Ascent of Sap
Short questions and answers
One mark questions and answers
1. Ascent of sap takes place as a result of
(b) transpiration pull
(c) root pressure
Answer: (b) transpiration pull
2. Upward movement of water in plants is
(d) ascent of sap
Answer: (d) ascent of sap
3. Force developed in cortex of root pushes water into xylem of root is
(a) root pressure
(b) turgor pressure
(c) osmotic pressure
Answer: (a) root pressure
4. The force of tension-cohesion exceeds root pressure on a
(a) rainy day
(b) sunny day
(c) foggy day
(d) full moon day
Answer : (b) sunny day
5. Wilting of plants occurs when
(a) phloem is blocked
(b) xylem is blocked
(c) both phloem and xylem are blocked
(d) a few old roots are removed
Answer: (b) xylem is blocked
6. Which one of the following is the most widely accepted explanation for ascent of sap?
(b) atmospheric pressure
(c) pulsation activity
(d) transpiration pull
Answer: (d) transpiration pull
7. The water is translocated apart from the roots through
Answer: (a) xylem
8. Exudation or bleeding is associated with
(c) hydrostatic pressure in the root
(d) pulsation in the innermost layer of cortex
Answer: (c) hydrostatic pressure in the root
9. The most widely accepted explanation for the ascent of sap in trees is
(b) atmospheric pressure
(c) pulsating action of living cells
(d) transpiration-cohesion theory of Dixon
Answer: (d) transpiration-cohesion theory of Dixon
10. In plants water supply is due to
(d) cohesive force
Answer: (a) osmosis
11. Xylem transport of water in tall trees appears mainly due to
(a) metabolic activity in xylem cells
(b) root pressure
(c) capillarity rise in xylem open pipes
(d) transpiration pull and cohesion of water molecules
Answer: (d) transpiration pull and cohesion of water molecules
12. Flowers cut underwater remain fresh for longer time because
(a) it has sufficient supply of water
(b) water column is not blocked by air bubbles
(c) these transpire very rapidly and hence should be cut underwater
(d) there is no vascular supply to the leaf
Answer: (b) water column is not blocked by air bubbles
13. The cohesion-tension theory was first introduced by
(a) Dixon and Jolly
(d) JC Bose
Answer: (a) Dixon and Jolly
14. Which of the following will immediately reduce the ascent of sap in a tall tree?
(a) inhibition of root pressure
(b) closure of hydathodes and lenticels
(c) removal of bark of the trees
(d) spraying phenyl mercuric acid on the leaves
Answer: (d) spraying phenyl mercuric acid on the leaves
15. Ascent of sap in tall trees is due to
(a) tensile strength of water and root pressure
(b) tensile strength of water and transpiration
(c) pulsation of cortical cells and transpiration
(d) root pressure imbibition of water by vessel walls
Answer: (b) tensile strength of water and transpiration
Two marks questions and answers
1. What is ascent of sap?
Answer: The process of translocation of sap from the roots to the top of the plant is called ascent of sap. Once the water is absorbed by the root hairs it is translocated to various parts of the plant. The fluid that moves upward in the stem is not pure water. It is a dilute solution of mineral ions absorbed from the soil and is referred to as sap.
2. What is the difference between root pressure and transpiration pull?
Answer : Root pressure is a pressure developing in the elements of the xylem as a result of the metabolic activity of the roots where as transpiration pull is a pressure developing at the top of the plant due to the evaporation of water from the leaves.
3. Define root pressure. How does it help in ascent of sap?
Answer: Root pressure is an osmotic phenomenon. Root pressure can be defined as a pressure developing in the tracheary elements of the xylem as a result of the metabolic activity of the root. It is a positive pressure in the roots which pumps the sap up in the xylem ducts.
4. Mention the demerits of Sir J.C. Bose’s pulsation theory.
Answer: The demerits of Sir J.C. Bose’s pulsation theory are as follows:
(1). There was no relation between the pulsatory activity and the rate of translocation.
(2) The pulsatory movement of sap, according to theory, involves transfer through 200-400 living cells per second. But comparison of the maximum pumping capacity of the most rapidly pulsating rate found by Bose and the actual rate of sap flow under maximal conditions of transpiration has shown that the latter is 8000-30,000 times as rapid as would be possible under the theory proposed by Bose.
2. Why was the relay pump theory discarded?
Answer: The theory was discarded because of the following reasons:
- Water moves upwards even in the absence of living cells.
- Metabolic inhibitors affecting the living cells do not change the rate of ascent of sap.
- Wood anatomy does not show the arrangement of living and xylem cells as proposed in Godlewski’s theory.
Three marks questions and answers
1. Who proposed the pulsation theory? Explain the pulsation theory.
Answer: Sir J.C. Bose (1923) proposed this theory. He invented an electrical probe by which the activity of cell inside the plant body can be detected. He inserted the needle of an electric probe up to certain distance in the stem, i.e., up to cortex and found pulsating movement in the cortical cells. He believed that when the cells of this zone expanded, they suck water from outer surface and pumped it into the inner cell on contraction. He came to the conclusion that cell associated with the xylem show pumping action and pump it into the xylem.
2. What is the relay pump theory? Who proposed it?
Answer: Godlewski (1984) proposed the relay pump theory. He suggested that ascent of sap takes place through xylem parenchyma and medullar rays whereas the vessels and tracheids of xylem serve as reservoir. Upward movement of water takes place due to the periodic increase or decrease of permeability of living cells.
According to him, parenchymatous cells first draw water from the vessel below it and put it into other vessel above it. Thus, by alternative passage through living and nonliving cells, water raises. That is why the theory is known as relay-pump theory.
3. Explain Boehm’s capillarity theory. Mention its demerits.
Answer: This theory was proposed by Boehm (1809). It is a well known fact that capillaries exist in plant body and finer the capillaries greater the raise of liquid through them. This is the basis of capillarity theory proposed by Boehm.
However, the calculations of various workers have clearly revealed that even very fine capillary cannot lift water for long distance. A capillary of 0.03 mm diameter carries water up to 4 feet. The diameter of thinnest plant is 0.63 micron. Hence, this theory is discarded. Moreover, in gymnosperm, vessels are absent and tracheids have numerous septa which offer resistance to capillary movement. Capillarity also requires free surface and direct contact with soil water.
4. What do you mean by transpiration pull? Explain.
Answer: Transpiration causes the evaporation of water from mesophyll cells of the leaf. This results in an increase in the diffusion pressure deficit (DPD) or suction pressure.
Since the water in the mesophyll cells is in contact with the xylem sap of stems and roots through tracheids in the veins, the diffusion pressure gradient gradually passes down to the silence of the root and water is pulled up. Thus due to transpiration there is a constant pull or tension on water column in upward direction. This is called transpiration pull or tension in the water column of xylem due to transpiration.
5. Explain the cohesion-tension theory.
Answer: This theory was proposed by Dixon and Jolly (1894). The water molecules have a strong mutual attraction i.e., they tend to stick to each other. This is called cohesion. They also tend to stick to the wall of the xylem elements; this is called adhesion. The cohesive and adhesive forces do not allow the water column to break or pull away from the walls of the xylem.
Evaporation due to transportation results in an increase in the diffusion pressure deficit. The diffusion pressure gradient gradually passes down to the xylem of the root. Due to transpiration pull there is a constant pull or tension on water column in the upward direction. The continuity of water column is maintained even under conditions of considerable pull because of strong cohesive forces between the water molecules.