Page:Popular Science Monthly Volume 18.djvu/529

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monocotyledonous ones which he observed. Omitting particulars, the cuts (Fig. 17) will show the effects produced in four instances.

In the case of A, the apex of the radicle is so much bent away from the square as to form a hook. At B, the irritation of the card, aided, perhaps, by geotropism, has formed a circle. At C, the tip in forming a loop has rubbed off the attached bit, and the circle has contracted, while at D, the apex, in making a second turn, passed through the first loop and so rubbed off the card, and, growing downward, tied itself into a knot.

Mr. Darwin believes that the tips of all radicles are similarly sensitive, and transmit an influence causing the upper part to bend. Moreover, the tip distinguishes between harder and softer objects, and between moisture and dryness. It is also sensitive to light and gravitation, and the course of the radicle in the ground is determined by the tip. The volume concludes with the following sentence: "It is hardly an exaggeration to say that the tip of the radicle thus endowed, and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain, being seated within the anterior end of the body, receiving impressions from the sense-organs and directing the several movements."

By Professor H. S. CARHART.

FROM the earliest periods the flash of lightning and the peal of thunder have excited curiosity, stimulated awe, and inspired fear in man; and according to his mythological, religious, or poetic habit of mind has he regarded the latter as the bolt of Jove, the voice of God, or the conscious utterance of the heavens. The explanation of these appearances in the sky is most curious and fantastic, even after the introduction of the modern inductive method. In a "Compendious System of Natural Philosophy," by J. Rowning, M.A., London, 1744, we find the following: "As vapors exhaled from the surface of water are carried up into the atmosphere, in like manner the effluvia of solid bodies are continually ascending thither. Now, we find by experiment that there are several inflammable bodies which, being mixed together in due proportion, will kindle into flame by fermentation alone, without the help of any fiery particles. When, therefore, there happens to be a mixture of the effluvia of such bodies floating in the air, they ferment, kindle, and, flashing like gunpowder, occasion those explosions and streams of fire which we call thunder and lightning."

Ever since Franklin identified lightning with the electricity of the frictional machine, an inquiry has been prosecuted into the origin of