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.” [Illustration: 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: