What is your reaction if you unexpectedly step on a sharp object, such as a thorn, while walking without shoes, or you accidentally touch a hot cup of tea? List three stimuli (plural of stimulus) and your body's instant response to them.
Answer
I pull my foot (or my hand) away at once, before I have even thought about it — and I usually cry out.
A stimulus is anything or any event that prompts living beings to respond. What the body does about it is the response.
Stimulus
My body's instant response
Stepping on a thorn
The foot is lifted away instantly; I shout
Touching a hot cup of tea
The hand jerks back before I feel the pain properly
Bright sunlight falling on the eyes
The eyes shut and the pupils become smaller
Pepper or dust in the nose
A sneeze
Smell of food when hungry
The mouth waters
A sudden loud noise behind me
I turn round and my heart beats faster
Why the response is so fast: for a dangerous stimulus the body does not wait for you to decide. The message travels from the skin to the spinal cord and the order to move comes straight back, all in a fraction of a second. That is why the hand is already away from the hot cup before you say “ouch”.
Q2.
Do plants also respond to stimuli? Find a few more plants in your neighbourhood which fold their leaves after sunset.
Answer
Yes, plants also respond to stimuli. The book gives two clear examples.
Touch-me-not (mimosa, chhui-mui, lajjalu) — the leaves fold up the moment you touch them, and open again after some minutes.
Sleeping leaves — the leaves of certain plants facing each other tend to come together after sunset. This is seen in the amla (Indian gooseberry) tree.
Other plants in an Indian neighbourhood whose leaves fold after sunset:
Plant
Where you find it
Tamarind (imli)
Roadsides, school grounds
Rain tree (gulabi siris)
Parks and avenues — the whole canopy droops at dusk
Gulmohar
Along roads; the tiny leaflets close in pairs
Sesbania (agasti / hadga) and subabul
Field bunds and hedges
Groundnut, bean and gram plants
Kitchen gardens and fields
Clover / oxalis (khatti-booti)
Lawns and damp corners
Check it yourself: pick one branch of an amla or imli tree that you can see from your window. Draw it at 5 p.m. and again at 7 p.m. on the same day. The second drawing will show the leaflets pressed together in pairs.
Q3.
Why do the leaves of chhui-mui and amla plants respond in this way? Which stimulus could be responsible for their behaviour?
Answer
The two plants respond to two different stimuli.
Plant
Stimulus
Response
How it helps the plant
Chhui-mui (touch-me-not)
Touch — and also a shake, a hot object or a strong wind
Leaflets fold and the whole leaf droops within a second or two
A grazing animal suddenly sees a shrivelled, dried-up looking twig instead of fresh green leaves and moves away; insects sitting on the leaf are shaken off
Amla (and imli, rain tree)
Darkness — the fall of light at sunset, along with the drop in temperature
Leaflets facing each other come together and the leaf folds up for the night
Less water is lost at night, the delicate leaflets are protected from cold, dew and wind, and the folded leaf offers less surface for insects
How the movement is produced: at the base of each leaflet there is a small swollen cushion of cells. When the stimulus arrives, water moves out of the cells on one side of the cushion. That side goes limp, the other side stays firm, and the leaflet is pulled shut. When water returns, the leaf opens again — which is why chhui-mui reopens after a few minutes and amla opens again at sunrise.
Did you know? This response of plants to stimuli was proved with instruments by the Indian scientist Jagadish Chandra Bose, whom you meet on page 194 of this chapter.
Q4.
Have you seen young ones of cats, dogs or other animals? List young ones of five different animals. Have you seen young ones of any non-living things such as a pencil, a chair or an electric bulb?
Answer
Yes, I have seen young ones of animals — and no, non-living things never have young ones.
Animal
Its young one
Cat
Kitten
Dog
Puppy
Cow
Calf
Hen
Chick
Goat
Kid
Frog
Tadpole
Butterfly
Caterpillar (larva)
Non-living things: a pencil has never produced a small pencil, a chair has never produced a stool and a bulb has never produced a little bulb. New pencils, chairs and bulbs are all made in a factory by people — they are not born from an older one.
All living beings reproduce. Reproduction is the process of producing new ones of one's own kind.
Q5.
Why is reproduction necessary?
Answer
It is necessary for the continuity of life. That is the book's own answer, and it is worth unpacking.
Every individual has a fixed life span and finally dies — an adult mosquito lives 10 to 15 days, a bean plant a few months, a person several decades.
If no new individuals were produced, each kind of living being would disappear from the Earth within one lifetime.
Reproduction replaces the individuals that die, so the kind continues generation after generation — the seeds of this year's bean plant become next year's bean plants.
Notice what reproduction does not do: it does not keep the individual alive. The parent bean plant still yellows and dries after fruiting. What survives is the kind. That is exactly why the summary of this chapter says “the process of reproduction maintains the continuity of its kind”.