410 



MOVEMENTS. 



the new position. The diversity of positions can be only imper- 

 fectly indicated by the accompanying illustrations. 



According to Pfeffer, the sleep-movements of leaves and of 

 cotyledons depend upon increased growth on one side of the 



median line of the 

 petiole and midrib, 

 followed after a 

 certain interval of 

 time by a corre- 

 sponding growth 

 on the opposite 

 side. Thus in 

 ordinary leaves 

 which droop at 

 night the depres- 

 sion is produced 

 by a slightly in- 

 creased growth on the upper side, and the rise in the morning 

 by a similar growth on the under side. But in the most striking 

 cases there is a 

 distinct appara- 

 tus at the base 

 of the leaf-stalk, 

 which accom- 

 plishes the same 

 movement by 

 simple turges- 

 cence of the op- 

 posite sides. 



The apparatus 

 consists of an 

 enlargement 

 formed of cellu- 

 lar tissue in 

 which there is 

 often an appre- 

 ciable difference 

 between the 

 character of the 



cell-walls on the upper and under side of the swelling. This 

 swelling, known as the pulvinus, permits the movement to be 



Fio. 183 Leaflets of Averrhoa bilimbi at night. (Darwin.) 



FIG. 184. Leaf of Acacia Farnesiana during the day and at night. (Darwin. ) 



