Question

According to Dixon and Joly, cohesive force of water molecules is of the magnitude of
A. 1-10 atm
B. 10-15 atm
C. 45-200 atm
D. 15-45 atm

Answer 1

The attraction between molecules of the same kind, like attraction between water molecules, is called cohesion. The strong hydrogen bonds that water molecules form is the reason why the molecules can adhere to each other. Adhesion is the attraction between molecules of a different kind. Adhesion, surface tension, and cohesion forces work in tandem to cause capillary action.

Complete answer:
Water and nutrients in the soil are absorbed through the roots and transported to the other parts of the plant namely the leaves and the stem. Plant tissues that enable this transport are xylem and phloem. Xylem transports water and water-soluble nutrients from the roots to other parts of the plant. This movement of water and nutrients through the help of xylem is called the ascent of sap. Here the xylem acts as capillaries.
The water vapor is expelled by a leaf opening called stomata. This is known as transpiration. The water deficiency that the leaves experience is responsible for pulling water from the roots to the stems and leaves. This is a passive mode of transport.
In 1894, Henry Dixon and his colleague John Joly gave the cohesion- tension theory that explained the transport of water from the roots with help of xylem to leaves and stem. They proposed that when water is lost in the form of water vapor by the leaf stomata, it creates tension in cells of the leaf (mesophyll cells). This tension causes water to be pulled from the roots. Now, water molecules can adhere to each other (cohesion) and the xylem (adhesion).
In plants, 1 atm pressure can pull water to about 32 feet high. In trees, it usually requires about 75 atm to pull water to a height of 400 feet.

Here the correct option is (C), 45-200 atm is the magnitude of the cohesive force of water molecules.

Note: Cohesion tension theory so far is the only one that can properly explain with evidence the ascent of sap. Transpiration rate is affected by various factors such as-

Temperature- higher the temperature more is the transpiration rate as the plant loses water rapidly from the stomata.
Humidity- more humid the environment less will be the rate of transpiration.
The dearth of water in the soil will decrease the rate of transpiration.
The number of stomata and the number of leaves will affect the transpiration rate.