LEAF DICHOTOMOUS KEY
Name: __________________________ Hr: ____
Background
You have learned that there are distinct biotic regions on Earth that are called biomes.
Each biome has distinct vegetation or plant life that has adapted to the abiotic condition
that existing there. For example, grasslands are dominated by grasses that have
adapted to the lack of moisture and periodic fire. In the following activity you will be
making observations of the leaves from deciduous trees that you collect. From these
observations you will demonstrate your ability to construct a dichotomous key.
Objective
1. You should be able to construct a dichotomous key based on observed differences
between leaf collected on school property.
Procedure
1. Read the following information so that you understand what the definition of a leaf is.
A leaf can be identified on the tree because its petiole always connects to the stem where a bud is formed.
2. Bring your textbook, a piece of paper, and something to write with outside. Collect
tree leaves from 10 different species (it might be wise to collect leaves that you think
can easily be distinguished). As you collect them place them in your book for
temporary storage and note the ‘phyllotaxic’ arrangement of the leaves on the stem
and a piece of paper. Phyllotaxic arrangement is described below:
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.
3. Once you have returned to the classroom spread your leaves out on a lab table and
begin to analyze and record information about each leaf, identifying whether it is
‘simple or compound’, its “venation”, and “margin”. I suggest numbering the leaves
and with a piece of tape as well so that you know which you are writing about.
Information on leaf type, etc… is described below:
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.
Venation
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.
Margins
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. “Doublyserrated” leaves have these projections in pairs. “Lobed” leaves usually have fairly deep rounded indentations,
while “Cleft” leaves have indentations creating points on the lobes. Leaf that are smooth around the whole edge
are considered “Entire”.
4. Once you have identified the characteristics of the leaves you have collected,
construct a dichotomous key for the identifying your leaves in the space below (if
you don’t know the name of the tree, and I don’t have time to tell you the names,
then just identify them with the numbers that you have given them).