Like people, plants need water to survive. However, plants naturally lose some of their water as the result of environmental factors. This process of water loss is called transpiration. It occurs mainly in the leaves, branches and stems of plants, but also happens in the root system. You must counter the environmental factors that impact transpiration to some degree if you wish to keep your plants healthy.
Plants have tiny openings on their leaves called stomata. These stomata let a plant take in the carbon dioxide it needs to go through photosynthesis. As explained by The Encyclopedia of Earth website, stomata react to light, opening wider the more light there is. When stomata are open, the plant loses some water to evaporation. However, mechanisms within the plant keep the opening of the stomata balanced so the plant can take in enough CO2 without dehydrating severely.
Temperature impacts the transpiration process. Water evaporates more readily at higher temperatures. Thus, plants lose more water as the temperature increases. Many plants can survive hot days because temperatures cool at night and moisture condenses on and around the plant, permitting it to replenish some lost water.
Molecules in a very dense area typically will try to move to an area of less density until they are evenly distributed -- this is one of the principles of osmosis. When it is very humid, there are a greater number of water molecules around the plant than during dry conditions. Subsequently, fewer molecules in the plant are lost. Conversely, when humidity is low, moisture loss is high for the plant. For this reason, plants in rainforest plants usually suffer less water loss than do those in the desert.
Soil contains vitamins, minerals and elements plants need to go through physiological processes. Some of these substances actually help the plant take up water, while others, such as salt, cause reverse osmosis to occur and draw water out of the plant. Soil testing and additives help correct soil imbalances, as can natural decomposition of organic materials that mix with the soil.
Wind affects transpiration indirectly. When it is windy, humidity levels go down because the wind carries some of the moisture in the air away. When air is stagnant, humidity is higher. This means plants lose more moisture on windy days than on non-windy days. Indoors, ventilation systems can circulate air and cause the same results. This is why you may need to water your plants more often when you run your heating or cooling system.