Leaves occur in various arrangements in plants. All plants seem to have leaves on branches to normal folks. We, botanists, should see it differently though. Many plants have several leaves in a branch called leaflets and the whole branch is called a leaf! These are called compound leaves. So basically there are two types of compound leaves in a broader sense. A compound leaf may be either pinnately compound or Palmately compound.
What’s special about this post? I’ve tried to use ferns as examples for compound leaves as much as possible. I hope this would be a slightly different approach to learning about compound leaves and you will enjoy reading this!
Before actually getting into the topic, one should know what s simple leaf looks like. To be simple, a simple leaf will contain only one leaf that is attached to the stem by a petiole.
Whereas a compound leaf will have many leaflets attached to a stem through a petiole. The branch that contains the leaflets is called a rachis.
Pinnately compound leaves
These leaves contain leaflets arranged in opposite arrangement on the rachis (the extended petiole). The leaflets may be evenly paired or oddly paired. In the evenly paired pinnate leaves, the leaflets are arranged in an opposite manner and the rachis ends with two leaflets and is called Paripinnate. In oddly paired pinnate leaves, the leaflets are arranged in opposite fashion with a trailing leaf at the anterior end of the rachis and is called imparipinnate.
The leaves may be of three types based on the number of times of pinnation as follows,
Single leaflets are present on the rachis in the opposite fashion.
When the single leaflets of the unipinnate leaf get replaced with unipnnate leaves themselves become bipinnate leaves.
When the single leaflets in the unipinnate leaves get replaced with bipinnate leaves, it is called as tripinnate leaves!
When the single leaflets in the unipinnate leaves get replaced with tripinnate leaves, it is called as quadripinnate leaves!
These leaves do not have a pattern to count the number of times compound. The arrangement is compound but is not in a definite manner and can’t be classified under uni, bi, tri, or quadripinnate leaves.
Palmately Compound Leaves
These are a form of compound leaves that appear as a single leaf but appearances may be deceptive! Multiple leaflets arise from a common point that is at the end of the petiole. Based on the number of such leaflets, they are classified as follows…
These leaves contain a simple leaf blade with multiple lobes.
These leaves will contain two leaf blades attached to the petiole at a single point.
These appear to be three leaves originating from the anterior end of the petiole.
Four leaves would be fused to a petiole and appear to be a single leaf.
When more than four leaflets are attached to a petiole to form a single leaf, it is called a multifoliate palmately compound leaf.
Read about the genetics behind Compound Leaves in this interesting research article – https://dev.biologists.org/content/131/18/4401