MOVEMENTS OF GROWING LEAVES AND FLORAL ORGANS. 875 



be exposed to light for a longer time in order that a movement may take place when 

 they are again in darkness. Each movement produced by darkness is followed by a kind 

 of rebound ; thus the downward curvature of the leaf of Impatiens is followed by a rise. 

 The rise takes place more quickly in the light. 



The rigidity of the leaves of Impatiens is not materially affected by their assuming the 

 nocturnal position. 



A rise of temperature at 3 a.m. distinctly accelerated the assumption of the diurnal 

 position by the leaves of Impatiens, but it seemed to have little or no effect upon other 

 meteorically sensitive leaves {Chenopodium), 



(c) Flowers (a). Pfeffer made measurements to ascertain the amount of growth 

 during curvature upon flowers which perform movements at all times in consequence 

 of slight changes of temperature (see infra). He found the Crocus (jvernus and luteus) 

 to be especially adapted for this purpose. In this flower the zone of curvature of the 

 six segments of the perianth lies above their separation from the tube, and occupies 

 from one-fourth to one-sixth of each segment of the perianth. A portion of this zone 

 about 3 mm. in length is especially capable of curvature, and upwards and downwards 

 from this the capacity gradually diminishes. Black marks were made upon the most 

 motile portions to serve as a means of measurement. The measurements were made 

 with a magnifying power of eighty diameters. One division of the micrometer eye-piece 

 was equivalent to 0*0076 mm. The following table may serve as an illustration : — 



Crocus 'vernus. 

 Lengths of the marked portion. 

 On the external surface. 

 Closed. Open. 



214 divisions. 214 divisions. 



217 „ 218 „ 



187 „ 188 ,» 



209-5 » 210 „ 



196 „ 196 „ 



On the internal surface. 



214*5 „ 220 „ 



181 „ 185 „ 



208 „ 210-5 „ 



205 „ 212 „ 



191*5 „ 196-5 » 



In the closed position, the portion measured was nearly straight; in the open position, 

 it was concave outwards, the curvature having a radius of from 15 to 30 mm. 



These and other measurements made upon Tulipa, Oxalis, 'Taraxacum, Leontodon, and 

 Fenidium show that the side towards which the movement takes place undergoes no 

 perceptible elongation, whilst the other side increases considerably in length. 



It is possible, in the case of Crocus, to remove the epidermis of one side and to 

 determine that the same movements are effected as before in consequence of varia- 

 tions of temperature. This took place even when the epidermis of both sides was 

 removed. The negative tension and the elasticity of the epidermis therefore play 

 but an unimportant part in producing the movement. 



The growth of the motile zone is by no means at a standstill when the flowers 

 are not performing any movements ; on the contrary, it is continuing slowly and 

 uniformly in the two antagonistic halves, and may be retarded or accelerated in either 

 half at any time by a variation of the temperature or of the intensity of light: a 

 movement (curvature) will of course be the result. 



(3) The effect of Fariations of Temperature may become apparent at any time in 

 many flowers, such as those oi Crocus and of Tulipa. If they be placed first in a cold 



