UNIT Living World of Plants - 6 Plant Physiology Learning Objectives At the end of this unit the students will be able to understand, that Plants too have certain autonomic movements how do plants produce their food through the process of photosynthesis? that Plants are the primary producers that feed the rest of the living organisms Introduction (sunflower) follows the path of the sun from dawn to dusk, (from east to west) and Animals move in search of food, shelter and during night it moves from west to east. for reproduction, even microorganisms The dance of Desmodium gyrans (Indian show movement. telegraph plant) leaf is mesmerizing. Do plants show such movement We see a branch of a tree shaking in heavy thunder storm; and leaves dancing in gentle wind. These movements are caused by an external agency. Do they Move on their 6.1 own Accord In short, do plants have spontaneous movements without external agency? Do they breathe? In this chapter we will study some of the biological functions of plants. 6.2 Do Plants Move The leaves of Mimosa pudica (touch-me- not plant) closes on touching, and in like manner, the stalk of Helianthus annuus Mimosa pudica 6 . L i v i ng Wo l d o P a nt P a nt P y s i o og 133 IX_Science Unit-6.indd 133 27-03-2018 12:31:36 Activity 1 Roots grow down and shoots grow up You can do this experiment with some of your friends. Step -1 It is easy and fun. Take four or ve earthen cups or small pots and ll them with soil from a eld. Add a little cow Helianthus annuus dung to the soil. Make sure the cups have a hole at the bottom. Label them as A, B, and C. Put some seeds of green gram in all the cups, water them daily. As soon as the seeds sprout, select a shoot of same height from each of the cups. Remove all the other shoots from the cups. Step-2 Desmodium gyrans You will need a rectangular box with These movements are triggered by an a small window in one of its sides. A external stimuli. Unlike animals, plants shoe box would be ideal. Cut a 1 × 1 cm may not move on their own from one window in one side. When the shoots location to another, but can move their are 10 cm long, cover the plants in all the body parts for getting sunlight, water and cups except cup A with the rectangular nutrients and are sensitive to external box. When covering the cups, place the factors like light, gravity, temperature etc. box such that opening in each of them face different directions. Keep all the More to Know cups including ‘A’ in open sun light. In Desmodium gyrans (Indian telegraph Step-3 plant), the compound leaf shows three leaets, one terminal large leaet and two Wait for three to four days. Remove very small lateral opposite leaets. e the box and look at the plants again. two lateral leaets move up, move back Are the plants tilted? To which side are and then move down and nally back to the plants tilted? With opening facing the original position showing rhythmic different direction, did all the plants tilt movement. Desmodium gyrans is also the same direction? called the dancing plant. is plant was used by Indian scientist Jagadish Chandra Compile the results and discuss Bose for his experiment. with your friends. Mimosa pudica is called Perhaps you have done such as Thotta surungi and experiments in earlier classes and have Desmodium gyrans is called as ozhu kanni in Tamil. found that plants do grow towards the sunlight. 6 . L i v i ng Wo l d o P a nt P a nt P y s i o og 134 IX_Science Unit-6.indd 134 27-03-2018 12:31:37 Do Plants Actually developed Darwin’s experiments. He found 6.3 Sense Light that if he removed the coleoptiles, plant growth stopped. en he inserted a piece of agar and then placed the coleptile on top of it You would have studied in earlier classes about the plant again was able to curve towards the seed germination. In monocot plants like grass, light. Hence this chemical was predicted to there is only one rst leaf (called cotyledon). be water soluble. He reasoned that chemicals Once germinated, the embryo sends a primary that controlled the light sensitive movement root downward into the soil and pushes the (phototropism) moved through agar. primary shoot to the soil surface. e tip of the embryonic shoot (plumule) is covered by a protective sheath called coleoptile. It is the coleoptile covered embryonic shoot that grows above the ground. In one of the experiments initially Darwin noticed that germinated seeds also grow towards light. If, in a dark room light is placed in one end, the growing seedlings bend towards the light. In the next experiment, he carefully removed the tip from growing seedlings. e shoot did not bend towards the light. When he covered the tip (Coleoptile) with foil, preventing light from entering, the shoot did not bend. In the next step, he covered the elongating part of the shoot, but le the tip open. Now the bending could be In his next experiment he replaced the noticed. Covering the elongating part of the agar block with butter. e above mentioned shoot did not a ect the response to light at water soluble chemical did not dissolve in all! Darwin concluded “some ‘in uence’ is butter, as expected the chemical did not pass transmitted from the tip to the more basal through butter. is water soluble “in uence” regions of the shoot thereby regulating growth was later identi ed as the plant hormone and inducing curvature”. is demonstrated Auxin. In response to light, auxin elongates clearly that something within the tip of the the cells on the dark side of a stem so that the coleoptile controlled growth of the plant. plant literally bends towards the light source. C a r l e a r w n Pete Bo s en J en en What is this in uence? In 1913 Peter Boysen-Jensen, a Danish Botanist further Bend ng o t e te tow r d i g t 6 . L i v i ng Wo l d o P a nt P a nt P y s i o og 135 IX_Science Unit-6.indd 135 27-03-2018 12:31:38 Types of Tropisms in Tropism is generally termed 6.4 Plants “positive” if growth is towards the signal and “negative”, if it is away from the signal. While the shoot in a plant moves Tropism is a growth movement, the direction towards the light, the roots move away in of which is determined by the direction of the the opposite direction. us the shoots are stimuli. Unidirectional movement of plant said to have positive phototropism, while part to light stimulus is called Phototropism. the roots have negative phototropism. Heliotropism or motion of plant parts like Can you think of an example of a negative owers or leaves in response to the direction hydrotropism? of the sun is a kind of phototropism. e plant may also respond to gravity and it is termed Geotropism or Gravitropism. e response to water is called Hydrotropism. Climbing vines have to nd a suitable support shortly a er germination. Once the shoot touches a suitable support, it grows towards the surface it is touching. is is called igmotropism. Po i t v e h otot o i s m Growth or movement of a plant in response to chemical stimuli is called ( neg t v e y geot o i c Chemotropism. During fertilization, pollen tube grows down the style in response to the sugars in the style is an example of chemotropism. Neg t v e h otot o i s m ( p o i t v e y geot o i c Activity 2 P otot o i s m Experiment to demonstrate phototropism Info bits i) Take pea seeds soaked in water Tropism allows plant to get the best overnight. conditions for its survival. ii) Wait for the pea seeds to germinate. Tropic movements are slow, iii) Once the seedling has grown put it directional movements towards in a box with an opening for light or away from the stimulus and it on one side. depends on growth. iv) After sometime, you can clearly Nastic movement is an immediate see how the stem has bent and action. grown towards the light. 6 . L i v i ng Wo l d o P a nt P a nt P y s i o og 136 IX_Science Unit-6.indd 136 27-03-2018 12:31:38 Info bits Some halophytes produce negatively geotropic roots (E.g. Rhizophora). ese roots turn 180° upright for respiration. Activity 3 Water or gravity When a seed germinates the primary shoot, Experiment to demonstrate (plumule) goes up while the primary Geotropism root, (radicle) goes down. What stimulus i) Two Clinostats are taken and a is making the primary root to go down? potted plant on each is fixed on a Is it gravity or water that is principally horizontal position. responsible for the direction of the growth ii) One Clinostat is rotated and the of a primary root? We can do an interesting other is kept stationary. experiment to find out for ourselves. Observations made after sometime will What do you need? Glass trough, show that the shoot of the plant fixed on sand, flower pot, plugged at the bottom, the stationary clinostat bends upwards pea or bean seeds and water.