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This article speculated on the evolutionary origins of thermogenesis and observed how it predominates in ancient lineages of flowering plants like magnolias and water lilies. But thermogenesis goes back much farther than this, for it also occurs in cycads, nonflowering plants that arose in the Paleozoic. The male cones of some cycads, when mature, may maintain themselves at an astonishing 20 or more above the ambient temperature. This, as in the dead-horse arum, helps volatilize what is (to us) a foul smell. It is considered antisocial in Guam to have male cycads in one’s garden, but for certain insects, apparently, this smell is a delicious one, for they are attracted to the hot cones and help distribute their pollen.

Oliver Sacks
New York, N.Y.

The last section of this article briefly contrasts the rewards flowers give pollinators: heat versus pollen and nectar. It should be pointed out that the techniques are essentially identical, in that the plant is rewarding the pollinators with energy. In one case, the energy is passed as chemical energy. In the other case, it’s radiant energy. The media are different, but the message is the same: an incentive for the pollinators to behave in a way that benefits the plant. The chemical technique may have a better ratio of energy expended to results produced (pollination and offspring). If so, selection pressures would tend to maximize its occurrence.

George Almond
Ridgewood, N.J.