Tag: botany

In ringing experiment the bark of a tree is removed at a height above the soil surface as a ring. This leads to accumulation of sugars just above the ring of bark removal because sugars are transported through phloem. The ascent of sap which occurs through xylem continues normally because xylem is not part of the bark and bark consists of secondary phloem along with several other tissues.The experiment clearly demonstrates the function of xylem.

In soil, the minerals can be present dissolved in soil solution or adsorbed on charged clay particles. Plants obtain minerals both from the soil solution (through absorption) as well as directly from clay particles (through ion exchange mechanisms). 

According to the most widely accepted theory, ascent of sap occurs through 'Cohesion Tension' or 'Cohesion Transpiration pull'. The cohesion and adhesion forces give rise to a continuous water column in xylem elements, which is lifted by the transpiration pull. Thus, ascent of sap is purely physical process and does not involves energy input.

Xylem tissue is a complex permanent tissue specialized to transport water in plant body. Xylem tracheids are found in a number of plant groups like angiosperms, gymnosperms and pteridophytes. Bryophytes do not possess vascular tissues. Hence, bryophytes do not attain height. Xylem tracheids are present in pteridophytes and gymnosperms. These plant groups lack vessels. Vessels are characteristics of angiosperms only. In angiosperms, xylem vessels form the main channel for ascent of sap.

Vital force theories were propagated to explain the ascent of sap. These theories believed that living cells are involved in the ascent of sap e.g., relay pump theory by Godlewski, 1883 and pulsation theory by Bose, 1923. Bose believed that the inner most cortical cells pump water into the interior, that contains xylem channels. The vital force theories have been discarded because as proved by Boucherie in 1840 and Strasburger in 1891 that water rises up the stem, even when all the living cells are killed by poison or heat.

Ascent of sap occurs through non living xylem tracheids and vessels. The force responsible for pull of water to great heights is the transpiration pull. The properties of adhesion, cohesion and capillarity helps in ascent of sap through non living tracheids and vessels.

Plants absorb water through the entire surface - roots, stems and leaves. However, mainly water is absorbed by roots. They also absorb minerals like nitrogen, phosphorus, etc., from the soil through their roots. The root hairs which are in close contact with the thin film of water surrounding the soil particles absorb the mineral salts such as nitrates, chlorides, sulphates, phosphates, etc., dissolved in this water. However, the concentration of minerals in the soil is very low.

Water moves from a region of its own high concentration to a region of its own low concentration. If the cell sap is highly concentrated as compared to soil solution- it means water molecules are present in high concentration in the soil solution and in low concentration in the cell sap. In such a situation water molecules will move along their concentration gradient and readily move inside the root hairs.

"Pulsation" or "Vital force" theory was a proposed mechanism for the ascent of sap through the xylem tissue of plants according to which, the conduction of water up the xylem vessel is a result of vital action of the living cells in the xylem tissue.
Jagadish Chandra Bose suggested this mechanism for the ascent of sap in 1927. He found electrical pulsations or oscillations in electric potentials, and came to believe these were coupled with rhythmic movements in the telegraph plant Desmodium. However, this theory has now been rejected by majority of plant physiologists.

Dixon and jolly demonstrated the process of translocation of  water and minerals from base to apex of the plant and is known as the ascent of sap as proposed by Dixon and Jolly. The complex tissue xylem in plants is responsible for translocation water and minerals in plants. In blockage experiment, two small shoots of a woody plant are taken. Their cut ends are so prepared that in one shoot phloem is blocked with wax, while in the other xylem is blocked. Now the cut ends are submerged in water. After some time, the shoot with its xylem blocked shows wilting, but the shoot with its phloem blocked remains unaffected.