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.
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Ascent of SAP:
The water that is taken in by the roots is transported throughout the plant, with any extra water being lost through transpiration. To reach the highest parts of the plant, the water needs to move upwards through the stem. This upward movement of water is known as Ascent of Sap.
The process of sap movement upwards can be examined in two main ways:
What Does Ascent of Sap Refer To?
It is widely accepted that the upward movement of sap occurs within the xylem.
The stem of a balsam plant, which has a semi-transparent appearance, is cut underwater to prevent air bubbles from entering. It is then placed in a beaker filled with water that contains eosin, a dye. Over time, colored lines can be observed moving upwards within the stem. 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 twig is submerged in water and placed in a beaker filled with water, if the ring of bark (the outer tissues surrounding the vascular cambium) is removed from the stem, it can be observed that the leaves above this ringed section of the stem stay fresh and green. This happens because water is consistently being transported to the upper part of the twig through its xylem.
(B) Mechanism of Sap Ascent:
The process of sap ascent is easily explained in small trees and herbaceous plants. However, it becomes more challenging to understand how water rises to great heights in tall trees like Australian Eucalyptus and giant Sequoias. These towering trees can reach heights of 300-400 feet, presenting a problem for the mechanism behind sap ascent. Despite not fully grasping this mechanism, several theories have been proposed to explain it.
Understanding the Ascent of Sap: A Closer Look
Proponents of vital theories believe that the upward movement of sap is regulated by essential activities within the stem.
There are two commonly proposed theories, but they lack strong evidence and are not very persuasive.
The idea was considered only theoretical and was later disproven by the experiments conducted by Strasburger in 1891 and 1893. He showed that the upward movement of sap still occurs even in stems where living cells have been destroyed due to the absorption of toxins.
In his experiment, Bose employed an electric probe connected to a galvanometer. As he inserted the probe needle into the stem gradually, the galvanometer needle displayed slight oscillations. However, when the probe reached the innermost layer of cortex, the galvanometer needle exhibited intense oscillations. Bose attributed this phenomenon to the pulsating behavior of these cells.
What Does Root Pressure Theory Mean?
However, the force of root pressure in the xylem of the roots is not considered to be a significant factor in the upward movement of sap for several reasons.
The root pressure is relatively weak, 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 observed in gymnosperms.
Understanding the Concept of Ascent of Sap
Different physical forces can play a role in the upward movement of sap.
It is unable to affect the water that is already in the xylem of roots.
If it is functioning properly, it will still be unable to lift water higher than 34 feet.
Sachs (1878) believed that the ascent of sap occurred through imbibition, where water is absorbed by the walls of xylem. However, it is now widely accepted that this imbibitional force plays a negligible role in the upward movement of sap. Instead, it has been established that the primary pathway for sap ascent occurs through the lumen or inner space of xylem elements, rather than via their walls.
To study the ascent of sap, a leafy twig is submerged in water 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 twig is then placed in a beaker filled with water. As a result of wax plugging the lumens of xylem elements, the twig quickly loses its turgidity and wilts.
In plants, the xylem vessels are arranged in a stacked manner, creating a continuous pathway that resembles 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 developing and there is a higher demand for water, the wood contains wider elements. Conversely, in autumn, when the supply of water decreases, the wood consists of narrower elements. This contradicts the concept of capillarity.
In Gymnosperms, the presence of vessels is generally not observed. The other components of xylem do not create uninterrupted pathways.
What is the Meaning of Ascent of Sap?
The concept of the 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 significant acceptance among researchers and is now widely supported by many scientists.
The cohesive and adhesive characteristics of water molecules allow them to combine and create a connected column of water within the xylem.
(ii) The water column is pulled upwards due to the process of transpiration.
Water molecules are able to stick together because of the hydrogen bonds formed between them. These bonds occur when a hydrogen atom is sandwiched between two electronegative atoms, creating a connection represented by dotted lines. In the case of water, the positively charged hydrogen atoms from one molecule are bonded to the negatively charged oxygen atoms of other water molecules through these hydrogen bonds.
Although hydrogen bonds are relatively weak, they become significantly stronger when present in large quantities, such as in water. This leads to a strong cohesive force between water molecules, allowing them to form a continuous column within the xylem. The magnitude of this force can be quite high, sometimes reaching up to 350 atmospheres. As a result, the continuous water column in the xylem is resistant to being broken by gravity or any internal tissue obstructions that may hinder its upward movement.
The cohesive nature of water, which refers to the attraction between water molecules and the walls of xylem, 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. Additionally, more water is released from the mesophyll cells into the intercellular spaces within the leaves. These mesophyll cells obtain their water supply from the xylem present in the leaf.
As a result of these processes, there is a build-up of pressure in the water within the xylem vessels located in the leaves. This pressure is then transferred downwards through the xylem vessels found in the roots via the stem and petiole. Consequently, water is drawn upwards as a continuous and uninterrupted column to reach the surfaces where transpiration occurs, ultimately reaching even the highest points of plants.
According to some researchers, one criticism of this 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 break the continuous flow of water through other parts of the xylem.
What do root pressure and sap ascent mean?
Root pressure is a form of osmotic pressure that occurs in the roots and aids in the upward movement of sap towards the leaves. It facilitates the absorption of water by root hair cells from the soil, resulting in an increase in pressure within the root system. This increased pressure then propels the sap to move upwards through xylem vessels, ultimately reaching various aerial parts of plants such as stems and leaves.
The role of sap in transpiration
Xylem sap plays a crucial role in the transportation of soil nutrients, such as dissolved minerals, from the roots to the leaves of a plant. This process is known as ascent of sap. The xylem vessels act like tiny pipelines, allowing water and nutrients to move upwards against gravity. As water evaporates through small openings called stomata on the leaf surface during transpiration, it creates a pull or suction force that helps draw more water up from the roots.
In contrast to xylem sap, phloem or sieve-tube sap carries sugars produced during photosynthesis from the leaves to other parts of the plant during summer months. Unlike xylem vessels which transport fluids upwardly, phloem tissue transports them both upwards and downwards throughout various regions of plants using specialized cells called sieve tubes.
What does the ascent of sap mean for grade 10?
The process of water and mineral movement from the roots to all other parts of a plant is called the ascent of sap. This essential process allows plants to absorb nutrients and distribute them throughout their entire structure. The xylem vessels, which are like tiny tubes in plants, play a crucial role in conducting water and minerals from the roots upward.
During the ascent of sap, water is absorbed by the root hairs present on the surface of plant roots. These root hairs have a large surface area that helps in efficient absorption. Once inside the roots, water moves into small cells called xylem parenchyma cells through osmosis.
From there, it enters into long hollow tubes known as xylem vessels or tracheids that make up the xylem tissue. These vessels act as conduits for transporting water and dissolved minerals upwards towards leaves and other aerial parts of the plant. This upward movement occurs due to several factors including transpiration (water loss through stomata) and capillary action (the ability of liquids to flow against gravity).
Overall, understanding how plants transport water and minerals through their system is important for studying their growth, development, and overall health. The ascent of sap ensures that necessary nutrients reach every part of a plant efficiently so it can thrive in its environment.
