THE VEGETABLE CELL. 149 



diverted from it by the influence of light falling obliquely on it, 

 or artificially thrown upon it from below, the leaf constantly 

 striving to turn its upper face to the light. 



The movements which the leaves make under these circum- 

 stances frequently take place so quickly, that the leaves of many 

 plants follow the daily course of the sun ; at the same time they are 

 often far more extensive than those movements we observe in the 

 stem. Not only is the leaf in general far more capable of curva- 

 ture of its flat and extended substance, in consequence of the 

 greater pliancy of this than the axial organs, but the movement 

 of the whole leaf is favoured by the circumstance that in a great 

 number of leaves, there lie, both at the base of the petiole^ and 

 in compound leaves, also at the base of each leaflet, little enlarge- 

 ments (articulations) composed of soft, succulent parenchyma, 

 winch, on account of the abundance of their cellular tissue, and 

 because, at the same time, the vascular bundles passing down 

 through the middle of the articulation can oppose but slight re- 

 sistance to the curvature, are capable of a far stronger degree of 

 curvature than the other stem-like parts of the plant* 



That the various positions to which the terms waking and 

 sleeping are applied, are produced by the alternating influence 

 and absence of light, and that the diminishing temperature and 

 increasing moistm-e of the air coming on with night, do not play 

 any essential part in this, is shewn especially by the experiments 

 of De CandoUe, who succeeded in reversing the periods of the sleep- 

 ing and waking of plants, by illuminating them at night and 

 keeping them in the dark by day. In very sensitive plants, also, 

 artificial withdrawal of the light, even for a short time, as the 

 dim light which exists during a great eclipse of the sun, suffices 

 to make the leaves go to sleep ; while, on the other hand, many 

 plants, especially the different species of Qxalis, require bright 

 sunshine to make them expand their leaves fully. 



The movements made by leaves in going to sleep difier in the 

 highest degree in diflerent plants : sometimes they consist of a 

 sinking, sometimes of an elevation, of the leaf, in compound leaves 

 at once of sinking, elevation or twisting, sometimes of folding to- 

 gether of the leaflets ; in general the leaves present a smaller 

 expansion during sleep than by day, not, however, so that we can 

 say with E. Meyei", they always seek to return to that position 

 which they possessed in the bud ; since not unfreqnently, for ex- 

 ample in Oxalis, the position of the sleeping leaf differs essentially 

 from its position in the bud. JsTeither must the term sleep lead 

 to the assumption that the movements by which leaves pass into 

 their nocturnal position depend upon a relaxation, since, on the 

 contrary, the parts from which the motion issues, that is the joints, 

 are in a state of considerable tension during the sleep of leaves. 



The flowers of a large quantity of plants exhibit changes of 

 position by night analogous to those of leaves, the corollas fold* 



