While taking a look at the different plants on earth, what makes us easily identify the differences between one plant from the others?... It's simply the leaves!
Leaves are mostly green in colour, but the differences in their structures make not just their identification but a whole plant identification easier. Also the morphological aspect we take note are just the shape, venation. But in this article, we will be taking a look at some different leaf structures that we might not have noticed before.
Before that, let's give some little explanations on the terms to be used later for easy understanding.
- Leaf: this is a vital organ in a plant, it is either simple or compound depending on whether the incision of the leaf touches the midrib.
- Leaflet: a leaf whose incision touches the midrib converting a leaf to numerous leaflets. Presence of two or more leaflets on the same petiole makes a leaf.
- Midrib: this is a thick, large single vein along the middle of a leaf or a leaflet, it is from it that other veins are attached to.
- Petiole: this is the leaf stalk, it connects the leaf to the stem
- Petiolule: this is a leaflet stalk, it connect a single leaflet to the rachis
- Base: the bottom of a leaf
- Apex: the tip of a leaf
- Rachis: this is a modified midrib found only in compound leaves. It is also a stalk on which the leaflets are arranged.
- Axile: it is the angle formed between a petiole and the stem.
- Axillary bud: it is a bud developing at the axile.
- Leaf blade: the surface area of a leaf.
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| Fig1a: Diagram of a leaf |
Note: in compound leaves, the petiole is the part of a stalk where there is absent of leaflets, while the remaining part(of the stalk) where leaflets are attached to is the rachis.
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| Fig1b: Diagram of a compound leaf (bipinnate) |
TYPES OF LEAF STRUCTURES
There are two major types, which are;
SIMPLE LEAF: this is a single leaf which is connect to the stem of a plant by a petiole, it might have incision but this incision(s) do not touch the midrib of the leaf, there by no leaflet is formed. Also there is present of axillary bud.
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| Fig2: Diagram of a simple leaf |
COMPOUND LEAF: Here, the leaf blade is divided completely into a number of leaflets, with all leaflets connected in such a way that they possess a single leaf base. The leaf base is connected to the stem by a petiole.Also, in-between the leaflets is the rachis.
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| Fig3: Diagram of a compound leaf |
A compound leaf is further classified into two and these are;
- Pinnately compound leaves
- Palmately compound leaves
- Pinnately compound leaves: this occurs when the leaflets are on either sides of the rachis with each leaflet arising from different point.
Based on the number of leaflets( either an even or odd numbers of leaflet), a pinnated compound leaf can be further classified into;
- Paripinnate compound leaves; which is when the tip(Apex) of the rachis ends with two leaflets, making an even number of leaflets on the particular rachis, e.g., Tamarind
- Imparipinnate compound leaves; this is when there is only one leaflet found at the tip of the rachis, making an old number of leaflets on the rachis when counted, e.g., Neem
Based on the numbers of rachis found on a particular branch, we have;
- Unipinnate; this is a simple pinnated compound leaf, here, only one rachis can be counted on a particular branch, e.g., Cassia, Azdirachta indica, e.t.c
- Bipinnate; with bi--- meaning two, there are presence of two rachis on a branch. The first rachis is called the primary rachis, while the second, secondary rachis, e.g., Acacia, Mimosa pudica, honey locust.
- Tripinnate; tri= three,so three rachis occurs on a branch, the last rachis is the tertiary rachis, e.g., moringa
- Decompound; here, number of rachis on a branch is more that three, e.g., coriander
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| Fig4: Diagrams showing different types of pinnately compound leaves. |
- Palmately compound leaves: here, the leaflets arises from a single point, usually at the tip of the petiole. No rachis is found here. This is further classified into;
- Unifoliate; there is only one leaf-like structure present on the tip of the leaf stalk. This leaf-like structure isn't a simple leaf due to the absent of an auxillary bud usually found in simple leaves, e.g., citrus
- Bifoliate; presence of two leaflets arising from the same point, e.g., prinsepia, balanites, zornia, e.t.c.
- Trifoliate; presence of three leaflets arising form the same point on the stalk e.g., strawberry, soybeans, clover, oxalis, e.t.c.
- Quadrifoliate; presence of four leaflets originating from the same point, e.g., marsilia
- Multifoliate; more that four leaflets originated from the same point, e.g.,bombax, gynandropsis.
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Fig5: Diagrams showing the types of palmately compound leaves
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Note: Dear reader, I believe we can now differentiate the leaf of a particular plant from another using not only the shapes and venation.
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