An Excerpt from Jim Conrad's
Naturalist Newsletter

from the January 16, 2011 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO
GUTTATION

Nowadays at dawn the plants around my hut are wet with dew. As morning wears on the dew evaporates leaving behind on certain leaf margins lines of water droplets that seem too evenly spaced to have been left by dew. Such droplets on a blade of Boat Lily are shown below:

Guttation on Boat Lily leaf

The droplets are formed by the process of guttation, something completely different from dew formation.

At night, most plants have their stomata closed, so transpiration -- natural loss of water -- doesn't occur. If the plant is growing in soil with high humidity content, excess water enters the plant because of a slight root pressure. The excess water, which mixes with various chemical compounds, then may make its way to leaf tips or edges where it is exuded into the air by special structures called hydathodes. That's guttation.

Maybe the most mysterious part of guttation concerns the slight root pressure that pushes water upwards even when the plant's stomata are closed.

Root pressure results when water in plant roots contains a higher concentration of mineral nutrient ions than water in soil outside the roots. Water molecules pass through root cell membranes from areas of higher water concentration (the soil) to areas of lower water concentration (inside the root, where water is diluted by so many mineral ions), causing pressure inside the root. The pressure pushes water up into the plant, even though the stomata are closed and thus unable to rid the plant of excess water the usual way.

Thus, the need for guttation...