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Page:Popular Science Monthly Volume 18.djvu/514

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THE POPULAR SCIENCE MONTHLY.

DARWIN ON THE MOVEMENTS OF PLANTS.
By ELIZA A. YOUMANS.

SINCE the time of Linnæus, men have wondered and speculated about what are known as the spontaneous movements of plants, and in recent years the causes of these movements have been carefully investigated by botanists. The subject in its various bearings now forms a large part of the science of vegetable physiology. The periodical and irritable motions of plants, and those due to light and gravity, have been closely studied in connection with the mechanical laws of growth, and many of these phenomena have been more or less satisfactorily explained.

But it has been reserved for Mr. Charles Darwin to go deeper into the facts and philosophy of the subject than any of his contemporaries. In 1875 he published a book upon "The Movements and Habits of Climbing Plants"; and he has since extended his inquiries so as to include the movements manifested by the entire vegetable series, except the lowest flowerless plants, and upon these he is now engaged. He has just published an account of these researches in a volume of six hundred pages, uniform with his other works.

One of the movements of plants long ago observed was described by the term nutation, which simply means nodding. The motion of a flower in following the apparent movement of the sun from the east in the morning to the west in the evening is an example of nutation, and this kind of motion has been found to be much more extensive in plants than was formerly supposed.

When we observe the growing stem of the hop, after the first two or three joints are formed, we see it bend to one side and travel slowly round toward all points of the compass, and continue these revolutions day and night. This spontaneous gyrating motion of stems and tendrils was first remarked by Palm and Mohl, and Professor Sachs gave it the name of revolving nutation.

Mr. Darwin has found that this kind of motion is ever present in the growing parts of plants, so that it must be regarded as a universal property of growing vegetation, and he suggests for it the better term circumnutation. He has proved that even the buried stems and rootlets of germinating seeds make this movement so far as the surrounding pressure will permit.

By the most ingenious and delicate contrivances, and his own constant coöperation, Mr. Darwin has made it possible for the circumnutating organs themselves to indicate approximately the direction and extent of their movements. His arrangements for enabling organs to record their motions varied somewhat; but we give his own account of the general process: