The Groundbreaking Vital Theory Unveiling the Ascent of Sap

Vital Theory Of Ascent Of Sap

We will delve into a comprehensive examination of the rise of sap. By the end of this article, you will gain knowledge on (A) The route taken by sap as it ascends (B) How sap rises and the various theories explaining its ascent.

Theory of Sap Ascension

After being taken in by the roots, water is spread throughout the entire plant (with any extra amount being released through transpiration). To reach the highest parts of the plant, the water must travel upwards through the stem. This upward motion of water is referred to as Ascent of Sap.

The phenomenon of sap rising in plants can be examined from two perspectives.

The Pathway of Sap Ascension:

It is widely accepted that the upward movement of sap occurs within the xylem.

A branch from a balsam plant with a see-through stem is submerged in water to prevent air bubbles from entering through the cut end. The branch is then placed in a beaker filled with water mixed with eosin, a dye. Over time, colored lines can be observed moving upwards within the stem. If sections of the stem are cut at this point, only the xylem elements will appear to contain colored water.

When a leafy branch is submerged in water and placed in a container filled with water, if a ring of bark (the outer tissues) is removed from the stem, it can be observed that the leaves above the ringed area stay fresh and green. This occurs because water continues to be supplied to the upper part of the branch through the xylem.

(B) Mechanism of Sap Upward Movement

The process of sap ascent is easily explained in small trees and herbaceous plants, but it becomes more challenging to understand in tall trees like the Australian Eucalyptus and certain conifers such as the Sequoia. These towering trees can reach heights of 300-400 feet, requiring water to travel up several hundred feet. While the exact mechanism of sap ascent remains unclear, various theories have been proposed to explain it.

Theory of Sap Ascent: An Essential Perspective

Advocates of vital theories believe that the upward movement of sap is regulated by essential activities within the stem.

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There are two commonly proposed theories, but they lack strong evidence and do not seem to be very persuasive.

This theory appeared to be only speculative and was later disproven by the experiments conducted by Strasburger. In his studies in 1891 and 1893, he showed that the upward movement of sap still occurs even in plant stems where living cells have been destroyed due to the absorption of toxins.

In his experiment, Bose utilized an electric probe connected to a galvanometer. As the electric probe needle was gradually inserted into the stem, there were slight oscillations observed in the galvanometer needle. However, when the electric probe reached the innermost layer of cortex, intense oscillations were detected by the galvanometer needle. Bose believed that this phenomenon was caused by the pulsating activity of these cells.

Root Pressure Theory

While root pressure can elevate water to a specific level within the roots, it is not considered a significant force in the upward movement of sap. This is because of various reasons that hinder its effectiveness.

The root pressure is quite low, measuring around 2 atmospheres.

(ii) Even without the force of root pressure, the upward movement of sap still occurs. For instance, if a leafy branch is cut and submerged in water, it will remain fresh and vibrant for a considerable period of time.

Root pressure is not commonly observed in gymnosperms.

Theory of Sap Ascension: Physical Force Explanations

Different physical forces might play a role in the upward movement of sap.

It is unable to have an effect on the water that is already in the xylem of the roots.

(ii) Even if it is functioning properly, it will still be incapable of lifting water higher than 34 feet.

Sachs (1878) advocated the idea that sap could rise through the xylem walls via imbibition. However, it is now widely recognized that imbibitional force plays a negligible role in the ascent of sap since it primarily occurs through the interior space of xylem elements rather than their walls.

To investigate the ascent of sap, a submerged leafy twig is taken and its cut end is coated with melted paraffin wax. A small portion of the stem near the cut end is carefully removed to expose the cell walls. The treated twig is then placed in a beaker filled with water. As a result of this procedure, the lumens (inner spaces) of xylem elements become blocked by the presence of wax, causing the twig to quickly lose its turgidity and wilt.

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In plants, the xylem vessels are arranged in a stacked manner, creating a continuous pathway similar to long capillary tubes. It was previously believed that just as water rises in capillary tubes due to capillary force, the ascent of sap also occurs in the xylem.

During spring, when new leaves are growing and there is a greater need for water, the wood contains wider elements. Conversely, in autumn, when the water supply decreases, the wood consists of narrower elements. This contradicts capillarity.

In Gymnosperms, the absence of vessels is common and there are no continuous channels formed by other xylem elements.

(D) Transpiration Pull and Water Cohesion Theory

The theory of ascent of sap was first put forward by Dixon and Joly in 1894. Over time, Dixon further developed and strengthened this theory in his works from 1914 to 1924. This theory has gained widespread acceptance and support from numerous researchers.

The cohesive and adhesive characteristics of water molecules enable them to create an unbroken column of water within the xylem.

(ii) The water column experiences a pull due to transpiration.

Water molecules are held together by hydrogen bonds, which occur when a hydrogen atom is sandwiched between two electronegative atoms. In the case of water, the positively charged hydrogen atoms from one water molecule form bonds with the negatively charged oxygen atoms of other water molecules.

Even though hydrogen bonds are relatively weak, when they exist in large quantities like in water, they create a strong cohesive force between water molecules. This force allows the water to form a continuous column in the xylem, resisting gravity and other obstacles that may hinder its upward movement. The magnitude of this force can be quite high, sometimes reaching up to 350 atmospheres.

The stickiness of water, which refers to the attraction between water molecules and the walls of xylem (the container), helps maintain a continuous flow of water in the xylem.

During the process of transpiration, water is lost from the upper parts of a plant through evaporation in the leaves. This occurs when water evaporates from the spaces between cells and exits into the atmosphere through small openings called stomata. Additionally, more water is released into these intercellular spaces from specific cells within the leaf known as mesophyll cells. These mesophyll cells draw water from a specialized tissue called xylem that transports water throughout the leaf.

As a result of these processes, there is a build-up of tension within the water present in the xylem vessels of the leaves. This tension is then transmitted downwards through the xylem vessels found in the stem and petiole, eventually reaching the roots. Consequently, water is drawn upwards in an uninterrupted column to reach the surfaces where transpiration occurs, ultimately extending all the way to the topmost parts of plants.

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According to some researchers, a major criticism of the theory is that air bubbles in the conducting channels could disrupt the flow of water. However, others argue that there are either no air bubbles or if they do exist, they would not interrupt the continuous movement of water through other components of the xylem.

The essential principle of water transportation

According to the vital force theory, the upward movement of water in xylem vessels is attributed to the active functioning of living cells within the xylem tissue. These living cells play a crucial role in facilitating the ascent of sap. The relay pump theory and pulsation theory also support this active mechanism for explaining how sap rises through plants.

– Vital force theory: Proposes that living cells in xylem tissue actively contribute to the conduction of water upwards.

– Relay pump theory: Supports the idea that certain specialized cells act as pumps, aiding in pushing sap up through plant tissues.

– Pulsation theory: Suggests that rhythmic contractions and expansions of cell walls help propel sap upwards.

The role of sap ascent

The process of ascent of sap in plants plays a crucial role in their survival and growth. It serves multiple functions that contribute to the upward movement of water through the xylem vessels, allowing it to reach great heights within the plant structure. Additionally, it aids in creating a suction force known as transpiration pull, which facilitates the absorption of water by roots from the soil.

Another significant function is how the ascent of sap contributes to root absorption. Through transpiration pull, when moisture evaporates from leaf surfaces into the atmosphere, it generates a suction force that draws up additional water molecules from roots into stems and leaves. This mechanism helps maintain an adequate supply of water throughout different parts of plants and supports various physiological processes necessary for growth.

What is sap ascent and can you give an example?

– The ascent of sap is when water moves upwards in plants.

– Water creates a continuous column in tracheary elements.

– This column extends from root hairs to the tip of the plant.