Juicy cacti: spilling the succulents′ secret | Global Ideas | DW | 20.03.2015
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Juicy cacti: spilling the succulents' secret

Ever wondered how cacti manage to stay fresh and juicy even in the middle of a hot, dry desert? Here's the answer.

Photo: Cactus plant (Photo: CC BY NC SA 2.0: Eric Wagner/flickr)

Cacti usually grow in places where rainfall is rare and have developed coping mechanisms. Deep roots are just one of those adaptations.

For people lacking a green thumb, cacti can make the perfect houseplant. Even after long spells without water, a cactus will manage to stay juicy (and alive) and is one of the few plants that can actually thrive in arid desert environments.

But how do cacti, pineapples and other certain succulents do it? Their secret lies in how they metabolize.

Most plants absorb carbon dioxide (CO2) through their leaves' stoma - tiny holes on the underside of each leaf - during the day while the sun is shining and convert it directly into energy that is then stored in the form of sugar. This process might be familiar to you from school biology lessons as photosynthesis.

But if plants like cacti opened their stoma to "take a deep breath" of CO2 during a sweltering desert day, they’d most likely dry out as these tiny apertures not only absorb air but also allow water to be lost more easily via condensation.

Breathing? Only at night!

Photo: A pineapple

Pineapples are the most well-known plants using this special form of photosynthesis, called Crassulacean Acid Metabolism (CAM), which is named after the Crassulaceae plant family.

To keep their precious water inside, certain succulents like cacti and pineapple plants have developed a workaround: they open their stoma at night when the sun is not shining and it's cooler. Towards sunrise they close their stoma and "hold their breath" throughout the day. Once the sun starts shining, the stored CO2 is then depleted and converted into energy.

This two-step procedure whereby CO2 uptake occurs at night and energy conversion occurs during the day is what is distinguishes the cacti’s method of photosynthesis - called crassulacean acid metabolism - from "normal" photosynthesis, where both processes usually occur simultaneously.

Imagine taking a deep breath and holding it - you'd also look bigger than usual. The same goes for cacti and a number of other succulent plants. The absorbed CO2 is hoarded in the form of malic acid in the cells’ storage units, known as vacuoles, meaning they look juicy and can survive even in arid areas.

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