Define Guttation

Guttation is the process of extruding liquid droplets from leaves through specialised structures called water stomata or hydathodes. The small droplets that appear on the vein endings or grasses, or other leaves, are guttation. It’s mainly visible in the early mornings of summer when the relative humidity is high.

High relative humidity is the most prominent requirement for guttation. When relative humidity is high, transpiration rate is lowered, which will significantly increase root pressure. Therefore, guttation is seen in plants where root pressure is signified.

The result of guttation appears like water or dew drops. These drops contain sugar, enzymes, organic acids, minerals, etc. When water evaporates from the leaf surface, it leaves solutes, which causes salt burning. Such salt burning is called guttation burn.

What are Hydathodes?

Hydathodes are special openings found at the apex or margins of leaves.
It is a simple pore on the epidermal layer, surrounded by parenchymatous tissues called epithem.
Cells of epithem are isodiametric, loosely packed, and enclose intercellular spaces.
Epithem is also the point where the leaf vein terminates.
Epithem cells have several finger-like projections that help increase the surface area for water transport.

The Work Mechanism

When root pressure increases, water is given to the epithem by the xylem of the veins.
From epithem the water is released into the cavity.
When this cavity is filled with the watery solution, the latter begins to ooze out in the form of watery drops through the water pore.

Only very few taxons of plants lose water through guttation. Plants like Colocasia, grasses, tomatoes, members of Cucurbita, etc, are some of those 150-250 plants that exhibit guttation.

Differences Between Transpiration & Guttation

Transpiration	Guttation
In transpiration, water is lost from aerial parts of plants in the form of invisible water vapour.	It occurs only in some angiosperms, such as strawberry, garden nasturtium (Tropaeolum), Colocasia, tomato, etc.
It occurs in all vascular plants.	It occurs only in some Angiosperms such as strawberry, garden nasturtium (Tropaeolum), Colocasia, tomato, etc.
Transpiration occurs mostly through stomata. It may also take place through the cuticle and lenticels.	It occurs only through hydathodes (Water Stomata).
Usually it takes place throughout the day, its rate being maximum at noon.	Usually takes place only early in the morning when root pressure and the rate of water absorption are higher.

References

Jain, V. K. (1974). Fundamentals of Plant Physiology (19th ed.). S. Chand Publishing.
S. C Bhatla, M. A. Lal, Plant Physiology, Development and Metabolism, https://doi.org/10.1007/978-981-13-2023-1_1