Guttation is a loss of water from plants in liquid form through hydathodes. The force responsible for guttation is positive root pressure. Guttation is easily observed when transpiration rate is low and soil is well watered.

Root pressure helps to pull the cell sap through a plant stem to the leaves. It occurs in the xylem of some vascular plants when the soil moisture level is high either at night or when transpiration is low during the day.

When transpiration is low and soil is generally well watered, a positive root pressure develops. Under the influence of positive root, pressure water oozes out from ends of xylary elements in plants. The main reason for the development of positive root pressure is the active absorption of minerals and water by roots.

Transpiration is defined as loss of water as gas or vapors from aerial parts, like leaves through the stomatal pore. If there is less of transpiration as due to high atmospheric humidity and the soil is well watered; there will be an increase in root pressure. The increased root pressure leads to oozing out of a solution of minerals from hydathodes (called as guttation) and other openings (called as bleeding). 

Root pressure can be defined as the pressure exerted by the cortical cells of the root upon the sap in the xylem vessels which pushes the sap upward if the stem of the plant is cut just above the root water seeps or exudes out of the cut end of the stem for a considerable period of time. This indicates that there is a force pushing water up the stem from the roots.

One atmospheric pressure can raise water to a height of more than $32$ feet. so a pressure of about $13$ atmospheres would be required to raise water at the top of a $416$ feet high tree. The tension developed in the xylem sap of trees that are actively transpiring can be measured only indirectly and has been found to be high as $-77$ bars.

One atmospheric pressure can raise water to a height of more than $32$ feet. So a pressure of about $13$ atmospheres would be required to raise water at the top of a $416$ feet high tree. The tension developed in the xylem sap of trees that are actively transpiring can be measured only indirectly and has been found to be high as $-77$ bars.

Root pressure provides a force which causes pushing of water in a stem. A continuous flow of water is maintained in the plants because the leaves transpire water through stomata and to replace the lost water the water drawn by the roots moves up.

Root pressure can be defined as the pressure exerted by the cortical cells of the root upon the sap in the xylem vessels which pushes the sap upwards if the stem of the plant is cut just above the root, water seeps or exudes out of the cut end of the stem for a considerable period of time. This indicates that there is a force pushing water up the stem from the roots.

Root pressure provides a force which causes pushing of water in a stem. A continuous flow of water is maintained in the plants because the leaves transpire water through stomata and to replace the lost water, the water drawn by the roots moves up.

The root pressure can be measured by attaching a suitable mercury manometer to the cut end of the stem. Root pressure provides a force which causes pushing of water in the stem. A continuous flow of water is maintained in the plants because the leaves transpire water through stomata and to replace the lost water the water drawn by the roots moves up.

Root pressure is the positive pressure that develops in the roots of the plants by active absorption of nutrients from the soil. The active absorption depends on the active accumulation of solute in xylem sap. Root pressure in maximum when transpiration is very low and absorption is high because transpiration is the output of water from a plant, and absorption is the input of water into a plant. If the output is low and input is high, the pressure will be at its greatest. 

Root pressure is defined as the osmotic pressure in the root cell through which the water is translocated into leaves from the stem. The term root pressure was given by Stephan Hales. Thus the correct answer is option A.

Stephentiales (1727) coined the term root pressure. Root pressure is developed when rate of water absorption is more than the rate of transpiration.

When there is no transpiration then due to lack of transpiration pull water does not move upwards. At this time through osmosis water is absorbed from the soil and it accumulates in xylem region of the root .The accumulation through osmosis causes root pressure and water moves upwards.
Thus, the correct option is (A).

Transpiration causes a negative pressure region in the xylem . The leaves and other apical parts require huge amounts of water as it is lost in Transpiration. So the stomata takes up the water from the intercellular spaces of the neighbouring cells that in turn draw water from the xylem . Hence a tension or negative pressure region is created in the xylem tissue. This process significantly drops the water potential in the xylem as well as the root region and the water potential of the soil is higher than the roots. So the roots start absorbing water passively as water flows from high water potential to low water potential. Hence Transpiration causes passive absorption of water in roots.

Root pressure that is developed in the roots pushes the water into the xylem which is then distributed to different plant parts required for growth.

The exudation of liquid drops from the margin of the leaves is called guttation. This happens due to the development of high root pressure as a result of active or more absorption of water at night or early mornings and low transpiration rate. The positive pressure forces the liquid to come out through the special openings called hydathodes.

• The process by which water and minerals translocate from base to apex of the plant is known as the ascent of sap. It takes place through complex tissue xylem. Most important forces for the ascent of sap are; root pressure and capillary action.
• Active transport of water takes place through xylem by a pressure called root pressure. It is responsible for pushing up water to small heights in the stem.
• The tubes in the plant stem are very tiny and the movement of water through tiny tubes is called capillary action. This occurs when the adhesion is stronger than the cohesion. Water molecules are naturally attracted to each other. 

Root pressure is retarded or become absent under conditions of starvation, low temperature, drought and reduced availability of oxygen.

Root exudates function in below ground plant defence. Plant roots are constantly exposed to a variety of natural enemies, including pathogenic microorganisms and root-feeding arthropods. These organisms inhabit the soil surrounding the root system, which is designated the rhizosphere. 

Chemicals secreted into the soil by roots are referred to as root exudates. By doing this root changes the chemical and physical property of soil and inhibit the growth of other plant species. Root pressure can be demonstrated by means of exudation or bleeding caused due to root pathogens.

so the correct answer is 'Exudation'.

Different processes can be measured by various apparatus.