Question

The most widely accepted explanation for the ascent of sap in tree is (a) Capillarity (b) Roll of atmospheric pressure (c) Pulsating action of living cells (d) Transpiration cohesion theory of Dixon

Short answer

The correct answer is (d) Transpiration cohesion theory of Dixon.

Step-by-step solution

Understanding the options

The four options represent different theories to explain how sap ascends in a tree: capillarity (a physical process in which liquid moves through a medium due to the forces of adhesion, cohesion, and surface tension), role of atmospheric pressure, pulsating action of living cells, and the transpiration cohesion theory of Dixon. It is important to understand all these theories and eliminate the less probable ones based on scientific evidence.

Selecting the right option

The most widely accepted theory about the ascent of sap in a tree is the Transpiration Cohesion Theory of Dixon. The pressure difference created by transpiration (evaporation in plants) pulls water from the roots to the leaves. Coupled with the cohesion (mutual attraction) and adhesion (attraction to the xylem walls) properties of water, this theory accounts for the transportation of sap from the root to the canopy of the tree.

Key concepts

Transpiration-Cohesion Theory

The Transpiration-Cohesion Theory, often attributed to Henry Dixon, is a cornerstone of our understanding of how trees transport water against gravity from the roots to the crown. This theory relies on the fact that water molecules have a natural tendency to stick together—scientists call this 'cohesion'—and also to the walls of the xylem vessels, known as 'adhesion'. When water evaporates from the leaves during transpiration, it creates a tension, like a continuous 'pull'. Thanks to cohesion, this pull is transmitted down the column of water all the way to the roots, allowing for the continual movement of water upward. This process is akin to pulling up a chain where each link represents a water molecule.

The efficiency of the theory is further supported by the continuous structure of the xylem vessels, which allows for an uninterrupted pathway for the water to travel. Understanding this theory can often illuminate not only the basics of plant transport, but broader concepts in plant physiology and the integral role of water in plant life.

Capillarity

Capillarity, or capillary action, is another physical phenomenon often encountered in discussions about plant water transport, yet it is not the primary mechanism for the ascent of sap in trees. Capillarity describes the ability of a liquid to flow in narrow spaces without the assistance of external forces, and is a result of the liquid's adhesive and cohesive forces. In regards to plants, the concept explains how water can move upwards in the tiny xylem vessels of the plant.

While capillarity plays a role in assisting the movement of water in plants, it is insufficient on its own to account for the ascent of sap across the tremendous heights of tall trees. The diameter of xylem vessels varies greatly and capillarity is more effective in very thin tubes. This process can be particularly relevant in the initial uptake of water from the soil or in smaller plants. It's a supporting process within the larger context of water transport, but not the primary driver in the ascent of sap.

Xylem Water Transport

Xylem water transport is a fundamental process in plant biology, involving the movement of water and some dissolved nutrients from the roots to the rest of the plant. Xylem vessels are like the plumbing system of a plant, consisting of a network of tubular structures that facilitate this movement.

The xylem's design, with its hollow, dead cell structures, provides a low resistance pathway for water transport. This system is amazingly suited to handle large volumes of water necessary for photosynthesis, growth, and maintaining plant structure. It's fascinating to note that despite the lack of an active pumping mechanism like a heart in animals, plants efficiently move water through xylem vessels using the physical properties of water and the ingenious mechanisms of transpiration pull and root pressure.

Plant Physiology

Plant physiology encompasses the study of how plants function, covering various aspects such as nutrient uptake, water transport, photosynthesis, and response to environmental stimuli. The ascent of sap in trees is a specific area within this field that illustrates how numerous physiological processes are interconnected.

For instance, the evaporation of water from the leaf surface not only drives the transport of water but also facilitates the uptake of minerals and cools the plant. Moreover, understanding how plants manage water use is crucial not only for botany students but for agricultural and environmental applications as well. Plant physiology can be quite complex, yet by breaking down each process and examining it closely, students can gain insights into the ingenious ways plants sustain life and grow.