Phenomenon of Guttation in Plants. There are two types of hydathodes in plants. They are: (1) Epithem Hydathode and (2) Active Hydathode.
Most water is lost from plants in the form of vapors. However, some water may be exuded in liquid form, a process called guttation.
The phenomenon of guttation can be observed in Nasturtium sps. Lycopersicum esculentum (tomato), Vitis vinifera (grape), Colocasia esculenta (taro) and Impatiens balsamia (balsam) etc. (Fig. 4.9). It was first studied by Bergerstein in 1887.
Guttation occurs usually when environmental conditions are such as to check transpiration, particularly during cool nights following hot days when the air is very humid. Drops of water appear along the margins of leaves, usually as a result of exudation through special glands known as hydathodes. Hydathodes (see Fig. 4.10) are modified stomata, located at the ends of veins. Unlike stomata, hydathodes remain open throughout the day and night.
Haberlandt reported two types of hydathodes in plants:
1. Epithem hydathode:
That function due to excessive root pressure.
2. Active hydathode:
They exude water due to the force developed within the cells themselves. Guttation liquid is a solution, slightly acidic containing many minerals such as nitrates and salts of S, P., Mg, Mn, Na, Al, CI, hexoses and even enzymes like catalase, amylase and peroxidase. Root pressure is the main cause of guttation.
Conditions reducing the root pressure such as cold, dry aerated soil, bring down the guttation rate. The mineral deficiency also reduces guttation rate. No significant role has been attributed to the phenomenon of guttation. The guttation occurs when there is ample water in the soil and the humidity of air is high.
The phenomenon of guttation can be demonstrated by a simple experiment (see Fig. 4.9). Take a small potted plant of Nasturtium and place it over a glass plate. Now cover the plant with a bell-jar; the mouth of the bell-jar is connected through a bent glass tube to an aspirator. All the joints are made perfectly air-tight by applying Vaseline on them. Air of the bell-jar is now sucked with the help of aspirator. After some time water drops appear at the endings of the veins on the leaves.
It is the exudation of sap or watery solution from the cut or injured parts of the plant, e.g., Agave, Acer, Vitis. It occurs due to root pressure, phloem pressure, local pressure in xylem (stem pressure) and latex or resin.