Question

Доказательством напряжения жидкости внутри сосудов ксилемы служат суточные колебания диаметра древесных стволов, измеряемые дендрографом. Натяжение столба воды вызывает втягивание внутрь стенок сосудов древесины, что дает небольшую «усадку» ствола. Какое свойство молекул воды обеспечивает напряжение жидкости? Объясните феномен натяжения водного столба и сужения сосудов ксилемы. В какое время суток диаметр ствола должен быть минимальным и почему?

Answer 1

Ответ:

Жесткость стенок обеспечивается лигнином. Доказательством того, что жидкость внутри ксилемных сосудов сильно напряжена (растянута), служат суточные колебания диаметра древесных стволов, измеряемые инструментом под названием дендрограф. Минимальный диаметр отмечен днем, когда интенсивность транспирации наивысшая.

Answer 2

The Tension of Water in Xylem Vessels and Daily Fluctuations in Stem Diameter

The tension of water in xylem vessels is demonstrated by the daily fluctuations in the diameter of tree trunks, which can be measured using a dendrometer. The tension created by the water column in the xylem vessels causes the inward pulling of the vessel walls, resulting in a slight shrinkage or "contraction" of the stem. This phenomenon is known as the tension of the water column.

The Property of Water Molecules that Enables Liquid Tension

The property of water molecules that enables liquid tension is called cohesion. Cohesion refers to the intermolecular attraction between water molecules, which allows them to stick together. This cohesive force is due to the hydrogen bonding between water molecules.

Water molecules are polar, meaning they have a positive and negative end. The oxygen atom in water has a slightly negative charge, while the hydrogen atoms have a slightly positive charge. This polarity allows water molecules to form hydrogen bonds with each other.

In the xylem vessels, water molecules are pulled up through the plant due to transpiration, which is the evaporation of water from the leaves. As water evaporates from the leaves, it creates a negative pressure or tension in the xylem vessels. This tension is transmitted through the water column, causing the water molecules to be pulled upward.

The cohesive forces between water molecules allow them to form a continuous column in the xylem vessels, even against the force of gravity. This cohesion creates tension within the water column, which helps to maintain the upward movement of water in the plant.

Daily Fluctuations in Stem Diameter and the Timing of Minimum Diameter

The daily fluctuations in stem diameter are a result of the tension created by the water column in the xylem vessels. During the day, when transpiration rates are high, water is being pulled up through the xylem vessels at a faster rate. This creates a greater tension within the water column, causing the stem to shrink or contract slightly.

At night, when transpiration rates are lower, the tension within the water column decreases. As a result, the stem expands slightly, and its diameter increases.

Therefore, the diameter of the stem is expected to be at its minimum during the daytime when transpiration rates are highest and the tension within the water column is greatest. Conversely, the diameter of the stem is expected to be at its maximum during the nighttime when transpiration rates are lowest and the tension within the water column is reduced.

In summary, the tension of water in xylem vessels is due to the cohesive forces between water molecules, which enable the water column to be pulled upward against gravity. This tension causes daily fluctuations in stem diameter, with the minimum diameter occurring during the daytime when transpiration rates are highest.