which passes off with the decline of day—the greatest intensity always occurring during the shedding of the pollen. Any great increase of temperature is necessarily prevented by the equalizing effect of evaporation from the expanded surfaces of the leaves, and the water which pervades all the substances. It is only during a period of unusual energy, as in blossoming, that the heat becomes apparent.
The light and heat set free by combustion we recognize as merely the expression of chemical change and a giving back to original elements the forces that were stored up by vegetative activity in the coal-beds of the past or in the woody fibers of later growth. In the mysterious circle of Nature's means and mechanism, light, heat, and chemical combination are alternately cause and effect—not, it is true, in the abstract sense of cause, but, being mutually convertible or corelative, maintain necessarily reciprocal action. Now, the fact that they are thus reactive and interchangeable as modes of motion offers a very simple explanation of the giving out of light by plants at a moment when the surplus amount of these garnered forces is thrown off during the vital processes of reproduction.
That increased chemical activity exists at the period of flowering is shown by the exhalation of an unusual amount of carbonic acid, and this increased action supplies the additional heat for the elaboration of the reproductive agents, whose preparation seems to be the highest expression of energy in vegetable organization. Some doubt exists as to the proximate cause of the manifestation of light by flowerless or cryptogamic plants, in which are embraced the mosses, fungi, etc. A study of the conditions under which it is presented will, I think, enable us to refer it directly to similar chemical action.
These plants were ingeniously named by Linnæus because the concealed organs of reproduction offer great diversity in structural relations—a diversity so great that they can not even be presented under one common type; we must, therefore, look for modifications in the expression of force directed by these different forms. The root-hairs which form in the germination of one of the liverworts (Hepaticæ) have been observed to be luminous in the dim light of caverns. It gathers principally upon schists, and derives its name (Schistostega Osmimdacæ) from a miniature resemblance to the royal fern Osmimda. This plant, like the true cavern-mosses, is emerald-green, and develops into root, stem, and leaf. The dainty fern-like leaves or fronds are of the very simplest organization, and a slender, threadlike stem rises from the apex, bearing upon its summit a valvular case or capsule which contains the reproductive cells or sporules. The root-hairs which give out the light appear like the tangled meshes of a spider's web; and, as the same effect has been noticed in these tenuous structures, some naturalists have attributed the appearance to reflected light. But analogy in the condition of this and other light emitting plants leads to the conclusion that it is self-luminous.
