Water is essential for all plants, so survival in arid environments is a real challenge. However, some of the ways in which desert flora have adapted are ingenious. Generally, they either avoid or endure periods of low rainfall. Ephemeral or annual plants survive periods where water is in short supply because their seeds germinate only after heavy rain, grow rapidly, and live their whole life-cycle in just a few days. These plants often produce large, brightly coloured flowers to attract the insects that are essential to their pollination.

Perennial plants take the alternative route, enduring the dry periods and making the most of the scarce water supplies. These plants utilise a number of interesting methods to survive.

Grasses
Grasses are extremely hardy plants and are found in most of the world's environments. They have extremely large and complex root systems that enable them to collect water over a wide area. During extremely hot and dry periods the parts of the plants that are above the surface may wither and die, but the root systems remain alive. They reproduce by growing new stems. The extensive root systems of grasses play an important role in keeping the sparse desert soils together.

Geophytes
These plants survive the harsh desert conditions by remaining underground for most of the time, often as bulbs. When rain does come they quickly produce stems and flowers.

Succulents
These plants take in large amounts of water during times of plenty and store it for use during times of drought. They have a low surface area in comparison with their volume, which reduces water loss due to evaporation. Succulents typically have vast, shallow root systems in order to make the most of any water reaching the ground.

Like most other plants, succulents use photosynthesis to convert carbon dioxide into organic compounds by means of the energy of sunlight The best-known succulents are cacti. These are found almost exclusively in North and South America, and are often spectacular in shape and size. The most celebrated are the saguaro, which can grow up to 15 metres over many years. The African equivalents of the cacti are the euphorbia, which resemble their American counterparts in many ways.

Shrubs and stunted trees
Unlike succulents, these plants do not differ greatly from those found in other regions, and use more conventional methods of surviving drought. They are small trees or shrubs, normally with very small leaves that are often shed during the hottest part of the year. Their root systems are extensive, and can often penetrate as far as 50 metres into the earth in search of water. Sometimes they have short thick trunks that act as reservoirs for excess water taken up during wet seasons. Examples of this kind of plant are the tamarisk, the creosote bush, the acacia, the mesquite, and some species of eucalyptus. Like succulents, they grow slowly.

In coastal deserts where fog is typical, specially adapted plants grow to take advantage of the moisture. One such plant is the welwitshia, which consists of a large semi-underground root with strap-like leaves protruding from it.

Desert plants often protect themselves against being eaten by animals by growing spikes or other deterrents.

photo courtesy of WWF-UK

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