242 C1RCUMNUTATIOX OF LEAVES. CHAP. IV. 



seem to be continually opening and shutting to a very small 

 extent. A nearly full-grown leaf (afterwards proved to be 

 highly sensitive to contact) stood almost horizontally, so that 

 by driving a long thin pin through the foliaceous petiole close 

 to the blade, it was rendered motionless. The plant, with 

 a little triangle of paper attached to one of the marginal spikes, 

 was placed under a microscope with an eye-piece micrometer, 

 each division of which equalled ^ of an iuch. The apex of 

 the paper-triangle was now seen to be in constant slight move- 

 ment ; for in 4 h. it crossed nine divisions, or g-^ of an inch, 

 and after ten additional hours it moved back and had crossed 

 5^ in an opposite direction. The plant was kept in rather 

 too cool a place, and on the following day it moved rather less, 

 namely, -^ in 3 h., and -^ in an opposite direction during the 

 next G h. The two lobes, therefore, seem to be constantly 

 closing or opening, though to a very small distance; for we must 

 remember that the little triangle of paper affixed to the marginal 

 spike increased its length, and thus exaggerated somewhat the 

 movement. Similar observations, with the important difference 

 that the petiole was left free and the plant kept under a high 

 temperature, were made on a leaf, which was healthy, but so old 

 that it did not close when its sensitive hairs were repeatedly 

 touched, though judging from other cases it would have slowly 

 closed if it had been stimulated by animal matter. The apex of 

 the triangle was in almost, though not quite, constant movement, 

 sometimes in one direction and sometimes in an opposite one ; 

 and it thrice crossed five divisions of the micrometer (i.e. y^ of 

 an inch) in 30 m. This movement on so small a scale is hardly 

 comparable with ordinary circumnutation ; but it may perhaps 

 be compared with the zigzag lines and little loops, by which the 

 larger ellipses made by other plants are often interrupted. 



In the first chapter of this volume, the remarkable oscillatory 

 movements of the circumnutating hypocotyl of the cabbage 

 have been described. The leaves of Dionaca present the same 

 phenomenon, which is a wonderful one, as viewed under a lovr 

 power (2-inch object-glass), with an eye-piece micrometer of 

 which each division (^ of an inch) appeared as a rather wide 

 space. The young unexpanded leaf, of which the circumnutating 

 movements were traced (Fig. 106), had a glass filament fixed 

 perpendicularly to it; and the movement of the apex was 

 observed in the hot-house (temp. 81 to 86 F.), with light 

 admitted only from above, and with any lateral currents of air 



