terminology which is an integral part of the identification process.
Leaves are plant organs which are very important to the photosynthetic
process. A broad leaf usually consists of the blade (lamina) which is the flattened
photosynthetic portion. This type of leaf is usually considered to be deciduous,
that is, the leaves drop during the fall and winter. The petiole is the stem-like
portion of the leaf, found between the blade and the branch or stem. Leaves without
petioles are considered to be sessile. Stipules are leaf-like structures found near the
base of the petiole. A leaf is complete if the blade, petiole, and stipules are present.
It is incomplete if one or more of the parts are missing.
Evergreens, or the gymnosperms, are plants which typically do not lose a
a major portion of their leaves during a single season. These leaves have been
modified to conserve moisture and may be found as long and narrow needle-like
leaves. They also may be rounded or flattened in cross section, while others may
appear to be scale or fan-like.
Simple and Compound Leaves
A simple leaf has a single blade on a petiole, which in turn, is attached to a
woody stem. A leaf is considered to be compound if the blade is divided into a
number of leaflets which are attached to a single petiole. A compound leaf with
leaflets on either side of the extension of the petiole (rachis) is considered to be
pinnate. A branching form of pinnately compound leaves are considered to be
bi- or doubly pinnate. The leaflets of palmately compound leaves arise from the
terminal end of the petiole. This gives the appearance of fingers emanating from
the palm of a hand.
Phyllotaxy refers to the arrangement of leaves on the stem. There are three
basic types of leaf arrangement: alternate, opposite, and whorled. Trees with
alternate arrangement have one leaf at each node. Two leaves at the node, opposite
to each other, demonstrate opposite arrangement. Three or more leaves on a node
are considered to be whorled.
Veins are strands of vascular tissue which appear prominently on the blade.
Venation deals with the arrangement of veins. Netted venation appears as a network
of smaller veins emanating from one or more larger veins. Pinnately (netted) veined
leaves have one major vein from which smaller veins diverge on either side. These
veins appear to be feather-like. Palmately (netted) veined leaves have several main
veins, each having many small branches. Parallel venation is common to monocot
leaves and the veins are usually of equal size and parallel with one another.
Dichotomous venation is common to the ferns and Ginkgo biloba. These veins are
nearly parallel at the base, but diverge into two branches near the margin of the leaf.
Leaf margin deals with the morphology or shape of the blade. A leaf with an
entire margin may be more or less round without any indentations. Serrated leaves
have fine sawtooth-like projections on the margin. Doubly-serrated leaves have these
projections in pairs. Lobed leaves usually have fairly deep indentations, but may also
have pointed or rounded portions. Pinnately-lobed leaves may appear to be shaped
like a feather, while palmately-lobed leaves may appear to be shaped like a hand.
After reviewing the terminology associated with leaf classification we may now
proceed to our "Key to Selected Trees and Shrubs". Read through the text of the key
and click on the genus and species names to see a picture of each of the trees. Use the
back button to return to the key and scroll down the page to continue. The third and
fourth pages of text contain historical and economic information relavent to each of
the trees. We may now attempt to identify five different tree leaves. As you observe
the leaves jot down: leaf type, leaf arrangement, venation, margin, and whether it is
simple or compound. During the selection process, read the text of the key and select
one of the two choices concerning each characteristic of the leaf. If the leaves are
broad you may need to scroll through nearly the entire key. If the leaves are
evergreen you may go to number twenty-one.