So much by way of illustration regarding the phosphorescence exhibited by animals, terrestrial and marine; but it ought to be noticed that there are also a few cases in which the same phenomenon is to be witnessed in plants.  These are not so numerous as was at one time supposed, the property having been mistakenly ascribed to some plants not really luminous.

[Illustration:  A PHOSPHORESCENT SEA.]

In some instances the mistake appears to have been due to a subjective effect produced by brilliantly colored (red or orange) flowers, such as the great Indian cress, the orange lily, the sunflower, and the marigold.  The fact that such flowers do give out in the dusk sudden flashes of light has often been stated on the authority of a daughter of Linnaeus, subsequently backed by the assertions of various other observers.  But most careful observers seem to be agreed that the supposed flashes of light are in reality nothing else than a certain dazzling of the eyes.

In another case, in which a moss, Schistostega osmundacea, has been stated to be phosphorescent, the effect is said to be really due to the refraction and reflection of light by minute crystals scattered over its highly cellular leaves, and not to be produced at all where the darkness is complete.

Among plants, genuine phosphorescence is to be found chiefly in certain fungi, the most remarkable of which is Rhizomorpha subterranea, which is sometimes to be seen ramifying over the walls of dark, damp mines, caverns, or decayed towers, and emitting at numerous points a mild phosphorescent light, which is sometimes bright enough to allow of surrounding objects being distinguished by it.  The name of “vegetable glow-worm” has sometimes been applied to this curious growth.

Among other phosphorescent fungi are several species of Agaricus, including the A. olearius of Europe, A.  Gardneri of Brazil, and A. lampas of Australia, and besides the members of this genus, Thelaphora caerulea, which is the cause of the phosphorescent light sometimes to be seen on decaying wood—­the “touchwood” which many boys have kept in the hope of seeing this light displayed.  The milky juice of a South American Euphorbia (E. phosphorea) is stated by Martins to be phosphorescent when gently heated.  But phosphorescence is evidently not so interesting and important a phenomenon in the vegetable as it is in the animal kingdom.

The whole phenomenon is one that gives rise to a good many questions which it is not easy to answer, and this is especially true in the case of animal phosphorescence.  What is the nature of the light?  What are the conditions under which it is manifested?  What purpose does it serve in the animal economy?

As to the nature of the light, the principal question is whether it is a direct consequence of the vital activity of the organism in which it is seen, of such a nature that no further explanation can be given of it, any more than we can explain why a muscle is contracted under the influence of a nerve-stimulus; or whether it is due to some chemical process more or less analogous to the burning of a candle.