Water is the foundation of every major process a plant undertakes in its life. All species of plants, from the lowliest mosses to the tallest trees, depend on water for every facet of their existence. The xylem, or main central column for water transport in a plant, shunts water through countless veins in a plant at every moment of its life, not unlike the circulatory system in humans and animals.
From the moment a seed is planted, whether by nature or by a gardener, transpiration plays a role. Roots take up water into the plant, acting as a support mechanism that makes the plant stand upright. This is why plants that are not getting enough water tend to wilt. The leaves of all plants have tiny pores on their surfaces called stomata, which take in carbon dioxide and release a small amount of water. The path of the water, from roots to expulsion through the leaves, is called transpiration.
Water gives plants the strength to open their stomata and pull in the carbon dioxide necessary for photosynthesis, which converts ultraviolet light, usually from the sun, into energy. When water amounts are not sufficient to support a plant, it is not able to open its stomata to draw in sufficient carbon dioxide. This, in turn, leads to decreased levels of photosynthesis, which leads to decreased vigor and possible wilting of plants.
Minerals necessary for any plant's health are absorbed through water uptake into a plant's system. Water acts as a carrier not only for these minerals, but also for converted sugars (stored energy) within a plant. Water is constantly moving throughout a plant, and is comparable in importance to the bloodstream in humans and animals.
Plants have different preferred operating temperatures, depending on their species. Tomato plants, for example, fare better in hot weather. Other plants are either indifferent to or prefer colder weather, such as members of the evergreen family. In all cases, water's constant movement throughout plants helps them to maintain their desired temperature. Other factors, such as ambient temperature and sunlight, may play a role. However, as with people and animals, water acts as an important internal thermostat for plants.
Plants are most commonly thought of as growing in soil. However, they can survive without it, provided they are given the nutrients they require. This is why hydroponic and aeroponic growth systems are possible and successful. However, another way in which plants are similar to every other living creature on Earth is this: They cannot survive without water.