In several plants the root pressure is manifested as guttation which is the exudation of droplets of water from the margin and tips of leaves (Fig. 8-1). The exudation takes place through groups of leaf cells called hydathodes. The energy involved in secretion of water is supplied by these glandular cells themselves.
In a leaf when the general surface is smeared with mercuric chloride or alcoholic solution no liquid exudation is observed. A hydathode is an opening or pore in the leaf epidermis around (Fig. 8-2) which several thin walled parenchyma cells are situated. The pore is like an incomplete stoma having lost the ability to close or open.
Guttation of water is easily observed in the young barley seedlings or serrate margin of the garden nasturtium leaves (Fig. 8-1). Guttation water is rich in organic and inorganic solutes. The organic solutes accumulate due to leakage of adjacent parenchyma cells. The inorganic solutes were absorbed by the root hair and passively carried to the hydathodes by the upwards xylem.
Guttation water is less concentrated in inorganic solutes compared to xylem vessels. The numbers of water pores associated with each hydathode vary from one to many. Below the pore a small cavity is present below which several loosely arranged parenchyma cells called epithem are present. In some plants hydathodes function during early period of the plant life. Table 8-2 shows differences between transpiration and guttation.
Losses caused by guttation are slight, though some tropical plants may lose as much as 100 ml of water per leaf in a night.