Question

What are the physical properties of water responsible for the ascent of sap through the xylem in plants?

Answer 1

Xylem is one of the vascular tissues and phloem being the second. Xylem is a vascular tissue that transports water from roots to stem and the leaves. It is also known for the supply of nutrients along with water.

Complete answer: Xylem is composed of long distinctive cells called tracheary elements that transport water. It also contains vessel elements that are shorter and connected into long tubes which are called vessels. Xylem also contains two other forms of cells which are parenchyma and fibers. The main function of xylem is the transport of water; it forms a network in which xylem, vessels, tracheids of the roots, stems, and leaves are connected. This system transports water and many of the soluble minerals from roots to the aerial parts of the plants. Xylem sap mainly consists of water, soluble inorganic ions. This transport is passive. Transportation of sap is as such difficult because of the height of the plants and gravity in function.
There are many phenomena which help in the transport of sap:
Pressure flow hypothesis- Sugars produced in leaves and many other green tissues are present in the phloem system creating a solute pressure differential versus xylem system carrying a far load of water and mineral ions.
Transpiration pull- The evaporation of water from the leaves and the aerial parts of the plants create a negative pressure at the top of the plant. Due to this millions of minute menisci are formed in the mesophyll cell wall. The resulting surface tension causes a negative pressure which makes xylem to draw water from the roots and transport upwards.
Root pressure- If the water potential of root cells becomes negative than that of the soil, it causes osmosis of water from the soil to the roots. Root pressure is highest in the morning as the stomatal pores are opened and transpiration is high.
Cohesion- Mutual attraction of the water molecules causes the movement of these molecules up the stem. This property of water is known as cohesion.
Adhesion- It is defined as the attraction of the water molecules to the polar surfaces such as those of tracheary elements.
Surface tension- It is responsible for the greater attraction between the water molecules which is more in a liquid state than the gaseous state.
Capillary movement- It is defined as the rise of water in thin tubes along with the movement of water.
These physical properties of the water makes it possible for the transport of xylem sap to the tip of high place These give water higher tensile strength that helps to resist the pulling force I.e. gravity and also high capillarity. The thin tubes of xylem have capillary-like action after rises in these tubes.

Note: Xylem and phloem being vascular tissues of the plant’s help in the growth of the plants possible as they help reach the water and food to so high plants. Also, it is shown that vascular plants grow higher than other plants due to rigidity because of the xylem which supports the plants.