You know that it didn't rain before, yet the grass is wet as if it did. Some of this water condensed from the air as it touched cool surfaces. Some of the water came from inside the plant.
Plants give off water through stomata, which are tiny openings in their leaves.
In many plants, the majority of stomata are on the lower surface of the leaves. Each stomata is surrounded by 2 guard cells. When the plant has access to plenty of water, the guard cells are plump and swollen with water. In this condition. stomata are open and water can escape from the plant. When the plant does not have enough water, the guard cells become limp and the stomata close.