Chapter 10: Life Processes in Plants

A growing plant gets taller, its stem becomes thicker, and new leaves and branches appear. Unlike animals, plants do not eat food; they make their own. To find out what a plant needs for growth, we can set up three identical pots with marigold saplings of the same size. Pot A gets sunlight and water, Pot B gets sunlight but no water, and Pot C gets water but is kept in the dark. After about 15 days, Pot A has grown best, the plant in Pot B has wilted and may have died, and the plant in Pot C is pale, thin and weak. So plants need both sunlight and water for growth. The experiment is a fair test: only one condition is changed at a time and everything else (soil, pot size, sapling size) is kept the same.

Plants store food as starch, a type of carbohydrate. Starch is made in the leaves, which are usually broad and flat so that they catch plenty of light. Most leaves are green because of a pigment called chlorophyll, which captures sunlight. We test for starch with iodine solution, which turns blue-black when starch is present. To see this colour clearly, the green colour is first removed: the leaf is boiled in water to soften it, then warmed in a test tube of alcohol kept in a beaker of hot water until it becomes colourless; then drops of dilute iodine are added. Caution: alcohol catches fire easily, so it must never be heated directly, and the teacher does this demonstration. If a plant with leaves having green and non-green patches is kept in sunlight, only the green patches turn blue-black. A similar leaf from a plant kept in the dark for two days shows no blue-black colour anywhere. So chlorophyll and sunlight are both needed to make starch, and leaves are called the food factories of plants.

To find out whether air has a role, the teacher takes a green plant and keeps it in the dark for two to three days to destarch it, which means the plant uses up its stored starch. One leaf of this plant is then half-inserted through a split cork into a bottle containing caustic soda solution, which absorbs carbon dioxide, while the other half stays outside in the air. After a few hours in sunlight, the iodine test shows that the half outside the bottle turns blue-black but the half inside does not. Both halves had the same water, sunlight and chlorophyll; only carbon dioxide differed. So carbon dioxide is needed to make starch. Caustic soda can burn the skin, so only the teacher should handle it. The process by which plants prepare food using sunlight, chlorophyll, carbon dioxide and water is called photosynthesis. The leaf is the main site of photosynthesis, but other green parts with chlorophyll also perform it.

To find out what plants release, a water plant such as Hydrilla is placed under an upturned funnel in a beaker of water with an inverted test tube over the funnel stem. When the set-up is in sunlight, bubbles rise and collect in the test tube; in a similar set-up kept in the dark, hardly any bubbles appear. When enough gas has collected, the test tube is closed with a thumb, taken out, and a burning matchstick is quickly inserted. It burns more brightly, showing that the gas is rich in oxygen. So oxygen is released during photosynthesis, and light is needed. The food actually made is glucose, a simple carbohydrate that gives instant energy and is later converted into starch for storage. The word equation is: Carbon dioxide + Water = Glucose + Oxygen, in the presence of sunlight and chlorophyll. In India, the plant scientist Rustom Hormusji Dastur (1896 to 1961) studied how water, temperature and colour of light affect photosynthesis.

To see the openings, a thin layer is carefully peeled from the lower surface of a leaf of a plant such as rhoeo, money plant, hibiscus or any grass. The peel is kept in water, placed on a slide with a drop of water, given a drop of ink so that the pores show more clearly, covered with a coverslip and viewed under a microscope. We see tiny pores called stomata (one pore is a stoma). Stomata are present on the surface of leaves and allow exchange of gases. During photosynthesis, carbon dioxide enters the leaf and oxygen goes out through them. During respiration, oxygen enters and carbon dioxide leaves through the same pores. So stomata are the gateways of leaves for the gases of both photosynthesis and respiration.

The roots of a plant absorb water and minerals from the soil. Minerals are important nutrients for growth, and water is needed, among other things, for photosynthesis. To see how they travel, a fresh celery stalk with leaves is cut obliquely under water at its base and placed in a tumbler containing blue-coloured water; a second stalk is placed in plain water for comparison. Next day, the blue colour is visible in the stem and leaves of the first stalk, and a cut across the stem shows blue dots under a magnifying glass. The colour climbs through thin tube-like structures called the xylem, present in the root, stem, branches and leaves. In the same way, minerals dissolved in water travel up through the xylem to the leaves and other parts.

The food made in the leaves by photosynthesis is carried to all parts of the plant through another set of thin tube-like structures called the phloem. This is why the roots, stems, flowers and fruits also get food even though they do not make it themselves. Some food is used at once for energy and growth; the rest is converted to starch and stored in parts such as seeds, roots and underground stems. A potato, for instance, stores starch which was made in the leaves and reached the potato through the phloem. So a plant has two sets of transport tubes: the xylem carries water and minerals from the roots upwards, and the phloem carries food from the leaves to the rest of the plant.

Plants also respire. To check this, soaked moong seeds are kept on wet cotton in a flask closed with a cork that has two tubes, and left in the dark for 24 hours. Air from the flask is then passed through lime water in a test tube, and a second test tube of lime water is kept open to the air for comparison. The lime water connected to the flask turns milky, showing that the seeds released extra carbon dioxide. In respiration, glucose is broken down using oxygen to release energy: Glucose + Oxygen = Carbon dioxide + Water + Energy. The energy is used for growth and development. All parts of a plant, green or non-green, respire. The Indian scientist Kamala Sohonie (1911 to 1998) studied respiration in plants and also worked on the coconut palm sap called Neera as a nutritious drink. Photosynthesis and respiration together keep the balance of oxygen and carbon dioxide in nature.