All plants inhale carbon dioxide through their leaves. This gas is used in photosynthesis. As the plant opens its leaf pores to take in carbon dioxide, some of the moisture in the leaf can escape. Thus the plants sweat water vapor into the air whenever they breath.
Dry air causes plants to transpire moisture much more rapidly than does humid air. Water
in the leaves evaporates very quickly into air, causing the plant to lose moisture at a rapid rate. When leaves begin to lose water faster than the roots can absorb it – disaster strikes. It is an evil the plant inflicts on itself, in self defense. In order not to lose more water to the air, the plant will almost completely close its leaf pores. This slows down the flow of moisture from the plant effectively, but unfortunately it also reduces the intake of carbon dioxide. Without supplies of carbon dioxide, the cells begin to die and the plant looks tired and ill.
The important point to remember is that dry air pulls water out of the leaves faster than the
roots can supply the leaves. Under these conditions, it doesn’t matter how much you water such a plant it doesn’t help. Over watering only reduces the amount of air in the soil and invites root rot.
When plants have the right humidity they thrive, because they open their pores completely and so breath deeply without threat of excessive water loss. When the air is moist, there is little water lost from the leaf. Damping down the benches and surrounds, also misting leaves will keep the air moist. Rapid temperature rises damage orchids too. It means that
the plant’s leaves become warm and physiologically active, while the root system in it’s solid rooting medium, is still cold and consequently Physiologically dormant. The active leaves are demanding large quantities of water and nutrients which the root system cannot possibly supply.
Under these conditions, photosynthesis, transpiration and the other vital plant processes, are severely restricted and as a result, developing flower growth and new growth are damaged. Rapid rises in temperature on sunny days can be avoided by opening vents or doors early in the morning – and letting the greenhouse warm gradually.
A humid atmosphere that is not moving, is particularly undesirable also. Damp, stagnant conditions encourage mould and bacterial diseases. A constantly moving, light and buoyant atmosphere keeps orchids vigorous and healthy.