Tuesday, June 12, 2012

Plants that Generate Heat

A few weeks ago I took you on a field trip to the marshy meadows around Longmire, in Mt. Rainier National Park in Washington.  The most spectacular early spring wildflowers there are the western skunk cabbage. These exotic-looking members of the mostly tropical aroid family (Araceae) seem totally out of place here in the cool woods of the Pacific Northwest.  In May and June, it's still cold at night, the snow has just recently melted, and the water in the streams and marshes is frigid, yet these extravagant plants are in full bloom.

The eastern skunk cabbage (Symplocarpus foetidus)
generates enough heat to melt its way through snow
in early spring.   Photo by Sakaori, from
Wikimedia Commons
The eastern skunk cabbage blooms even earlier, even poking up through the snow.   The inflorescences of these plants actually generate heat, enough to melt the snow above them and continue their reproductive activities when most other plants are still hunkered down underground.  Presumably, this allows them to take advantage of the pollination services of some of the earliest insects to emerge in the spring.

But how and why do these plants do this?  Surprisingly, this ability evolved before the ancestors of these tropical plants moved northward.  Many tropical aroids living in 365/24/7 jungle steam baths also generate heat.  They do it also to attract pollinators, and that is the original reason.  The ability to melt snow is a bonus. Heat generation in plants is similar to that in animals, by the burning of carbohydrate or sometimes lipid reserves.

Philodendron bipinnatifidum (formerly
P. selloum) generates heat to attract
pollinating beetles.  Photo by
Tekwani, Wikimedia Commons
Tropical aroids that produce heat include species of  Philodendron and Amorphophallus, the dead horse arum (Helicodiceros muscivorus), and voodoo lily (Typhonium [Sauromatum] venosum).  The practice is not confined to aroids, but is also found in some waterlilies (famly Nymphaeceae), sacred lotus (family Nelumbaceae), and in a number of other plant families.  It is most often found associated with pollination by flies and beetles, dull-witted insects that find flowers primarily by scent.  The heat in some cases helps vaporize and disperse the scent of the flowers, and in the plants growing in cold places, it may provide a refuge for the insects, who sometimes spend the night where orgies of reproductive activity may take place. Temperatures up to 35 degrees C above ambient temperatures have been reported.  (See the essay by Roger Seymour at Plant Physiology online.)