406 MODIFIED CIRCUMNUTATIOX. CHAP. VII. 



made several little triangles at the rate of 43 m. 

 for each. Mimosa pudica, similarly observed, de- 

 scribed three little ellipses in 67 m. ; and the apex 

 of a leaflet crossed ^^ of an inch in a second, or 

 O12 inch in a minute. The leaflets of Averrhoa 

 made a countless number of little oscillations when 

 the temperature was high and the sun shining. The 

 zigzag movement may in all cases be considered as 

 an attempt to form small loops, which are drawn out 

 by a prevailing movement in some one direction. The 

 rapid gyrations of the little lateral leaflets of Des- 

 modium belong to the same class of movements, 

 somewhat exaggerated in rapidity and amplitude. 

 The jerking movements, with a small advance and 

 still smaller retreat, apparently not exactly in the 

 same line, of the hypocotyl of the cabbage and of 

 the leaves of Dionaea, as seen under the microscope, 

 all probably come under this same head. We may 

 suspect that we here see the energy which is freed 

 during the incessant chemical changes in progress in 

 the tissues, converted into motion. Finally, it should 

 be noted that leaflets and probably some leaves, whilst 

 describing their ellipses, often rotate slightly on their 

 axes ; so that the plane of the leaf is directed first to 

 one and then to another side. This was plainly seen 

 to be the case with the large terminal leaflets of Des- 

 modium, Erythrina and Amphicarpaea, and is probably 

 common to all leaflets provided with a pulvinus. 



With respect to the periodicity of the movements of 

 sleeping leaves, Pfeffer* has so clearly shown that 

 this depends on the daily alternations of light and 

 darkness, that nothing farther need be said on this 



* 'Die Periodischen Bewcgungen der Blattorgane,' 1875, p. 30, et 

 passim. 



