What is the Purpose of a Leaf in a Plant?
A leaf is the principal component of the plant as a lateral appendage i.e. a structure at the sides originating and attached to the main plant body. It is usually found above the ground forming part of the shoot along with the stem. A leaf is mainly a part of the vascular plant. The main function of leaves in plants is to produce food or nourishment to the plant via the process of photosynthesis.
So, what do leaves do? - It produces food for the plant as it is specialised to carry out photosynthesis. The features of a leaf are - they are flattened with well-defined different upper and lower surfaces that have a difference in colour, hairiness, number of stomata - openings in the basic internal anatomy of the leaf that facilitate the exchange of gases, and other various features.
One of the very characteristic features of a leaf is that most of the leaves found in the land plants are green in colour due to a compound called chlorophyll present in one of the main parts of a leaf which forms a part of the reaction centre for photosynthesis. This is one of the main functions of leaves in plants.
Basic Internal Leaf Structure and Function
As already stated above as to what is the purpose of a leaf on a plant, the peculiars of what does a leaf do for a plant are briefly explained in this section. Chlorophyll, which is present in one of the main parts of a leaf and is responsible for the green colour of leaves. One of the most recognisable features of a leaf is absorbing light energy to carry out photosynthesis.
The basic internal anatomy of leaf can be mainly classified into three tissue systems as given below:
The Epidermal Tissue:
The internal structure of a leaf is enclosed within and protected by the leaf epidermis which covers the upper and lower surface. This epidermis is continuous with the stem epidermis.
The Mesophyll Tissue:
The mesophyll tissue is an important part of the leaf structure and function. It consists of soft-walled, unspecialised cells of the type known as parenchyma. Also, almost one-fifth of the mesophyll tissue contains plastids known as chloroplasts which bear the chlorophyll compound responsible for green colour and photosynthesis.
The basic internal anatomy of leaf is given below and very well explained in the given figure:
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The Vascular Tissue:
The linings that are visible on the leaf surface are known as veins which are the vascular tissue. Located in the spongy layer of the mesophyll the vascular tissue is responsible for the conduction of nutrients and minerals in between the leaf and the plant body. The veins consist of vascular bundles at the core of which there are clusters of two types of specialised cells for this conduction: the xylem and the phloem. Most importantly, these vascular bundles are important for the transport of water to the leaves and nutrients to the plant. The veins are shown below:
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Function of Leaf Cells
For carrying out the photosynthesis, the chlorophyll absorbs sunlight and along with certain enzymes use the radiant energy for the decomposition of water into its natural elements - hydrogen and oxygen. The oxygen is then released into the atmosphere replacing the oxygen consumed.
This liberation of oxygen is carried out through stomata - small openings in the internal structure of a leaf. The hydrogen obtained from the decomposition of water is combined with carbon dioxide in various enzymatic processes of photosynthesis to form sugar molecules which form the basic nutrition for plant and animal life.
Thus, when asked what is the function of leaf cells - it can be easily said the function of leaf cells, apart from protection and transport is to carry out photosynthesis, liberate oxygen, and provide sugar and other nutrients that sustain plant and animal life.
Leaf Morphology and Modifications
Most notably a leaf consists of a broad expanded blade - the lamin that is attached to the plant stem by petiole - a stalk-like structure. In angiosperm leaves, there are a pair of structures known as stipules present on both the sides of the leaf base and may appear as scales, spines, glands or leaf-like structures.
Many of the features of a leaf are present in a wide range of variety. They are quite diverse in the forms of size, shape, blade margin, the venation i.e. the arrangement of veins, etc. When only a single leaf is inserted into the petiole it is called a simple leaf. A diagram of a simple leaf with major parts of a leaf is shown below:
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Leaf Margins
The leaf margins can be entirely smooth or be lob-shaped in different ways. For example, the teeth of dentate margins project themselves at right angles to each other whereas that of serrate margins point towards the leaf apex. There are two types of patterning of the lobs leaf margins. They are pinnate and palmate.
In the pinnately lobed leaf margins, the leaf blade or the lamina is spaced equally along each side of the midrib or the mid-vein (for example white oak).
In palmately lobed leaf margins, the leaf blade is spaced along each of the secondary veins (for example red maple).
There are also different types of leaf bases and leaf apex. Most particularly the leaves differ in venation. A plant can be categorised on the bases of venation. For example, a dicotyledon such as lettuce has net-like venation whereas a monocotyledon such as bamboo has parallel venation.
Leaf Structure and Function
Leaf structure and function have interesting correlations. Hence, many features of a leaf or parts of a leaf are modified to perform various functions. Sometimes the whole of a leaf or some of the parts of a leaf is often modified for special functions such as climbing and substrate attachment, storage, protection against predation or climatic conditions, or trapping and digesting insect prey. For example, in cactus plants, the spines are wholly modified leaves. In the cactus, the function of leaves in plants is to protect the plant from herbivores, radiate heat from the stem during the day and collect and drip condensed water vapour during the low temperatures of the night.
Leaves or parts of a leaf are modified to provide support as well. The most common of these types of modified leaves are tendrils and hooks. Carnivorous plants use leaf modifications to trap insects. Glands that are part of the basic internal anatomy of leaves secrete enzymes that digest these insects and the leaf in this case absorbs the nitrogenous compounds (amino acids) and other products of digestion. So, in this case, what do leaves do? - They not only provide for the nutrients by photosynthesis but also carry out the process of food-trapping by their modifications. This states the clear vitality of the function of leaves in plants.
Importance of Leaves in Plants
Having gone through the information, it is clear that leaves play an important role in the plants. Leaf structure and function contribute not only to nutrient production but also to the protection, growth and sustenance of the plants. This is underlined by the fact that even though small as compared to the size of the plant or a tree, they are numerous in number thus providing large numbers of reaction centres of photosynthesis and plenty of nutrient material.
Not only when the conditions are thriving but even in arid conditions such as in a desert there are leaf modifications that minimize water loss through transpiration. Conclusively, the entire internal structure of a leaf serves the purpose of the growth of the plant which in turn sustains animal life as well.
FAQs on Basic Internal Anatomy of Leaf
1. What are the three main tissue systems that make up the internal structure of a leaf?
The internal anatomy of a leaf is composed of three primary tissue systems:
- The Epidermal Tissue: The outermost protective layer, which includes the upper and lower epidermis and pores called stomata.
- The Mesophyll Tissue: Located between the two epidermal layers, this is where most photosynthesis occurs. It is often divided into palisade and spongy parenchyma.
- The Vascular Tissue: These are the veins of the leaf, containing xylem and phloem, which transport water, minerals, and food.
2. What is the function of the mesophyll tissue in a leaf?
The primary function of the mesophyll tissue is to perform photosynthesis. This tissue is packed with cells containing chloroplasts, the site of photosynthesis. It is typically differentiated into the palisade parenchyma, with tightly packed cells for maximum light absorption, and the spongy parenchyma, which has air spaces to facilitate the exchange of gases like carbon dioxide and oxygen.
3. What is the role of the vascular bundles, or veins, inside a leaf?
The vascular bundles, commonly known as veins, act as the leaf's transport and support system. They contain two main types of tissues:
- Xylem: This tissue transports water and minerals from the roots and stem into the leaf, which are essential for photosynthesis.
- Phloem: This tissue carries the sugar (food) produced during photosynthesis from the leaf to other parts of the plant for energy or storage.
4. Why are most stomata found on the lower surface of a leaf?
Most stomata, the tiny pores for gas exchange, are located on the leaf's lower surface to minimise water loss through transpiration. The upper surface is more directly exposed to sunlight and heat, which would cause excessive evaporation. Placing stomata on the cooler, shadier lower side allows the leaf to take in carbon dioxide for photosynthesis while conserving precious water.
5. How is the internal structure of a leaf perfectly adapted for photosynthesis?
The leaf's internal anatomy is highly specialised for photosynthesis. The upper epidermis is transparent to let light pass through to the palisade mesophyll, whose cells are packed with chloroplasts. The spongy mesophyll has large air spaces that allow carbon dioxide to easily diffuse to the photosynthesising cells. The veins (xylem) deliver a constant supply of water needed for the process.
6. What are the key differences between the internal anatomy of a dicot and a monocot leaf?
Dicot and monocot leaves show distinct differences in their internal structure. In a dicot leaf, the mesophyll is usually differentiated into palisade and spongy tissues, and the vascular bundles are arranged in a net-like (reticulate) pattern. In a monocot leaf, the mesophyll is often undifferentiated, and the vascular bundles are arranged in a parallel pattern. Monocot leaves also often have specialised bulliform cells in the epidermis that help with rolling during water stress.
7. How do the different internal tissues of a leaf work together as a single system?
The internal tissues of a leaf function as a highly coordinated system. The epidermis and its stomata act as a gateway, controlling the entry of CO2 and the exit of water. The xylem delivers water to the mesophyll, which uses it with sunlight and CO2 to create food. The phloem then transports this food away. This seamless integration ensures the leaf operates efficiently as the plant's primary food-producing factory.
