This means that the cells can balloon up very rapidly. Very little energy is required, basically the cells just enlarge with water. So a mushroom can increase in size as fast as water can be pumped into its cells. Almost overnight a mushroom can go from a pin head to a large mushroom. That is why a saucer and a humidity tent is included with Mushroom Pots TM .
Mushrooms have no skin so they can lose water to the atmosphere very easily.
Mushrooms need to breath just like humans do, except they do not have lungs. Mushroom cells exchange gases directly with the atmosphere. If the body of the mushroom is submerged in water it is comparable to drowning. It is almost the same with the mushroom fruit . If it is too dry they lose too much water and desiccate.