Category Science

For a pair of untrained eyes, leaves are nothing but a bunch of rustling foliage. However, a closer look will reveal much more than what meets our eyes. For example, in an ordinary rose leaf, the green colour is in fact inside the leaf rather than outside. If we observe it closely, we can perceive that the outer surface is colourless. We might even need a microscope to see this more clearly.

Much like a human or animal body, leaves too have a skin! A thin layer of epidermis, or skin cells, covers the outside of the leaf. Small openings called stomata are present in the lower epidermis, and sometimes in the upper epidermis. Air passes in and out of the leaf through these openings. In plants such as water lilies, all the stomata are in the upper epidermis as the lower epidermis rests on the water.

The opening of each stomata is controlled by two bean-shaped cells called the guard cells. They swell with water during the day and shrink at night.

These guard cells allow air to pass freely in and out of the leaf during the day and at night, when their cell walls straighten, the opening narrow.

There is layer of cells called spongy cells into which the air reaches through the stomata. Above the spongy cells, there are usually one or two rows of palisade cells. These are long, slender cells closely packed together.

Leaves have veins. They usually run through the middle of the leaf, between the spongy and palisade layers. The xylem and phloem tubes, which bring in water and carry away food, are found in the veins.

The spongy and palisade cells contain chlorophyll. This vital substance is located in hundreds of microscopic chloroplasts, or ‘green bodies.’ There are a few chloroplasts in the guard cells too.

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The green hands of a plant, the leaves, look so simple and plain at the first glance. However, there is intense activity going on in them, much like inside a factory!

Food for the entire plant is produced in the leaves, in the form of sugar. The special green substance called chlorophyll helps leaves in the process of food production. Chlorophyll needs sunlight, carbon dioxide from the air and water from the soil to produce sugar. The process of leaves producing food is called photosynthesis. Photosynthesis literally means ‘putting together light’.

Therefore, leaves manufacture sugar, like factories. The manufacturing process in the leaves is not simple. Much like in a factory, the plant collects raw materials such as carbon dioxide and water through their leaves and roots. The water from the roots goes to the leaves through the stem and branches.

How do they prepare the food, though? Chlorophyll, the green substance, helps the leaves transform sunshine into energy. Leaves need this energy in order to transform minerals in the water and carbon dioxide from the air to sugar. There can be waste material in the cooking process too! The remaining air along with extra oxygen thrown out through small opening called stomata on the underside of the leaf.

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Trees have long leaves. However, their leaves do not. They fall off the trees in course of time. Shedding of leaves occur due to many reasons.

In cold climates, the days are shorter during autumn and winter. Therefore, the leaves do not get sufficient sunlight to prepare food for the plant. In the event of scarcity of food for a tree, it has to conserve its energy. Leaves, which do not manufacture food and only consume it, are a liability for the tree now. The tree, then, has no other alternative but no shed its leaves!

In extreme cold, when trees are exposed to frost, shedding of leaves is kind of defense against the cold. If there were leaves, more area of the tree would be affected! In addition, trees need rest too. When there are too many leaves, they keep the roots of the plant busy by demanding more water and nutrients from soil for preparing food. However, when these ‘little food factories’ close down, the roots are much relieved! Evergreen plants shed leaves regularly as the leaves mature and get old and therefore have to be replaced by new ones.

During the season of autumn, which comes between summer and winter, temperatures gradually decrease. In the United States, autumn season is known as ‘fall’ because that is when leaves fall of trees.

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We usually think of leaves as being green in colour. Are they always? Not really. If you observe carefully, you will notice that from the time a leaf is unfolded, to the moment it falls of a branch, it undergoes a change in colouration.

In cold countries, leaves become yellow, brown, orange and red in the passage of seasons. This phenomenon usually happens at the turn of summer and fall or autumn seasons. Why does this happen?

We know that chlorophyll, the green pigment, causes the green colour in leaves. In cold countries, the leaves of trees become busy during summer manufacturing food for the entire tree with the help of chlorophyll.

However, when winter approaches, and days are shorter, food production in the leaves does not happen as often as it used to be. The tree starts using the food that it has stored in its trunk and branches.

In winter, preparing food becomes impossible for leaves and thus plants ‘shut down’ their food factory! As a result, the chlorophyll present in the leaves becomes of no use; it breaks down and disintegrates. Other pigments, such as xanthophyll- yellow in colour, carotin-orange in colour and anthocyanin-red, start appearing on the leaf. These colours were dominated by the green colour so far. When green is weak, the leaves shine in these bright and brilliant colours.

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Can you tell the difference between the leaf of a mango tree and that of a jack fruit tree by examining them? The shape and appearance of leaves varies from tree to tree. Some leaves are rough and jagged, while some others are smooth and soft. The blades of leaves may be simple and undivided or divided in multiple counts.

The leaves of a drumstick plant are very small. They are arranged in a single vein separately. There are other patterns of leaves as well. They may appear alternatively or in pairs. Leaves arranged in pairs may face the same way, or are at right angles to each other.

All trees have leaves; however, their uniqueness gives identity to the tree and adds beauty to nature.

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Leaves have very small opening on their surface. Such openings or pores are known as stomata. Stomata generally appear on the underside of the leaves. However, water lilies and other plants that have floating leaves, have stomata on top of their leaves. The purpose of these little pores is to allow gases to go in and out of the leaves. Guard cells control the passage of gases through stomata.

The amount of gases going in and out of a leaf is controlled by guard cells. These openings, or special pores called stomata, are placed on either side of the leaf’s open surface.

Leaves manufacture sugar with the help of chlorophyll in the guard cells during the day time. The sugar attracts water into the cells making them balloon out on the side opposite the pores. When this happens, the pores become open as more carbon dioxide is allowed to move in and more oxygen is released in to the air.

At night, the absence of sunlight prevents photosynthesis. Therefore, no sugar is produced, and the sugar level in the guard cells goes down. No water is allowed into the guard cells, and the pores remain closed. As a result, the movement of gases is limited.

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