Page:Popular Science Monthly Volume 18.djvu/518

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To determine whether the growing parts of mature plants circumnutate, Mr. Darwin experimented with the representatives of about twenty genera, belonging to widely different families and various countries. Several woody plants were chosen, as being less likely to circumnutate. Plants in pots were kept at a proper temperature, either in darkness or feebly illuminated from above. Diagrams showing the circumnutation in about fifty instances of runners or stolens, flower stems, leaves young and old, leaflets, fronds, etc., are given, and the volume contains about a hundred and fifty such diagrams, accompanied by explanations and comments bearing upon the argument of the book. The published instances are selected, for one reason and another, from an accumulation of similar material, the result of years of observation. The figures made by stems, which were always growing, are of course somewhat spiral, forming a succession of more or less irregular narrow ellipses, with their longer axes directed to different points of the compass at different times. They show that the course pursued is often interrupted by zigzags, loops, and small triangles. The rate of movement was different at different times and with different plants. Some made but one ellipse a day, and others four or five.

In studying leaves, he experimented with from thirty to forty widely distributed genera of dicotyledons, monocotyledons, and cryptogams. The seat of movement, he found, was generally in the petiole, but sometimes also in the blade. The extent of movement differed greatly. It is chiefly in a vertical plane; but, as the ascending and descending lines never agreed, there was always some lateral motion describing irregular ellipses. These observations were made upon healthy plants growing in pots, illuminated from above, many of them through ground glass, and they were also plants that do not sleep at night. The stem was always secured to a stick close to the base of the leaf under experiment. Besides his general conclusion that all growing parts circumnutate, many other important inferences are drawn by the author from these experiments and observations.

This movement, which in the case of climbing plants was believed to be due to increased growth of the side that for the time became convex, has more recently been proved to result from the circumstance that every part of a plant while it is growing, and in some cases after growth has ceased, has its cells rendered more turgescent and its cell-walls more extensile first on one side and then on another. Why this should be the case is not known, but Darwin suggests that the changes in the cells may require periods of rest, which accords with our knowledge of the rhythmical nature of motion.[1]

Under the microscope, this movement of circumnutation was seen in a few cases to be made up of sudden small jerks forward for ·002

  1. For an interesting and extended discussion of this subject, the reader is referred to the chapter on the mechanical laws of growth in Sachs's "Text-Book of Botany."