I'm pretty sure you're referring to '
transpiration.' Basically, water enters the roots and when the plant reaches a high enough moisture content, (probably has something to do with humidity in the air as well) the leaves lose water out of stomata in the leaves. This is usually noticed as droplets of water on the edges of leaves, especially in the evening time. Someone correct me if the details here aren't quite right....I've never taken botany, unfortunately since it wasn't offered at any of the schools I attended. Just know a bunch of plant crap from my own private research/interest in the topic.