Question

Root pressure theory cannot explain the ascent of stap because it is not found in
A. Bryophytes
B. All plants and in all seasons
C. Trees
D. Spring

Answer 1

Hint: Root pressure, in plants, is the force that helps to push fluids upward into the water-conducting vessels (xylem).

Complete Answer:
- It is primarily caused by osmotic pressure in the root cells and can be demonstrated by the exudation of fluid when the stem is cut off just above ground. It is partly responsible for the growth of water in plants.
- The root-pressure theory of the growth of the sap holds that the pressure in trees is often atmospheric, not nearly enough to carry water to the top of the tallest trees.In addition, root pressure appears to be lower when the water loss from the leaves (transpiration) is higher, precisely when the trees most need water.
- The lifting force produced by the evaporation and transpiration of the water from the leaves and the coherent and adhesive forces of the molecules in the vessels, and probably other factors, all contribute to the rise of the sap in the plants.
- After being absorbed by the roots, the water is dispersed to all parts of the plant (the excess of which is lost by transpiration). In order to enter the topmost areas of the plant, the water has to pass upwards through the stem. This upward flow of water is called the Ascent of Sap.

Although the root pressure that develops in the xylem of the roots can raise water to a certain height, it does not appear to be an effective force in the rise of the sap due to the following reasons:
i) Root pressure magnitude is very low (about 2 atms).
ii) Even in the absence of root strain, the absence of sap persists. For example, when a leafy twig is cut under water and put in a beaker full of water, it stays fresh and green for a long time to come.
(iii) Root pressure in gymnosperms has been rarely observed
The plants with and without roots show an ascent of sap.

Thus the correct answer is option(B) All plants and in all seasons.

Note: Root pressure is more common in spring seasons before the leaves grow and the rate of transpiration is rapid. The effects of root pressure can only be seen during the night and early morning, as the evaporation rate is very low.