ON THE POWER OF MOVEMENT IN PLANTS. 153 



nient series of shakes, and bravely kept its leaves extended in 

 spite of them. 



Anyone who wishes to see the poetical side of this phenomenon 

 cannot do better than read Shelley's exquisite poem, entitled 'The 

 Sensitive Plant.' 



Another wonderful instance of this curious movement is 

 afforded by the Telegraph-plant, Desmodiwn gyrans. It is a 

 trifoliate leaf, the terminal leaflet being very large, the lateral 

 leaflets very small (in this species). During the day the end 

 leaflet gradually follows the setting of the sun, sinking slowly 

 until its under surface lies quite back against its own stalk, or 

 petiole. The pair of side leaflets, on the other hand, are perpetu- 

 ally moving up and down with a jerky motion all day long, and in 

 experiments on the plant it has been ascertained that this goes on 

 until between 3 and 4 a.m., and commences again about 8.30 a.m. 



Both the end and lateral leaflets move in virtue of pulvini at 

 the base of their petioles. The end one would seem to be 

 influenced by the sun in some way, but the lateral leaflets seem to 

 be quite independent of external influences of all kinds. Their 

 movements are not absolutely perpetual, but in spasms, stopping 

 now and then as if to overcome some unseen obstacle. The 

 motile condition (but not the movements) seems dependent on 

 degrees of heat, Hght, and presence of water. The movements 

 themselves appear to result from the alternate lengthening 

 and shortening of one side of the petiole. Their motion is on 

 their own axis, being circular as well as up and down, and each 

 moves alternately with the other, the down stroke being rapid, the 

 up stroke much more steady. It is called the Telegraph-plant, 

 from the motion resembling the movements of the two arms of 

 the old-fashioned Semaphore-signal. One curious fact remains to 

 be noted; viz.^ that as the terminal leaflet falls, its petiole rises, so 

 that at night the leaves are all drooping and the petioles upright, 

 thus greatly reducing the diameter of the plant. 



There can be no doubt, that, in all instances, this sleep of 

 leaves is brought about in order to expose less of their surfaces to the 

 effect of chill by radiatio7i from those surfaces at ?iight. Hence, they 

 assemble themselves together as closely as they can, and nowhere is 

 this more beautifully seen than in Desmodiuin^ where the rising of 



