Summer World_ A Season of Bounty - Bernd Heinrich [99]
Animals’ quick responses, though based on physiology, are called behavior. Some plants also have relatively fast responses that, though based on different mechanisms, are behavior as well. Many flowers track the sun so as to be continually warmed. A Venus flytrap leaf closes in seconds to capture an insect that lands on it. When touched, mimosa leaves droop and fold together in seconds, thus presenting less of a target for predators that might otherwise browse them. I was intrigued to learn that the leaves of rhododendrons in the mountains of northern China were curled into rolls during the winter (Schaller 2007). I could not believe my eyes when I saw the leaves of rhododendron of two species planted on our campus also rolling up. At temperatures below the freezing point of water (there are many days like that in Vermont!) the leaves drooped and rolled themselves into tight little tubes like sucking straws, and as soon as they were a couple of degrees warmer they again unrolled and raised themselves to the horizontal, looking as they do in spring and summer.
After being tightly rolled up (at minus 10°F), they could unroll and be almost totally flat and “normal” within about two to four minutes at room temperature (around 65°F). The reverse reaction occurred as well, but it was several minutes slower. I never did figure out what the mechanism underlying the curling is, but it has something to do with water, because when I cut twigs at room temperature and left their cut stems without access to water, the leaves slowly curled within about a day. When I then tried to rehydrate the curled leaves by wetting them and leaving them in either cold or warm water, they failed to respond in the winter, but did respond in the summer.
Ignoring for the time being the proximate question of the mechanism of how the leaf behaves, there is also the evolutionary question of why leaves roll up. Is there an adaptive advantage, and if so, what is it? Comparative data about the responses of other trees give clues.
One might suppose that the curling and drooping (rather than dropping) of leaves in the rhododendrons is a general response of leaves, as such, to temperature. To find out, I raided some of our collection of house plants, taking a leaf from ten broad-leafed species and laying them outside at 0°F. A few minutes later I brought them back inside. As I had expected, all were frozen brittle-solid. A few minutes after thawing they were limp mush. None had curled. So the rhododendrons that survive at minus 30°F have a different physiology and a behavior that shows that frost-hardiness is possible in broad leaves. I called out-of-state relatives and had them send me twigs of broad-leafed magnolia from North Carolina and leatherleaf viburnum (Viburnum rhytidophyllum) from Pennsylvania (though viburnum is native to China). These plants’ broad leaves, like those of the rhododendron, also survived the Vermont winter temperatures. In short, not only is it possible for broad leaves to stay alive in winter, but some broad leaves even behave in ways that greatly reduce their surface area at the temperatures where rain turns to snow.
Fig. 39. Leaves of rhododendrons drooping and curling in response to near-freezing temperatures.
The New England snowstorm of October 2005 had dumped a modest eight to twelve inches of snow in Vermont. We gauged its severity by the duration of the power outages, which in turn reflected the number of trees and branches that had fallen onto electric lines. (On our road, the power outage lasted three days. At other places throughout northern New England, it extended to more than a week.) Our costs were modest and temporary, but the lasting cost was to the trees caught with their