Understanding the Concept of Sap Ascension

What Do You Mean By Ascent Of Sap

The ascent of sap in the xylem tissue of plants is the upward movement of water and minerals from the root to the aerial parts of the plant . The conducting cells in xylem are typically non-living and include, in various groups of plants, vessel members and tracheids.

Ascent of SAP:

After being taken in by the roots, water is transported throughout the plant, with any excess lost through transpiration. To reach the highest parts of the plant, the water must move upwards through the stem. This upward movement of water is known as Ascent of Sap.

The study of the ascent of sap can be categorized into two main aspects.

What is the Ascent of Sap?

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

When a leafy branch of the balsam plant, which has a partially transparent stem, is cut underwater to prevent air bubbles from entering through the cut end, and placed in a beaker filled with water containing eosin dye dissolved in it, colored lines can be observed moving upwards in the stem after some time. If sections of the stem are cut during this period, only the xylem elements will appear to be filled with colored water.

When a leafy branch is submerged in water and placed in a container, the outer layer of bark is removed from the stem. Over time, it becomes evident that the leaves above the area where the bark was removed remain healthy and green. This occurs because water is consistently being transported to the upper section of the branch through xylem tissues.

(B) Mechanism of Sap Ascension

The process of sap rising in small trees and herbaceous plants is easily understood, but it becomes more complex in tall trees like Australian Eucalyptus and large conifers such as Sequoias. These towering trees can reach heights of 300-400 feet, making the ascent of sap a challenging phenomenon to explain. While there is still much to learn about this mechanism, several theories have been proposed to shed light on how sap moves upward in these remarkable trees.

Understanding the Ascent of Sap: Theories Explored

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

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There are two commonly known theories that exist, however, they lack persuasiveness.

The hypothesis appeared to be only speculative and was subsequently disproven by the investigations conducted by Strasburger in 1891 and 1893. His experiments showed that the upward movement of sap persists even in plant stems where living cells have been destroyed due to the absorption of toxins.

(Bose, in his experiment used an electric probe which was connected to a galvanometer. When the needle of the electric probe was inserted into the stem slowly and slowly, the needle of the galvanom­eter showed some oscillations but when the electric probe needle reached the innermost layer of cortex, the needle of galvanometer showed violent oscillations. He attributed this to the pulsating activity of these cells.)

(B) Root Pressure Theory:

Even though the xylem in the roots can generate root pressure to lift water to a certain level, it is not considered a significant force in the upward movement of sap due to several reasons.

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

(ii) The flow of sap continues even without root pressure. For instance, if a leafy twig is cut underwater and placed in a beaker filled with water, it will stay fresh and green for a considerable amount of time.

Root pressure is not commonly seen in gymnosperms.

What is Meant by Ascent of Sap

Different physical forces can 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.

If it is functioning properly, it will still be unable to lift water higher than 34 feet.

Sachs (1878) proposed that the ascent of sap occurs through imbibition, where water is absorbed by the xylem walls. However, it is now widely accepted that imbibitional force plays a negligible role in the upward movement of sap. This is because sap primarily travels through the lumen or inner space of xylem elements rather than being absorbed by their walls.

To study the ascent of sap, a leafy twig is submerged in water and its cut end is covered with melted paraffin wax. A small portion of the stem near the cut end is removed to reveal the cell walls. The twig is then placed in a beaker filled with water. As a result of this experiment, we observe that the twig starts to droop because the lumens (inner spaces) of xylem elements have become blocked by the presence of wax.

In plants, the xylem vessels are arranged in a stacked manner, creating a continuous channel 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.

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

In Gymnosperms, the presence of vessels is usually lacking, and the other components of xylem do not create uninterrupted pathways.

Ascent of Sap: Transpiration Pull and Cohesion of Water Theory

The concept of the ascent of sap was first introduced by Dixon and Joly in 1894. Over time, it has gained significant support and further development from Dixon himself in 1914 and 1924. This theory has garnered widespread acceptance among various researchers.

The cohesive and adhesive properties of water molecules allow them to come together and form a connected column of water in the xylem.

(ii) The water column is pulled upwards due to transpiration.

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

Even though hydrogen bonds are weak individually, when there is a large number of them present in water, they create a strong cohesive force that keeps the water molecules together in the xylem. This force can be very powerful (up to 350 atmospheres), making it difficult for the continuous column of water in the xylem to be disrupted by gravity or internal tissue obstructions during upward movement.

The ability of water to stick to the walls of xylem, known as its adhesive properties, 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 the stomata in its leaves. This water evaporates into the atmosphere from the intercellular spaces within the leaves. Additionally, more water is released into these intercellular spaces by mesophyll cells. These mesophyll cells obtain this water from the xylem present in the leaf.

As a result of these processes, there is a development of tension in the water present in the xylem vessels located in the leaves. This tension is then transferred downwards to the water within the xylem vessels found in the roots through both the stem and petiole. Consequently, this creates a pulling force that allows for continuous upward movement of water as an unbroken column towards the transpiring surfaces, ultimately reaching even the highest points of plants.

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According to some researchers, one of the main criticisms against this theory is that air bubbles in the conducting channels could disrupt the continuous flow of water. However, other experts argue that either there are no air bubbles or if they do exist, they will not interrupt the water column as it remains uninterrupted through other components of the xylem (Figure 6.2).

What does the term “ascent of” mean?

The ascent of sap refers to the process by which plants and trees transport water and nutrients from their roots to other parts of their structure, such as leaves, stems, and flowers. This upward movement is crucial for the survival and growth of plants.

As water evaporates from leaves during transpiration, it creates a suction force known as tension or negative pressure within these xylem vessels. This tension pulls more water molecules upwards due to cohesion (the attraction between similar molecules) and adhesion (the attraction between different molecules). As a result, an unbroken column of water is formed within these vessels.

The significance of sap in transpiration

Starting from the root system, xylem sap travels through tiny tubes called xylem vessels. These vessels extend throughout the entire plant and provide a pathway for water and nutrients to reach all its parts. As water evaporates from the surface of leaves during transpiration, it creates a pull or suction force that helps draw more water up through the xylem vessels.

During late winter, maple trees exhibit an interesting variation in their xylem sap composition. Maple sap contains not only water and dissolved minerals but also some sugar. This sugary content gives maple syrup its distinct flavor when extracted from these trees.

P.S: It is fascinating how plants have developed such an efficient mechanism to transport essential nutrients against gravity through their vascular system. The ascent of sap ensures that every part of a plant receives adequate nourishment for growth and development.

The flow direction in phloem can be bidirectional depending on various factors including demand and availability across different parts of a plant. This flexibility allows plants to efficiently distribute sugars and other organic compounds required for growth, reproduction, and storage.

P.S: The intricate network of xylem and phloem vessels within plants showcases the remarkable adaptability of nature. Through these specialized transport systems, plants ensure a constant supply of nutrients and energy to sustain their vital functions in different seasons and environments.

The significance of sap in plants

In plants, sap is a fluid that is transported through xylem cells (vessel elements or tracheids) or phloem sieve tube elements. These specialized cells play a crucial role in distributing water and nutrients throughout the plant.