Ways in Which Plants Increase in Size 347 



glass horizontally a foot above it (figure 132), he can, by sighting 

 his reference marks, record on the pane the spot to which the fila- 

 ment is then pointing. But if, a half hour later, he sights again, 

 he finds that the filament, and therefore the tip of the plant, 

 points in another direction, and later in another, and so on. By 

 drawing straight lines through the points thus established, one 

 obtains a kind of polygon representative crudely of the magnified 

 course of the moving tip of the seedling; and a few of these records, 

 traced by one of my own students, are given herewith (figure 133), 

 while Darwin's book, The Power of Movement in Plants, contains 

 a great number. These are not by any means isolated cases, for 

 comparative studies have shown that such movements are dis- 

 tinctive of most if not all growing parts, stems, buds, leaves, 

 roots, tendrils, flowers and their parts, and many others, all of 

 which move during growth in slow, irregular, and jerky paths, 

 that are longer and more rapid the more active the growth of the 

 part. While the movement is thus well-nigh universal, it is not 

 popularly known because of its slowness. If its rate could be 

 magnified a few dozens of times, what a different aspect would 

 vegetation present! Then all the visible parts of all the growing 

 plants of a garden, a meadow, or a forest, would exhibit a con- 

 stant irregular movement, which collectively would seem of a 

 tremulous character, much, I imagine, as would be shown if 

 the plants were shaken by continuous little earthquakes. 



As to the cause of the circumnutation, that is known, in prin- 

 ciple at least. It results from the fact that all growing structures, 

 utilizing as they do osmotic turgescence for the expansion of their 

 tissues, are under strong internal pressures which hold them hi a 

 highly tense but unstable stiffness. Now the readjustment of 

 these pressures in growth cannot proceed with perfect evenness all 

 around the stems or other parts, whose great length and slender- 

 ness cause a large magnification of even the slightest disturb- 

 ances of the equilibrating tensions, and circumnutation results. 

 These movements, therefore, are simply an incidental by-product 



