Page:Popular Science Monthly Volume 16.djvu/839

 of inexorable law are as fully met in the pale glimmer of luminous plants, the flash of the fire-fly, or the radiance of the glow-worm, as in the light evolved by the heavenly bodies.

The discovery of the luminosity of plants has been attributed to the daughter of Linnæus. In the year 1762, during the months of June and July, she observed radiations of light from a cluster of garden nasturtiums (Tropœlum majus), which occurred in the morning or evening twilight. The same effect has been observed in other flowering plants, but principally in those bearing orange-colored bloom, as the corolla of the sunflower, the French marigold, and in the yellow lily. Some species of African marigolds also have manifested this property. Professor Haggern observed the luminous radiations to be most brilliant in the marigold, next in the nasturtium, and, third, in the yellow lily. A careful microscopic examination convinced him that the light did not depend upon the presence of any animal organisms. The rapidity of the flash indicated electrical action, and upon analysis he found that the light proceeded from the petals, the anthers being at an appreciable distance. He supposed that in the process of fecundation the elasticity of the anthers scattered the pollen on the petals and produced electrical disturbance by the impingement of these different substances. In bog or marsh land plants, of the genus Pandanus, the rupture of the spathe or shield of the flowers is sometimes accompanied by a loud, cracking noise and a spark of light. In 1857 M. Fries perceived light emitted by a group of poppies (Papaver orientate) in the botanical garden at Upsal, to which he called the attention of numerous witnesses.

The phosphorescence discovered by the daughter of Linnæus bore the character of an electrical spark which shot out from the corolla and was discernible at the same hour upon warm evenings when the air was surcharged with electricity. The radiance noticed by M. Fries also indicated a periodicity of movement, occurring always when the air was electrical, between ten and eleven o'clock at night. Not only the blossoms but the leaves of many of the flowering plants have been observed to emit a phosphoric light under favorable conditions of the air—even the milky juice of several vegetables becomes luminous in the dark; this was particularly noticed by M. Martins in a species of the phosphorescent spurge (Euphorbia).

The giving out of heat in the blossoming of plants was discovered by Lamarck more than a century ago in the European arum, which in opening "grows hot as if about to burn." It was afterward observed by De Saussure, and by the later appliance of the thermo-multiplico the heat generated in any cluster of blossoms is made appreciable. The development of this force is most remarkable in tropical plants, where a large number of flowers are crowded together under a covering hood of spathe. The temperature increases periodically, growing greater in the afternoon and appearing like a "paroxysm of fever"