Water is one of the most important environmental limiting factors for plants. Plants have evolved adaptations according to the supply of water in their environment.

• Hydrophytes: Plants adapted to excessive amounts of water

• Mesophytes: Plants adapted to moderate amounts of water.

• Xerophytes: Plants adapted to low amounts of water.

In addition there are halophytes which can grow in salty environments. Here water may be abundant but because of osmotic forces it may be unobtainable to the plant tissues without special adaptations.

These live in water or in soils which are waterlogged (soils so saturated with water that there are no air spaces).

• Submerged or floating plants do not need much support in tissue. Their mass is supported by the water.
• They need to be supple to move with water currents.
• Submerged leaves are often strap-like or dissected into thin strands, again an adaptation to water currents.
  
• Submerged leaves do not need a waxy cuticle they will not dry out and their surfaces do not have stomata. However, floating and emergent leaves do need protection from excessive evapo-transpiration.
  
• Their roots are not well developed, absorption of nutrients can take place over the whole surface.
  
• The roots of plants growing in waterlogged soils often have air spaces which may permit the passage of oxygen down into the root tissues from the surface. Waterlogged soils are usually low in oxygen so root tissues risk being subjected to anaerobic conditions.

Plants growing in mangroves of tropical, coastal regions have specialised roots called pneumatophores which grow upwards (the reverse of the normal behaviour of root tissue). When the break through the water surface their air channel provide a conduit for oxygen to the root tissues.

These are plants which live in arid environments. Arid environments can be described as those where the rate of evaporation of water is greater than the rate of precipitation.
For these plants the strategy is to avoid dry seasons or to endure dry climates.

Various ways of avoiding dry seasons include surviving as tubers and tap roots underground or a seeds in a dormant state until sufficient water stimulates germination.

Plants which resist arid conditions are typified by the cacti. Their adaptations are as follows:

• Reduced leaf surface, in the case of cacti, the leaves are reduced to spines.
• Reduced number of stomata.
• The stomata are often sunken in pits to prevent water vapour from being blown away. Grasses may roll their leaves up to enclose their stomata in the same way.
• A thick waxy cuticle.
• Succulent tissue which can store large amounts of water. A full grown saguaro cactus (12m tall) can store up to a tonne of water in its stem. The stem is folded like a concertina to expand and absorb water during the rains of spring and autumn.
• They may have an extensive rooting system.
• They may able to withstand an excessive desiccation of their tissues. The creosote bush of N and S America can drop to 30% of its fresh mass and survive, mesophytes usually need to remain above 50% to 75% of their fresh mass or they die.
• Many have a form of metabolism which permits them to open their stomata at night and combine CO₂ with organic acids in the dark. These acids then donate CO₂ to the photosynthetic processes during the day when the stomata are shut. (This is called Crassulation Acid Metabolism or CAM).
• Other drought resistant plants (C₄ plants) also have modifications to their photosynthetic metabolism. They have evolved an enzyme which fixes CO₂ with organic acids. This enzyme has a higher affinity for CO₂ than Rubisco (the enzyme normally used for carbon fixation) and it is not so easily inhibited by high levels of O₂. As a result these plants can photosynthesis with stomata that are not so wide open as the normal plants (C₃ plants), thus reducing water loss.

In addition to these adaptations which concern the problem of water stress directly, plants in arid regions are subject to attack by herbivores. Their spines are obvious but they also possess chemical weapons to dissuade would be grazers. Some camouflage themselves as stones to avoid attention.