Transpiration: 

Transpiration is a fundamental process in plants where water is lost in the form of vapor from the aerial parts, primarily through the stomata present on the surfaces of leaves and stems. This loss of water vapor occurs due to evaporation from the surfaces of specialized plant cells, known as mesophyll cells, followed by diffusion through the stomata and into the surrounding atmosphere.

Mesophyll Cells and Air Spaces: Within the leaf, water exits the mesophyll cells and enters the intercellular air spaces. The irregular shape of spongy mesophyll cells creates these air spaces, providing a large internal surface area for water vapor to escape. This expansive surface area enhances the rate of transpiration.

Stomata: Water vapor exits the leaf through tiny pores called stomata. These stomata are primarily located on the underside of leaves and are regulated by specialized guard cells. The size and number of stomata influence the rate of transpiration, with a higher density of stomata and larger stomatal openings facilitating increased water vapor loss.

Transpiration Pull: Water is transported upwards from the roots to the leaves through the xylem vessels. This upward movement is driven by a process known as the transpiration pull. As water molecules evaporate from the leaf surface during transpiration, they create a negative pressure gradient within the leaf. This negative pressure, combined with cohesion and adhesion forces between water molecules and the xylem vessel walls, results in the upward movement of water molecules in the xylem.

Forces of Attraction: Cohesion, the attractive force between water molecules, allows them to stick together, forming a continuous column in the xylem vessels. Adhesion, the attraction between water molecules and the xylem vessel walls, prevents the column of water from breaking apart. Additionally, transpiration creates a tension within the xylem, pulling up a cohesive column of water molecules held together by these forces of attraction.

Wilting:

Wilting is a physiological response in plants that occurs when the rate of water loss exceeds the rate of water uptake from the soil. When plants experience wilting, their cells become flaccid, and tissues become limp. This happens because the loss of water causes a decrease in turgor pressure within the cells, leading to a loss of rigidity and support in the plant structure.

Factors Affecting Rate of Transpiration:

Temperature:

• As temperature increases, the kinetic energy of water molecules within the plant also increases.
• This heightened kinetic energy accelerates the process of evaporation and diffusion from the mesophyll cells into the surrounding air spaces.
• Consequently, the rate of transpiration increases with higher temperatures, as more water molecules evaporate from the leaf surfaces.

Humidity:

• Humidity refers to the amount of moisture present in the air.
• Low humidity levels create a more significant difference in water vapor concentration between the leaf and the surrounding atmosphere.
• This steep concentration gradient facilitates the rapid movement of water vapor from the leaf into the drier air, increasing the rate of transpiration.
• Conversely, high humidity reduces the transpiration rate since the concentration gradient is less pronounced.

Wind Speed:

• Wind speed plays a crucial role in transpiration by affecting the boundary layer of still air surrounding the leaf.
• When there is a breeze or wind, it removes the water vapor molecules accumulating near the leaf surface, maintaining a steeper concentration gradient.
• This removal of water vapor molecules ensures that the air surrounding the leaf remains drier, allowing for a faster diffusion of water vapor from the leaf surface.
• Consequently, higher wind speeds lead to an increase in the rate of transpiration.