2. THE RAIN TREE
From time to time the newspapers publish accounts of a wonderful tree, said to grow wild in Peru or elsewhere, from the leaves of which falls a continuous shower of rain, even in the driest weather. The writers generally urge the introduction of this tree in regions where the rainfall is deficient, and a so-called “rain tree” has actually been sold for this purpose by nurserymen.
The story of this tree is very old. Early voyagers reported finding it in the East Indies, Guinea, Brazil, and especially the island of Ferro, in the Canaries. Nowadays the name “rain tree” is applied especially to a magnificent tree of tropical America generally known to botanists as Pithecolobium (or Enterolobium) saman. One of its common names is “guango.”
THE RAIN TREE
From “Voyageurs anciens et modernes”
(By E. T. Charton)
A legendary scene in the island of Ferro
That many plants spontaneously exude moisture under suitable conditions is well known. The phenomenon is called “guttation.” The moisture drawn up from the roots is usually transpired from the leaves in the form of invisible water vapor; i. e., it is evaporated on passing into the air. If, however, the humidity of the surrounding air is sufficiently high, or its temperature sufficiently low, to check evaporation, the water will collect on the surface of the plant in liquid form, and may ultimately trickle to the ground in considerable quantities. Guttation occurs chiefly at night, or in cloudy or foggy weather. In a very dry climate it does not occur at all; and for this reason, even if the so-called rain tree could be successfully introduced in such a climate, it would not help solve the problem of irrigation.
The dripping of moisture deposited on plants by drifting fog is another common process that may have contributed to the legend of the rain tree. A classic example of this process—technically called “fog drip”—is that described by Dr. R. Marloth, who has made actual measurements of the abundant moisture captured by the vegetation of Table Mountain, South Africa, from the driving clouds of the southeast trade winds during the nearly rainless summer months. Mr. Madison Grant, writing of a similar phenomenon witnessed in the redwood forests of California, tells us that “these forests are sometimes so wet that the dripping from the high crowns is like a thin rain, and in summer it is oftentimes hard to tell whether it is raining or not, so saturated with moisture are the foliage and the trunks when the fog darkens the forest.”
A copious production of “honeydew” by plant lice, scale insects, etc., may be at the bottom of some of the rain-tree stories. F. E. Lutz, in his “Field Book of Insects,” writes of “weeping trees,” which drip fluid of insect origin, and he says of the honeydew secreted by the pear psylla (Psylla pyricola): “When the psyllas are numerous the leaves and fruit become coated with this sticky substance and it even drops from them like rain and runs down the trunk.”
The following account of the Peruvian rain tree, quoted from the traveler Spruce, was published in “Nature” of Feb. 28, 1878, by Prof. Thiselton Dyer:
“The Tamia-caspi, or rain tree of the eastern Peruvian Andes, is not a myth, but a fact, although not exactly in the way popular rumor has presented it. I first witnessed the phenomenon in September, 1855, when residing at Tarapoto. I had gone one morning at daybreak, with two assistants, into the adjacent wooded hills to botanize. A little after seven o’clock we came under a lowish spreading tree, from which with a perfectly clear sky over-head a smart rain was falling. A glance upward showed a multitude of cicadas, sucking the juices of the tender young branches and leaves and squirting forth slender streams of limpid fluid.”