How Do The Leaves And Stems Of Cactus Plants Help Them Survive In Hot, Dry Weather With No Water?

Cactus plants survive in near desert conditions, on apparently no water. They manage this because of several adaptations to their stems and leaves that they have developed over their long evolution.

Cacti look swollen because they have adapted their stems and leaves to store as much water as possible and to lose as little water as possible by evaporation. They also have a very thick and waxy cuticle which cuts down water loss to the absolute minimum.

Their leaves are reduced to spines, again to lose less water (flat leaves lose loads of water by evaporation). The whole of the stem is green and contains photosynthetic cells, so the plant does not need broad leaves to pick up light. There is plenty of light around anyway, so the plant can photosynthesise very well for most of the day.

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Plants in the Cactaceae family are generally found in hot or cold deserts of the Americas, from Canada to Argentina.Their distinguishing feature, spines, are actually a form of leaves that have evolved into sharp, inedible points to protect the plant from browsers.

Cacti spines grow on leathery, liquid-filled "pads," stems, and joints that store water in drought. Thick skins prevent evaporation, and flowers are usually large and brightly colored -- or white, to attract nighttime pollinators like moths or bats.

While people usually associate cacti with deserts, they grow from sea level to the highest Andean peaks and even in jungles (as epiphytes). They range from a few inches to 50 feet tall, like the iconic saguaro with its large, branching "arms." It is the state flower of Arizona.
Cacti exist only where seasonal water supplies are plentiful, like the summer monsoon areas of the West's high deserts. Gravelly soil is necessary to quickly carry off accumulated water, and cacti have long, corky roots close to the surface to capture runoff.

Carbon dioxide accumulated in the pads or stems is gradually released, accounting for cacti's slow growth; it can take as many as 25 years for a nub to grow a foot on a saguaro.

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