Tag: botany

Sir J.C. Bose proposed a theory of ascent of sap known as pulsation theory. The pulsation theory states that water is absorbed by the plants due to the metabolic activity of living cells which are in constant state of rythmatic pulsation. But it was not accepted as water can be passively absorbed by the roots even the cells were dead.

Transportation is the process involving the transfer of materials from one place to another place.

According to the vital force theory, ascent of sap is due to active pulsation of the innermost layer of cortex. This theory was given by J.C Bose. The pulsation theory states that water is absorbed by the plants due to the metabolic activity of living cells which are in constant state of rythmatic pulsation. But it was not accepted as water can be passively absorbed by the roots even when the cells were dead.

The rupture or fractionation do not usually occur in the water column in vessels/ tracheids during the ascent of sap because of cohesion and adhesion. This is because continuity of water column is maintained by cohesion and adhesion. The conduction of water from root to aerial parts of plants is called ascent of sap. The water molecules remain attached to each other by cohesion force. The magnitude of this force is very high. This helps in the continuous water column in the xylem. The adhesive force is present between the water molecules and the walls of xylem. This also helps in continuous water column in the xylem.

The tensile strength of water is its ability to resist the effect of external stress and maintain it's flow. The transpiration all pull exerts great stress on the water column but due to its tensile strength, it doesn't break in between.

Godlewski proposed relay pump theory to explain ascent of sap. According to him there was a rhythmic change in the osmotic pressure of the living cells of xylem parenchyma and medullary rays, which brought about a pumping action of water in an upward direction.

The tracheids and vessels acted as water reservoirs. Janse supported Godlewski and showed that if the lower portion of branch was killed, the leaves above were affected.

Capillarity Theory: It is given by Boehm (1809). According to this theory, water is translocated because water molecules adhere to the surfaces of small, or capillary, tubes. This adhesion causes water to somewhat creep upward along the sides of xylem elements. In glass tubes this upward movement is visible as the curved or crescent-shaped (concave) meniscus. However, it was shown that capillarity (or capillary rise) alone in tubes of similar diameter as that of a xylem element raises water less than 1 meter.

Hopkins (1999) explained that the relationship between the rise of water in a capillary tube and the size of the tube is inversely proportional. This means that the thinner is the tube, the higher will be the rise of water.

The passage of absorbed water from root to the aerial parts of the plant is known as the ascent of sap. It occurs through xylem (lumen of tracheary elements or vessels and tracheids) with a rate of 10 to 75 centimetres per minute. This can be demonstrated by ringing experiments (conducted by Malpighi, 1672; Stephen Hales, 1727; and Hartig in 1837) or dipping the cut end of a fresh shoot (e.g., Balsam) in 2 percent eosin solution for about 30 minutes. Then, the veins of the leaves turn red. Sections through stem and leaves indicate that the xylem vessels/tracheids turn red.

Downward translocation of the organic solutes takes place through phloem. This can be proved by the ringing experiment. Ringing experiment was carried by Hartig, Malpighi and Stephen Hales.
They took a branch with leaves immersed in beaker which is filled with water. Now remove all the tissues which are present above the xylem [cortex, cambium and phloem]. Now placed this experiment in a suitable environment for 2 - 3 days. Then they observed that leaves did not wilt. This experiment makes it clear that water moves up only through vessels and tracheids of the xylem. 
Ringing [girdling] experiment has been done only in dicotyledons because their vascular bundles lies in a ring. While vascular bundles are scattered in monocotyledons, so that this experiment can not be possible.