How do Seeds sprout?
Seeds sprout when favourable temperature (15.38°C), enough oxygen and moisture are available. They absorb large amounts of water and swell. The moisture content of cells rises from 10 per cent to 90 per cent or more. The plant growth hormone, indole acetic acid which controls and promotes various stages of growth, sets into action the various life processes. Cells at the growing tips of the embryo within the seed divide rapidly and the primary root emerges through the soft seed coat. Further changes follow leading to emergence of the shoot Soil is not always necessary for the sprouting of seeds. Seeds will sprout in moist sand; saw dust, peat mosses, cloth or even paper if favourable conditions are available.
Lakes which appear frozen from the top never freeze completely. Only the top layer freezes and a thick layer of ice floats on water which remains liquid below and in which fishes and other aquatic animals survive. With the onset of winter water starts cooling. As it cools, it shrinks in volume and become denser, but only until it reaches 4°C. Below that temperature water expands and its density drops. This comparatively lighter water moves up and finally at 0°C turns into ice. Since the density of ice is less than that of water, the ice floats. Also, ice being a very poor conductor of heat, further cooling of the water below it stops. Thus the temperature of water below the ice layer remains at 4°C and fishes survive in it.
No animal can see in total darkness. However, some animals are capable of making use of faint light of the surrounding. Members of the cat family, like the tiger and cat or other nocturnal animals can see in near darkness because of the presence of large number of cells called rod cells in the retina. These cells are sensitive to dim light and help the animal to see in near darkness.
Spiders spin webs as a means of trapping insects. They have special organs called spinnerets located in their abdomen. Spinnerets are muscular organs which release the thread, independently of one another. Most spiders are equipped with three pairs of spinnerets. Each pair is fed by several silk glands which produce a sticky liquid that can be drawn into fine threads. The sticky liquid turns into a thread by solidifying on exposure to air.
Besides chemicals called pheromones, bees use a kind of dance language to tell other bees in a hive where the food is. When a worker bee locates a source of food such as a tree in bloom, it returns to the hive and performs a ‘dance’. If it is a round dance, that is, if the bee just goes round and round, it only says that food is nearby. A more complicated waggle dance, in which the bee executes a kind of ‘figure of eight’ movement, gives information not only about the distance and direction of the source of food but also the quantity of food expected. It is now known that the bees use gravity and not the sun as the reference point while dancing. That the dance language really works has now been demonstrated beyond doubt by using robot bees.
Honeybees suck nectar from a wide variety of flowers. But each bee usually keeps to a particular kind of flower until its supply diminishes. The nectar is collected by the forager bee in an organ called the honey sac. Here the nectar is broken down to two sugars, fructose and dextrose, by the action of a special enzyme. On returning to the hive the bee empties the nectar load by disgorging it onto the mouth parts of the younger bees. They evaporate some of the water and concentrate the nectar. Glandular secretions further transform it. After this, the nectar is deposited in an empty storage cell in the honeycomb, where the nectar gradually thickens into honey by evaporation. To speed up evaporation bees in the hive fan their wings (26,400 times a minute) to set up additional air currents. After the cells are filled with honey these are sealed with wax by the bees. 
Camels can survive for days in deserts with little food and no water due to their unique physiology. Contrary to the common belief, a camel does not store water in its stomach or its hump. Instead the hump stores fat which serves as a reserve for energy and a source of water when in need. When the fat is utilized by the camel’s body, hydrogen is released which combines with oxygen to form water. Camels also use water from other body tissues in a similar way. As body tissues dehydrate, it can lose as much as 25 per cent of its body weight without suffering any permanent damage. But when it does find water, a thirsty camel may drink up to 150 litres in 15 minutes. The water passes rapidly into its body tissues, making the animal fit for another desert crossing.
Electricity in fish is generated by the normal functions of nerves and muscles together with a chemical reaction. The electricity is produced in modified muscle tissue called electric organs. The nerves from these organs are joined to the brain. The electricity is produced by the passage of a stimulus along a nerve and a contraction in a muscle. The discharge goes from the belly to the back. It is a defensive mechanism in most cases, but is also used to kill the prey. Fishes are immune to their own shocks because of the insulating layers of fat.
It is not quite correct to say that a chameleon can change its colour to match coloured surroundings. A chameleon merely lightens or darkens its body colour in response to changes in the surrounding temperature and brightness. Its colour darkens in dim, cool surroundings and lightens when conditions are reversed. Chameleons also show lightened body colour if suddenly frightened or excited in which case the body colour may not blend with the environment.
All dogs have keenly developed senses of smell and hearing. Dogs naturally depend on their sense of smell to identify friends and foes. They also mark off their territory with their own urine which is individually recognizable to themselves and other dogs. Human beings also have individual ‘smells’ although they are usually unable to identify it. Each individual has a unique odour -the product of diet and subtle differences in metabolism.
Artificial insemination is a method used in animal husbandry to genetically upgrade stock by producing good quality off springs. In humans, the process is used to induce pregnancy in case of reproductive blocks.
Vitamin D is a group of about 10 fat soluble compounds. The form of vitamin D produced in humans is called vitamin and that in plants is called vitamin . Both vitamin and can be formed from their respective precursors in the presence of sunlight. In humans, exposure to sunlight leads to secretion of a compound called 7-dehydrocholesterol at the surface of the skin. The ultraviolet rays in sunlight convert 7-dehydrocholesterol into vitamin D by means of a series of chemical reactions. The vitamin D formed at the surface of the skin is reabsorbed into the blood stream.