We all know what leaves are, don’t we? However, when we are a student of biology, our knowledge has to be different from what others know. Do you agree? So, for you, Leaves are not the green parts of the plants that have chlorophyll in them. You are too old for that! In this chapter of plant morphology, we will look into greater depths of Leaves. We will look at their characteristics and types. However, before we proceed, let us first know what a leaf is!
What is a Leaf?
The leaf is a green, flat, thin, expanded lateral appendage of the stem. This part is borne at a node and bears a bud in its axil. It is exogenous in origin and develops from the leaf primordium of shoot apex. The green colour of the leaf is due to the presence of the photosynthetic pigment – chlorophyll. This pigment helps the various plants to synthesise their organic food.
You must have heard of the term foliage. Foliage refers to the green photosynthetic leaves of a plant. They are borne on the stem in acropetal succession. Now, we move on to the section where we look at the various characteristics of leaves.
Characteristics of a Leaf
- The leaf is a lateral dissimilar appendage of the stem.
- A leaf is always borne at the node of the stem.
- Generally, there is always an axillary bud in the axil of a leaf.
- It is exogenous in origin and develops from the swollen leaf primordium of the growing apex.
- The growth of leaf is limited.
- The leaves do not possess any apical bud or a regular growing point.
- A leaf has three main parts – Leaf base, petiole and leaf lamina. In addition, it may possess two lateral outgrowths of the leaf base, called stipules.
- The leaf lamina is traversed by prominent vascular strands, called veins.
It is the arrangement of veins and also the various veinlets in the leaves. Different plants show different types of venation. Usually, there are two types of venation:
- Reticulate venation: In a reticulate venation, the veinlets are arranged in a random fashion to form a complex network of veinlets. Ex: Dicotyledonous plants like a rose plant.
- Parallel venation: In this fashion, the veinlets are arranged parallel to each other. Ex: In monocotyledons like paddy.
Types of Leaves
You have possibly come across various plants with different shapes and sizes of leaves. They are categorized as follows:
- Acicular type: They are needle-shaped.
- Linear Type: They are long and relatively broader as compared to other leaves.
- Lanceolate Type: They are lance-shaped.
- Oblong Type: They are rectangular in their shape and structure.
- Reniform type: They refer to the leaves with the shape of kidneys.
- Cordate type: They are heart-shaped and have a deep notch at the base.
- Orbicular type: They are more or less circular in their shape.
- Saggitate type: They are shaped like arrow-heads.
- Elliptical Type: They have shapes of ellipses.
- Hastate type: The shape of these leaves is like saggitate. The two basal lobes are directed outwards.
- Ovate Type: They are oval or egg-shaped in their structure.
- Lyrate Type: They are in the shape of a lyre.
- Spatulate Type: They have shapes resembling spatulas or spoons.
- Centric Type: They are cylindrical and hollow.
- Oblique Type: The lamina of this leaf has unequal halves.
- Cuneate Type: They are wedge-shaped.
This is all about the characteristics and types of leaves. We will look at a few solved examples for a better understanding.
Solved Examples for You
Question: Explain how various leaf modifications help plants.
Solution: Though the main function of leaves is to carry out photosynthesis, in a lot of plants, the modification of roots performs various functions. Therefore, they become an important part of the plants. They include
- Tendrils: These help the plant in climbing.
- Spines: These act as an organ of defence. The common example includes cactus.