Question: Why are the water droplets arranged so evenly around the edge of this cucurbit leaf?
Answer:
The water droplets came out of pores that are at the edge of the leaf where a vein ends. The pores are called hydathodes. The droplets form through the process of guttation. Guttation is when the water pressure in the plant is high enough to force water out of the hydathodes. This occurs when soil moisture and humidity are high, typically at night during rainy humid weather.
The hydathodes may also serve as a means for bacterial pathogens to enter leaves. The water droplets can be drawn back into the plant as the sun comes out, relative humidity drops, and the leaf begins to lose water through transpiration. When bacterial diseases are present, these water droplets can be a major means of disease spread if people, animals, equipment, wind, or rain move water droplets containing bacteria from one plant to another.
For instance, see how the angular leaf spot on this cantaloupe leaf starts at the leaf margin and moves down the vein.
Black rot of cabbage and other crucifers also can enter through hydathodes and spread down the veins.
