Question

Your teacher has demonstrated various experiments of this chapter. She/he decided to organise a group discussion on these experiments. The students were divided into six groups and were asked to prepare for a particular experiment. They were further asked to learn about the latest experiments conducted related to this chapter and the scientists involved in it. A round of questionnaires would be given to each group to know about their concept and approach and to generate new thoughts towards 'Absorption by roots'.

Answer 1

Absorption by Roots: Group Discussion Plan

1. Experiments given to the six groups

 

1. Experiments Given to the Six Groups

Group	Experiment	What It Shows
1.	Potato Osmoscope: A hollow potato cup with sugar solution is placed in water. The level inside the cup rises.	It shows osmosis.
2.	Dry Raisins or Seeds in Water: Dry raisins or seeds are soaked in water. They swell up.	It shows imbibition.
3.	Rhoeo Leaf Peel: The peel is placed in strong salt solution, and its cell contents shrink. In water, the cells swell back.	It shows plasmolysis and deplasmolysis.
4.	Plant in a Test Tube with Oil Layer: A plant is kept in water covered with oil. The water level falls.	It shows that roots absorb water, as the oil stops evaporation.
5.	Cut Stem with Manometer: The stem of a potted plant is cut and fitted to a manometer. The liquid in the tube rises.	It shows root pressure.
6.	Balsam Twig in Red Ink: A balsam twig is placed in eosin or red ink. Red streaks appear in the stem.	It shows the ascent of sap through the xylem.

2. Scientists involved

Classic work

• Abbé Nollet (1748): Abbé Nollet was the first scientist to observe osmosis.
• W. Pfeffer (1877): W. Pfeffer measured osmotic pressure.
• Hugo de Vries: Hugo de Vries studied plasmolysis.
• Stephen Hales (1727): Stephen Hales showed the existence of root pressure.
• Dixon and Joly (1894): Dixon and Joly proposed the cohesion–tension theory, which is also called the transpiration pull theory.
• J.C. Bose (1923): J.C. Bose proposed the pulsation theory of ascent of sap.

Latest research

• Peter Agre (Nobel Prize 2003): Peter Agre discovered aquaporins, which are protein channels that let water pass through cell membranes. Root cells use these channels to take in water faster.
• Niko Geldner (2011): Niko Geldner found the CASP proteins. These proteins build the Casparian strip in the endodermis of the root.
• José Dinneny (2014): José Dinneny discovered hydropatterning. In hydropatterning, roots form side branches on the side that touches water.
• Malcolm Bennett (2018): Malcolm Bennett discovered xerobranching. In xerobranching, roots stop branching when they lose contact with water, and the hormone ABA controls this process.
• MRI and X-ray CT Imaging: With MRI and X-ray CT imaging, scientists can now watch water moving into living roots in the soil.

3. Sample questionnaire

1. Why does the sugar solution level rise in the potato osmoscope?
2. Why is oil poured over the water in the test tube?
3. Why does a plasmolysed cell recover when placed in water?
4. Root pressure is weak. What force then lifts water in tall trees?
5. What happens to water absorption if soil water becomes saltier than the cell sap?
6. How do aquaporins help root hairs absorb water?

4. New thoughts

• Too Much Fertiliser: When too much fertiliser is added, the soil solution becomes more concentrated than the cell sap. Water then moves out of the root cells by exosmosis, and the plant wilts.
• Drip Irrigation: Drip irrigation gives water only near the roots, which saves water. Hydropatterning also makes the roots branch towards that wet zone.
• Salty Soils: Crops could be bred with more aquaporins or salt-tolerance genes. This would help them grow in saline soil.
• Dry Spells: Knowing about xerobranching can help farmers choose crops that cope better with drought.