Desert Plant Adaptations (I) ~ Succulence

Plants that have adapted to living in dry habitats are called xerophytes, and they are the characteristic plants of deserts and semi-deserts. Over the next few weeks, I’ll be explaining some of the adaptations that allow them to survive in such a harsh desert environment. And it is harsh! Besides the limited supply of water, these plants must also survive high daytime temperatures, high levels of solar radiation, high levels of salinity, and strong winds. Water, however, is the most important ingredient in their lives and they must prevent water loss and overheating to survive. In general, to overcome these obstacles, desert plants can be classified into three main groups according to how they deal with the drought conditions – succulence, drought-deciduous, and drought-escaping. Today, I’ll talk about succulence.

Succulence is the ability of plants to store water in their leaves, stems, and/or roots. Cacti are a classic example. Succulent plants have shallow roots, allowing them to quickly absorb any available moisture, including dew. These plants can begin to grow 24 – 48 hours after rain. You may have noticed that there aren’t many cacti growing in the wadis of South Sinai, but there several species of succulents.

Species with Succulent Leaves

If you think back to your high school biology class, you’ll recall that photosynthesis is the process that allows plants to use the sun’s energy to convert carbon dioxide (CO2) and water into sugar. Basically, it’s how plants make their food. There are pores, called stomata, on the leaves that open and close to absorb carbon dioxide from the air and release oxygen. When these stomata open and close, water vapor is also released and evaporated. This release of water is called transpiration. It’s similar to sweating in humans and can help cool the plant. And we know that if we sweat a lot, we should drink more water to stay hydrated. However, desert plants do not receive a lot of rain to replace this lost water. So they have adapted to conserve as much water as possible.

Species with Succulent Stems

One way succulents do this is to use a different type of photosynthesis, one that allows the plants to make food without losing a lot of water to transpiration. It’s called crassulacean acid metabolism, or CAM photosynthesis. Plants that use CAM open their stomata at night when temperatures are cooler and keep them closed during the day. Instead of using sunlight to convert the CO2 to food right away, plants store the CO2 they absorbed at night as crassulacean acid. As the day begins and the temperature starts to rise, the acid is changed back into carbon dioxide and then used in photosynthesis. This allows the plants to conserve water and use it to make food instead of losing it to evaporation.

Species with Succulent Stems and Leaves

That’s a pretty fascinating adaptation, isn’t it? But it’s only the start! Stay tuned for my next post where I’ll discuss drought-deciduous and drought-escaping plants.

Desert Thumb

Desert Thumb (Cynomorium coccineum), aka Red Thumb or Tarthuth in Arabic, is not a common plant in Sinai. Growing in the spring only after a wet winter, this rare plant makes for a special spotting! I was intrigued when I came across these for the first time in 2014. I mistakenly thought they were mushrooms (so did the Maltese back in the 1600s so I’m not the only one!). With the help of my fellow nature lovers at Project Noah, I learned that these thumbs of the desert were actually plants.

cynomorium-coccineum-1

But they are no ordinary plants! They are parasitic; they have no chlorophyll and cannot photosynthesize. Instead, they live most of their lives underground as a rhizome, attached to the roots of another plant. In Sinai, Desert Thumbs most likely parasitize salt bushes (Atriplex spp.), but they are also known to live off the roots of Amaranthus and Tamarix species.

The stalk emerges in the spring, covered by clusters of tiny dark red to purplish flowers. The flowers are pollinated by flies that are attracted to their cabbage-like aroma. Once pollinated and mature, the spike turns black and produces small, black, nut-like fruit.

cynomorium-coccineum-3

Desert Thumbs are edible and are believed, throughout the region where they grow, to have an array of medicinal properties. In fact, Arab physicians during the Middle Ages referred to Desert Thumbs as “the treasure of drugs” because it was used to treat a range of problems from blood disorders to digestive and reproductive ailments. In Sinai, Bedouin have traditionally used these plants to cure colic.

cynomorium-coccineum-4

Bedouin in Saudi Arabia have been harvesting tarthuth for thousands of years as food for themselves and their camels. Tarthuth has been especially useful during long caravan treks through the desert as well as during times of famine. The spikes are cleaned and the outer skin is peeled off. The inner white flesh is reported to be like an apple – sweet, crisp, and juicy.

If I had known that three years ago, I definitely would have tasted this plant! We’ve had a relatively wet winter this year, so maybe I’ll get lucky and have another chance to sample a Desert Thumb this spring. I’ll be sure to let you know. 🙂

References and Further Reading:

Cynomorioum – Wikipedia

Cynomorium coccineum – Flora of Qatar

The Matelse Mushroom (Cynomorium coccineum): An Epic History of the Rise and Fall of the Treasure of Drugs

The Treasure of Tarthuth by Robert W. Lebling Jr. – Saudi Aramco World