on our
leaf menu page shows one Poison Ivy leaf consisting of three
leaflets. At the right you see the twice-compound leaf of the Honey
Locust. At the very top is a section of twig with a bud so small you can hardly see it.
Shooting straight down from the twig is the petiole holding the compound leaf. The
first featherlike thing leading off to the right is the leaf's first division (a pinna,
pl. pinnae), and this division is then itself divided into small leaflets (pinnules).
Notice that the left side of the compound leaf is more or less a mirror image of
the right side. This leaf was scanned in late summer, so some of the leaflets have fallen
off or been eaten by bugs.
- Most but not all leaves, even compound leaves, are shaped in such a way that they more
or less display bilateral symmetry. The three leaves above show this,
each leaf's pink side more or less being a mirror image of its other side. The
leaf on the picture's left is a simple (not compound) Sweetgum
leaf, the one in the middle is a twice-compound Honeylocust leaf with many
leaflets, and the leaf on the right is the digitally compound leaf of a Virginia Creeper,
showing five leaflets originating from one place like the digits, or fingers, of a hand.
Since shoots and leaflets usually do not show bilateral symmetry, thinking
in terms of symmetry is often a good way to decide whether the plant part before us is
leaf, sprout, leaflet, or something else. Because herbaceous plant typically don't have
buds, this bilateral symmetry trick is often the best for identifying their leaves.
- Finally, don't forget that some plants just don't have "standard leaves." The
above photograph shows one often-encountered "non-standard leaf" form,
that in which leaves are represented by much-reduced scales. This is a close-up of the
leaf scales of the Red Cedar, Juniperus virginiana, one of the world's many kinds
of juniper. If you look closely you can see that each leaf scale is sharply triangular. Yet another variation on the leaf theme is
presented by the pines, where needles serve the leaf function. the picture at the
left shows the 7-inch-long (18 cm) needles of the Loblolly Pine, Pinus taeda,
outside my door.The picture at
the right is a close-up showing how the needles, usually in clusters of 3, emerge from
little woody stump-like affairs arranged along the pine branch. The number of needles
arising from a stump often helps us decide the pine's identify. The needles of Eastern
White Pines arise in groups of 5; the Virginia Pine has 2; the Shortleaf Pine has clusters
of both 2 and 3.
Clearly, to dominate this leaf business you need to keep an open mind, and be ready for
just about anything!