502 THE POPULAR SCIENCE MONTHLY. 



To determine whether the growing parts of mature plants circum- 

 nutate, Mr. Darwin experimented with the representatives of about 

 twenty genera, belonging to widely different families and various coun- 

 tries. Several woody plants were chosen, as being less likely to cir- 

 cumnutate. 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, accom- 

 panied 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 grow- 

 ing, are of course somewhat spiral, forming a succession of more or 

 less irregular narrow ellipses, with their longer axes directed to differ- 

 ent points of the compass at different times. They show that the 

 course pursued is often interrupted by zigzags, loops, and small tri- 

 angles. 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 cryp- 

 togams. 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 grow- 

 ing parts circumnutate, many other important inferences are di'awn 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 circum- 

 stance 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.* 



Under the microscope, this movement of circumnutation was seen 

 in a few cases to be made up of sudden small jerks forward for '002 



* 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." 



