Monday, January 16, 2012

Darwin and the Rhododendrons

When you think of Charles Darwin, you think of rhododendrons, right?

Until today, I never would have, either.

It’s 11˚F at our house this morning ― that’s cold. And as usual in temperatures under 20˚F or so, the leaves on the rhododendron plants are curled and drooping:

I knew that the leaves curl in response to extreme cold (a phenomenon called temperature-sensitive or thermotropic leaf movements), but I’ve always wondered why and how.

I found a good article by ET Nilsen, published by the Arnold Arboretum at Harvard University, that offered some good information and interesting illustrations (citation below). The author included some interesting science history, including the fact that “Leaf movements in plants were first categorized by Charles Darwin in 1880 in his groundbreaking book The Power of Movement in Plants.” Nilsen writes:

“Darwin pointed out that ‘many plant parts, and particularly leaves, move in response to a number of extrinsic (environmental) and intrinsic (physiological) factors. The most important extrinsic factors are light intensity (phototropic), light direction (heliotropic), water content (hydrotropic), and temperature (thermotropic). The most frequently observed case of thermotropic movements occurs in plants in hot, dry environments where leaves move upward and become vertical to avoid excessive light absorption. The thermotropic leaf movements of Rhododendron are unusual because these movements are in response to cold temperatures and the leaves become pendent rather than vertical.’”Nilsen summarizes the various theories about thermotropic leaf movements in rhododendrons, describes his own research, and concludes that “the thermotropic leaf curling in Rhododendron may serve to prevent damage to cellular membranes during the process of daily rethawing that often occurs during the early morning.”
It’s an interesting, readable article.

Nilsen, Erik Tallak. “Why Do Rhododendron Leaves Curl? A physiological ecologist looks at the significance of temperature-sensitive leaf movements in Rhododendrons.” Arnoldia, 50(1):30-35, Fall 1990.

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