Nutrient Transport

• Plants contain vascular systems, networks of tiny tubes through which water travels to all their parts, from the roots to the stems and then to the tips of the leaves and blossoms. Aside from the sugars that plants need and manufacture themselves, they get most of their nutrients from the soil, and water provides a convenient vehicle toward that end. In the soil, water binds with nutrients and enters a plant through its fine root hairs and by osmosis, the process of a substance being absorbed through a thin membrane, which is in this case the roots' thin outer covering.
  
  

Plant Cell Growth

• Cellular division and expansion enable plants to grow, their stems stretching as more new cells form and divide. Water is crucial to this process, causing plant cells to swell. Water deprivation reduces cell size, resulting in stunted growth that's visible in smaller leaves and fruit, shorter stems and fewer roots, states the Missouri State University Extension. Cell expansion helps a plant's roots penetrate the soil and also stimulates its vertical growth up through the soil.
  
  

Cooling and Water Storage

• Like all other living organisms, plants have different levels of heat tolerance, with some plants, such as cacti, having the ability to retain water for longer periods than other plants. A plant is cooled by the water entering through its roots as well as the water with which it comes into contact externally, such as rain. Water also keeps plants clean by washing away impurities and pests that can cause disease and damage, in much the same way that wiping a houseplant's leaves with a damp cloth keeps it pest-free. Water applied to a plant externally also helps stabilize the humidity level, a beneficial step in areas where the air is normally dry.
  
  

Photosynthesis and Transpiration

• Water is a necessary part of photosynthesis, the process by which a plant manufactures its own food. Water molecules, made up of hydrogen and oxygen, are split using the sun's energy. The oxygen is discarded, and the hydrogen draws carbon dioxide from the air to produce sugar. A plant's leaves act as solar collectors, as well as the exit point for the water it has absorbed. During water shortages, this delicate balance is upset, causing photosynthesis to stop. In a fully functioning plant, the water it releases through transpiration returns to the air, forming roughly 10 per cent of the atmosphere's moisture.