Guttation is a physiological process in plants that permits the plant to emit droplets of xylem on the tips of the leaf margin. In roses, guttation appears to be very uniform droplets of liquid on each leaf serration early in the morning. Rose leaves have special organs or glands called hydathodes the allow this liquid to exude when evaporation is limited by high humidity or by a physiological limitation such as the closure of stomata at night, when transpiration does not occur.
Guttation may also be an indication of fertilizer salt build up in the soil. If the exuded xylem is very high in salts, it can damage or even destroy the hydathodes.