The idea that plants “sweat” is a common analogy for how they regulate internal moisture and temperature. This process has a precise scientific name: transpiration. Transpiration involves the movement of water through the plant and its release into the atmosphere, primarily as water vapor. This constant flow is an active, regulated system that drives the plant’s physiology.
The Scientific Term for Plant Water Loss
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems, and flowers. Water vapor is released mainly through microscopic pores on the leaf surface called stomata, which are controlled by guard cells. Stomata open to allow the plant to take in carbon dioxide for photosynthesis, but this action also allows water vapor to escape into the surrounding air.
The physical mechanism that pulls water from the roots to the leaves is known as the cohesion-tension theory. As water evaporates from the leaf, it creates a negative pressure or “pull” that extends downward through the plant’s vascular tissue, the xylem. Water molecules stick tightly together through cohesion, forming a continuous column drawn upward to replace the lost moisture. This passive pull, powered by the sun’s energy, drives water transport in the plant.
Essential Functions of Transpiration
This constant loss of water vapor serves two functions essential for plant survival and growth. One primary role is thermal regulation, a cooling mechanism similar to human perspiration. As water changes from a liquid to a gas during evaporation, it draws latent heat energy directly from the leaf surface. This evaporative cooling prevents the tissue from overheating under direct sunlight and maintains leaf temperatures at optimal levels.
The second function of the transpiration stream is the mass flow of mineral nutrients from the soil up to the leaves and developing tissues. Water is absorbed by the roots along with dissolved minerals, and the continuous upward movement carries these elements throughout the plant. Only a small fraction of the water absorbed by the roots is used for metabolism; the vast majority is lost to the atmosphere, maintaining this nutrient delivery system.
Guttation: When Plants Release Liquid Water
While transpiration releases water as vapor, plants have a separate mechanism for expelling liquid water called guttation. This process is typically observed when air humidity is high and transpiration rates are low, often in the morning or evening. Since the stomata are closed, roots continue to absorb water from the soil, building up positive hydrostatic pressure within the xylem.
This internal force, known as root pressure, pushes the water column up the stem and out through specialized leaf structures called hydathodes. Hydathodes are pores located at the margins or tips of leaves. The exuded droplets are not pure water but a dilute solution of xylem sap containing dissolved minerals. Guttation is most common in small herbaceous plants and provides necessary pressure relief when the atmosphere cannot accept more water vapor.