Sometimes when you head outside in the morning, you can see drops of what appear to be dew on the leaves of some plants. While dew commonly forms on plants as atmospheric water vapor condenses, what you see isn’t always dew. If you look at the garden strawberry above (Fragaria sp., Rosaceae) you might notice something odd about the “dew.” First, it seems to be present only on the tips of the leaf teeth. Second, it hasn’t all settled to the lowest spots on the leaves, as one might expect with dew. These observations suggest another natural process at work, known as guttation.
Plants move water through their structures via two mechanisms, osmosis and transpiration. With osmosis, water moves from areas of low solute concentration to areas of high solute concentration. Solutes can be sugars, salts, and minerals, among other things. Roots have a higher solute concentration than the surrounding soil, thus allowing the roots to “absorb” water.
Transpiration is similar to evaporation. Plants have openings on their leaves and shoots called stomata. Most plants open their stomata in the daytime to allow carbon dioxide, necessary for plant metabolism, to diffuse into the plant. When the stomata are open, water evaporates out of the pores. Since water molecules are very cohesive, this has the effect of “pulling” columns of water up the plant shoot as evaporation occurs.
In simplest terms, osmosis pulls water into the roots, and transpiration pulls water up the plant to the leaves, where it evaporates.
Guttation is a related phenomenon that occurs overnight when the soil is very moist. Most plants close their stomata at night, causing transpiration to cease. At the same time, osmosis continues to draw water into the roots. If the soil is sufficiently wet, a lot of water can be drawn into the plant. Since the stomata are closed, this creates a bit of pressure inside the plant. Because of this pressure, some of the water (more specifically, xylem sap at this point) oozes from the closed stomata. The end result is what you see on the strawberry above: little beads of sap around the leaf margins.