PLANT TISSUE CULTURE

In plant tissue culture, a very small tissue from a parent plant called as explant is placed in a test tube in a nutrient medium. The tissue may be taken from any part of the plant, that is, root, stem, leaf, anther or embryo. This is because all plant cells possess totipotency meaning a single cell can give rise to an entire plant. The nutrient medium used in tissue culture consists of sucrose apart from mineral salts and vitamins. Plant hormones such as auxins are used to help growth and cell division. The solidifying agent, agar-agar makes the medium semi solid otherwise the culture is done in suspension. The inoculated tubes are kept in an incubator to maintain sterile conditions and controlled temperature and light. After 2-3 weeks of incubation an irregular mass of cells called callus develops, which on sub culturing gives rise to small plantlets. These are potted and maintained in a green house and subsequently transferred to the field. PTC is aimed at engineering crop plants for good traits.

WHY DOES IT SEEM DIFFICULT TO CYCLE UP A STEEP HILL THAN TO PUSH THE BIKE UP AT THE SAME SPEED?

A bicycle is unusual among machines because it has a mechanical advantage (MA) of less than one-typically between 1/3 (in low gear)and 1/8 (in top gear). This helps to convert effort to speed but also means that forces acting on the machine are multiplied by the reciprocal of the MA. When riding on the level, the forces of wind and road resistance are small so not much effort is needed, but when going uphill the force of gravity is also multiplied up, making the journey more difficult. On steep hills it may therefore become easier to walk or run than to cycle. When you push the bike up a hill you tend to be doing about 4 to 5 kilometers per hour whereas while riding, even slowly it is at least 8 or 9 km/hr. Also while cycling up a hill our legs move at about half the rotational speed involved in walking, which means we will have to put in twice the amount of work. Mountain bikers overcome this problem by having extremely low gears on their machines. Cycling in a bike with low gears at 4km/hr is surprisingly easy, so it need not be more difficult.

WHY ONLY STARS BLINK AND NOT PLANETS?

The stars seem to twinkle, because we see the stars through the ocean of air, the atmosphere. The twinkling is caused by differences in temperature in the air. Some layers of air are hotter than others, and one layer is always swirling and moving through another. These different layers of air bend the star light in different ways, and at different angles. It is this passing through layers of air of different temperature that makes the light of the stars unsteady. The stars near the horizon seem to twinkle much more than those high in the sky. This is because the light of these stars has to travel a longer path through a thicker layer of atmosphere, and thus has more chance to become disturb. Sometimes the stars twinkle much more than they do at other times. This is true because at sometimes the atmosphere is not so still as it is at other times, or because there is not such a variation of temperature within its different layers. Planets do not twinkle, ordinarily, but seem to shine with a steady, unwavering light. Even through large telescopes, the biggest stars appear simply as tiny points of light, while the planets show very definite discs and surfaces. Hence, more rays come to us from the surface of a planet than from the surface of a star. The light from the planets does not waver as much as that from the stars the wavering of one ray of light is counteracted by the wavering of another ray in another direction. If one could climb up above the atmosphere surrounding the earth and then look at the stars, he would see them shining with a clear and steady light, with no suspicion of twinkling.