A Plant That Sweats

You and I keep cool in hot weather by evaporating water from our skin. For water to evaporate, it has to absorb roughly 540 calories per gram of water without changing its temperature. That removes a lot of heat from our bodies, allowing us to survive hot weather. Botanists have discovered that the carline thistle plant cools itself by evaporation. You could say it’s a plant that sweats.

Carline thistles grow in an area of southern Spain where almost no other plants can survive. In August, everything shrivels and dies in the arid region around the Sierra de Cazorla mountain range, except the carline thistle. By evaporative cooling, the flowers on this plant average nine degrees cooler than the surrounding air and may be as much as 18 degrees cooler.

To perform photosynthesis, leaves allow carbon dioxide to enter through tiny pores called stomata. During this exchange, some water in the leaf escapes, allowing some evaporation. However, the carline thistle is the only known plant that allows substantial quantities of water to move to the surface, making it a plant that sweats. That provides cooling to allow the carline thistle to survive the heat of Spain’s Mediterranean habitat.

Researchers still don’t know how the carline thistle can retrieve enough water from the ground to continue this process. They plan to study the root structure for novelties that might explain it. The carline thistle’s advantage is access to pollinators. Since all other plants die, the thistle has sole access to bees and other pollinators when it flowers in August. 

Scientists are interested in this plant’s ability to survive the heat as the planet enters a period of increased temperatures. Vast areas of Earth face drought conditions, and there have always been regions of high temperatures and a lack of precipitation. God has designed plants to survive every conceivable climatic condition, even a plant that sweats. We must use what God has given us intelligently, and the carline thistle deserves our study and attention.

— John N. Clayton © 2024

Reference: “This Flower Refrigerates Itself to Survive Scorching Summers” in Scientific American, May 2024, pages 18-19.