OF VITAL MOTION. 55 



to measure changes which are far beyond the per- 

 ceptions of the common instrument of this name. 



The phenomena of motion in other irritable 

 plants are of a similar character, and with few 

 exceptions, the contraction occurs at night, or at 

 times of contact with any foreign body, while the 

 opposite condition of expansion is cotemporaneous 

 with the day. The dijQference in other cases is, 

 that the movements are not susceptible of rapid 

 alterations, and merely respond slowly to the sun, 

 but still the general law is the same, and leaves 

 and flowers open in the day, and close at night. There 

 are some flowers, however, whose movements seem to 

 obey a different law, and to open at night ; but these 

 it must be observed do not close in the day, in the 

 same sense that the leaflets of the sensitive plant do, 

 but wither; and therefore the difference may be 

 accounted for by a greater sensibility in these plants, 

 the nocturnal stimulus being sufficient to awaken life, 

 while, on the other hand, the full beams of the sun 

 may be said to scorch and destroy the mobile textures. 

 Indeed, in some cases it may appear that the plants 

 which exhibit these exceptional peculiarities are those 

 which have been brought from climes less genial than 

 ours, to whose days our nights may be nearly equi- 

 valent, and it is possible, if the phenomena of motion 

 were observed in their own homes that these would 

 cease to differ from others. 



