Page:An introduction to physiological and systematical botany (1st edition).djvu/240

 210 likewise follow the sun in its course; a clover-field is a familiar instance of this.

Of all leaves those of pinnated leguminous plants are found most affected by light, insomuch that it appears, in several cases, the sole cause of their expansion, for when it is withdrawn they fold over each other, or droop, as if dying; and this is called by Linnæus the Sleep of Plants, who has a dissertation on the subject in his Amænitates Academicæ. The term Sleep may not really be so hyperbolical as at first sight it seems, for the cessation of the stimulus of light, and of the consequent restrained position of the leaves, may be useful to the vegetable constitution, as real sleep is to the animal. Another purpose is answered by the nocturnal folding of some leaves, that they shelter their flowers from the dew, the advantage of which we shall explain hereafter.

Some pinnated leaves display a more extraordinary sensibility, not merely to light, but to the touch of any extraneous body, or to any sudden concussion, as those of Mimosa sensitiva, and pudica, Oxalis sensitiva, and Smithia sensitiva, ''Ait. Hort.''