That's a great question! Plants do release moisture, through little pores in the underside of their leaves. These pores are called stomata, and they open up so the plant can absorb carbon dioxide from the atmosphere and use that carbon to grow. When they open their pores, water that is moving through the plants from the roots on up to the shoots and leaves gets evaporated out to the air. Therefore, when the plants are taking in carbon dioxide, they are also releasing moisture. That's why if you've ever been in a greenhouse, you'll notice that the air is both warm and moist. The moisture is released by the growing plants!

I study plants, so if you have any more questions on them, please contact me!