322 



MODIFIED CIRCUMNUTATIOX. 



CHAP. TIL 



as (see also 'Flora,' Oct. 1st, 1873, p. 437) that those ol 

 S. napcea fall at night, but 

 to what angle he cannot 

 remember. The leaves of 

 S. rhombifolia and retusa, on 

 the other hand, rise up 

 vertically, and are pressed 

 against the stem. \\e have 

 therefore here within the 

 same genus, directly op- 

 posite movements. Again, 

 the leaves of <S. rhombifoliu 

 are furnished with a pul- 

 vinus, formed of a mass of 

 small cells destitute of chlo- 

 rophyll, and with their 

 longer axes perpendicular 

 to the axis of the petiole. 

 As measured along this 

 latter line, these cells are 

 only -Jth of the length of 

 those of the petiole; but 

 instead of being abruptly 

 separated from them (as is 

 usual with the pulvinus in 

 most plants), they graduate 

 into the larger cells of the 

 petiole. On the other hand, 

 S. napcea, according to Ba- 

 talin, does not possess a 

 pulvinus; and he informs 

 us that a gradation may be 

 traced in the several species 

 of the genus between these 

 two states of the petiole. 

 >^ida rhombifolia presents 

 another peculiarity, of which 

 we have seen no other in- 

 stance with leaves that 

 sleep: for those on very 

 young plants, though they 



rise somewhat in the evening, do not go to sleep, as we observed 



Qidt rhom'nfolia : circumnutation and 

 nyttitropic (or sleep) movements of 

 a leaf on a young plant, 9 inches 

 high; filament fixed to midrib of 

 nearly full-grown leaf, 2jj inches in 

 length ; movement traced under a sky- 

 light. Apex of leaf 5| inches from 

 the vertical glass, so diagram not 

 greatly enlarged. 



