Leaf is lateral and aerial out growth of the stem and it is usually dorso-ventral and flat in structure. The main function of leaf is to manufacture food by the process of photosynthesis and typically it consists of stalk called petiole, a flattened blade, the lamina and the leaf base. Leaves are thin and flat, they maximize the surface area that is directly exposed to light and promotes photosynthesis.
The structures of the leaves vary from species to species depending on their adaptation to climate and availability of light. It also depends on factors like predating organisms, availability of nutrients and ecological competition. Other factors include balancing of water loss due to high temperature due to high temperature and low humidity. Leaves also store food and water and are modified to perform such specialized functions.
Leaf is defined as an organ of vascular plant that is borne above ground and is a specialized organ to perform photosynthesis. Leaves are typically flattened and thin organs, they are adapted in many ways to their environment such as succulent leaves, coniferous leaves, bulb scales and some do not perform photosynthesis like spines, cataphylls and cotyledons.
A typical dicot leaf consists of the following parts. The main parts of a leaf are the base, stalk or petiole and lamina or the leaf blade.
Leaf base is the portion of the leaf by which it is attached to the stem. In monocot leaves, the base of the leaves is sheathed type. In dicot leaves, the base of the leaf could bear two lateral outgrowths known as the stipules. In some plants like peas, beans, etc. the base of the leaf is swollen and is known as the pulvinus.
Two small flaps like structures grow at the base of the petiole is some dicot plants are known as stipules. In some plants, the stipules grow to enclose and protect the young leaf blade as it develops. In some plants like those of the willows and certain cherry trees, the stipules produce substances that act as insecticides and prevent insects attacking the developing leaf.
Petiole is the stalk of the leaf. It is the section joining the base of the leaf to the lamina. Generally petiole is cylindrical or semicircular in form. The petiole is stem like, within the petiole are tiny tubes that connect to the veins of the leaf. These tubes carry food and water to the leaf and some carry food that is prepared by the leaf. In most plants the petioles bend in a way where the blades can receive more sunlight, they also provide a flexible handle that enables the blades to twist in the wind to avoid damage. In some plants petioles are larger than their stems and are edible.
Leaf Blade or Lamina
The leaf blade or lamina is the expanded, broad, flat and green portion of the leaf. The process of photosynthesis occurs in this part of the leaf. The blade has many food making cells. There are many different forms of leaf blades based on the type of (a) edges, (b) patterns of veins, and (c) the number of blades per leaf.
Leaf characteristics like shape, margin, hairs, etc are important external characteristics that help identify different plant species. The type of leaf is usually the characteristic of a species, and classification of leaves depend upon many different criteria.
The basic types of leaves are:
- Ferns - leaves that have fronds.
- Conifer Leaves - These are typically needle or awl or scale shaped leaves.
- Angiosperm Leaves - They are typical, standard form of leaves that includes, stipules, a petiole and a lamina.
- Sheath Leaves - They are found in most of the grasses.
Leaves are of different types as follows based on the arrangement on the stem. They can be as follows:
- Alternate - Single leaves are attached at the nodes and they alternate in direction to a greater or lesser degree along the stem.
- Opposite - They are two structures one on each opposite side of the stem, they are attached in pairs on each node.
- Whorled - Here three or more leaves attach at each node on the stem.
- Rosulate - These leaves form a rosette.
- Rows - Leaves in this type of arrangement may be alternate or opposite in their attachment but not decussate.
Based on the divisions of the blade two basic forms of leaves can be described. They are simple leaf and compound leaf.
Simple Leaf - It has an undivided blade. The shape of the leaf may be formed of lobes but the lobes do not reach the main vein or the midrib.
- Compound Leaf - The leaf blade is fully subdivided and each leaflet of the blase is separated along a main or secondary vein. The middle vein of a compound leaf is called a rachis.
Compound leaf can be of the following types based on the leaflets:
- Palmately Compound Leaves - They have leaflets that diverge from the end of the petiole like the fingers off the palm of the hand.
- Pinnately Compound Leaves - Here the leaflets are arranged along the main or mid-vein.
- Bipinnately Compound Leaves - Here the leaves are twice divided and the leaflets are arranged along a secondary vein.
Based on petiole leaves are of two types:
Petiolate Leaves - They are also known as stalked leaves, these are the leaves that have a petiole. The petiole can be in different length from one plant to another.
Sessile Leaves - These are unstalked leaves and do not possess a petiole. The lamina of the leaf expands directly from the stem.
- Leaves contain cellular organelles chloroplasts which contain the pigment chlorophyll. Plants require chlorophyll to make their own food.
- Stomata of the leaves help in gaseous exchange. They help in entry of atmospheric carbon dioxide during the process of photosynthesis and they also aid in the removal of excess water in the form of water vapor during transpiration process.
- Leaves help in exchange of respiratory gases.
- In plants like Amaranthus, they serve to store food materials.
- The leaves also take part in vegetative propagation.