Keshav Jain

            All plants respond to touch, but Mimosa pudica, or the “touch-me-not” plant is more sensitive than the others. Whenever touched, the leaves of the mimosa plant close and drop immediately, appearing like a dead plant. Possibly it is a defense mechanism by which the plant protects itself against its enemies.

   The opening and closing of the leaflets and the entire leaf are controlled by fluid-filled sac-like structures found at the base of not only the compound leaf but of each leaflet as well. When the leaf is touched, a chemical signal immediately moves to the base of each leaflet making the fluid drain away from the sac which makes the leaflet fold. When the signal reaches the base of the whole leaf, it makes the entire leaf drop. If left untouched for some time, the sacs fill up and the leaflets open again.

            Lightning occurs when a massive electrical discharge takes place between two oppositely charged clouds or between a charged cloud and the ground. The charges develop in thunder clouds due to the friction of water droplets with air as the droplets move up and down with the rising and descending air currents within the cloud. During a bolt of lightning, thousands of amperes of electricity flows through the air in a fraction of a second. This rapidly heats up the air along its path which expands very fast producing shock waves which we hear as thunder. Although the lightning and thunder are produced at the same instant, we hear the thunder later because light travels faster than sound.

    The reason lies in the water cycle. The total quantity of water in the lakes, rivers and oceans of the world remains almost the same always; only the distribution changes periodically. The water cycle begins as the Sun evaporates water from oceans, lakes, rivers and soil. The vapour forms clouds which precipitate as rain or snow, 75 per cent of which falls on the oceans. The remainder falls on land. Some of this water seeps into the earth as groundwater while some is returned to the oceans by the rivers. Thus there is constant balance between water flowing into the oceans and water evaporating from them. That is why the oceans do not overflow.

            The Earth’s axis – the imaginary line around which the Earth turns daily is titled at an angle of  to the plane of its orbit. The seasons are caused by this tilt and by the fact that the Earth’s axis always holds the same orientation in space. As the Earth travels round the Sun, the North Pole is sometimes directed towards the Sun and sometimes directed away from it. When the North Pole is directed towards the Sun, sunray’s strike the northern hemisphere almost directly. This causes summer in the northern hemisphere and the southern hemisphere has winter. Three months later, the North Pole begins to slant away from the Sun. This causes the northern autumn and southern spring. After another three months the North Pole it tilted away from the Sun, this causes winter in the north and summer in the south. Finally the North Pole tilts towards the Sun again, bringing spring to north and autumn to south.

The main reason why some trees shed leaves is to conserve water during the winter months. When root activity and water absorption decrease, deciduous trees drop their leaves. This helps them keep water loss at the minimum. Unlike deciduous trees, which lose their leaves all at the same time, evergreen trees, mostly conifers, shed and replace leaves continuously. Before the leaves fall, triggered by changes in temperature and daylight, groups of the soft cells from around the base of the leaf stalk. These cells produce an enzyme which dissolves the stalk cell walls and causes leaf to detach. Discarding their leaves also helps trees dispose of toxic and waste products which accumulate in the leaves before they fall.

            Sea water is salty because it contains many dissolved salts, mostly sodium chloride with small proportions of potassium, magnesium, calcium and carbonates. The salt comes mainly with river water that flows through rocks eroded by frost and rain. The gradual wearing a way of mountains releases chemicals which are carried down by rivers to the ocean as dissolved salts. Some salt also enters sea water from the rocks beneath the sea bed. The river water carrying the salts to the sea does not taste salty because the salts are present in extremely small concentration. Whereas in the case of the oceans, water is continuously evaporating even as more and more salt continues to be added with river water. This, over millions of years has led to the high concentration of dissolved salts in sea water.

            During a total solar eclipse the Moon moves across the bright disc of the Sun from west to east. So, as the eclipse progresses, the Moon’s shadow too moves in the same direction. The Earth too spins from west to east. But the speed of movement of the Moon’s shadow is much greater than the speed at which the surface of the Earth moves from west to east. As a result, the Moon’s shadow still moves from west to east along the path of totality.

  All the planets of our solar system were formed out of the same cloud of gas and dust that gave rise to the Sun, some five billion years ago. As random collisions and gravitational forces compacted the gas and dust particles into the various planets, the heat generated brought the masses into a molten state. Since surface tension of a liquid tends to make the surface area minimum and the sphere has the minimum surface area for a given volume, all the planets in the molten state became spherical and have retained that shape as they cooled.

 The phase of the Moon as seen from the Earth arises because of the constantly changing position of the Moon relative to the Sun and the Earth. As the Moon goes round the Earth, one half of its surface that faces the Sun is always illuminated. But from Earth, the entire illuminated side is visible only when it faces us, that is, only at full Moon. At other times, depending on the position of the Moon in the sky relative to the Sun, only a part of the illuminated face becomes visible, which grows and shrinks in size, giving rise to the familiar phases of the Moon.