264 BOTANY 



uniformly rotated, with a velocity sufficient to preclude the continuous 

 operation of a stimulus on any one point long enough to occasion a 

 one-sided growth. As in that case, no one side will be exclusively 

 acted upon, but the growth of all will be equally promoted or retarded ; 

 the action of external influences, although exerted in only one direction, 

 will be equalised. On this account the "method of slow rotation," 

 originally instituted by Sachs, is of great assistance in the observation 

 and investigation of the phenomena of movements. By means of it, 

 heliotropic movements due to one-sided illumination may be prevented 

 without the necessity for either exposing the plants to the injurious 

 effects of continued darkness, or providing for an equal illumination on 

 all sides. This method is, moreover, of especial value in investigating 

 the movements due to the action of gravitation, for it is not possible 

 to exclude its influence, as it is those arising from light, definite 

 temperature, oxygen, etc. 



When plants are slowly rotated on a horizontal axis, the 

 one-sided action of gravitation is eliminated and geotropic 

 CURVATURE is THUS prevented in organs which react equally on 

 all sides. The rotations are best produced by the klinostat, 

 an instrument by means of which an exactly horizontal axis is 

 rotated by clock-work. That geotropic curvatures of radial organs 

 are, in fact, precluded by means of the klinostat, furnishes a re- 

 markable corroboration -of the result of Knight's experiments, and 

 may also be regarded as a further proof that such curvatures are due 

 to terrestrial gravitation. Through the equalisation of the action 

 of external directive influences, radial portions of plants exhibit on 

 the klinostat only such movements as arise from internal causes. 

 The most important of these autonomic movements are those resulting 

 in epinastic and hyponastic curvatures (p. 249), and the retrogression 

 of recently formed paratonic curvatures through longitudinal extension 

 (autotropism). 



Suoli autonomic movements should not be confused with those exhibited by 

 dorsiventral organs on the klinostat, in consequence of the unequal irritability of 

 their different sides. Through the special irritability of the dorsal side (p. 258) 

 of foliage leaves and zygomorphic flowers, it is during their rotation more strongly 

 acted upon by geotropic influence than the ventral side ; as a result of^his curva- 

 tures are produced which so closely resemble those resulting from epinasty that 

 they were for a long time actually considered as such. When stem-climbers are 

 rotated on the klinostat, their revolving movement ceases, the part of the stem 

 capable of growth unwinds and straightens, and afterward exhibits only irregular 

 nutations. It is thus evident from their behaviour that their winding and par- 

 ticularly their revolving movements are dependent upon geotropisni. 



E. Curvatures induced by Contact Stimuli 



The protoplasm of plants, like that of animals, exhibits an irritability 

 to contact, whether momentary or continuous. This is apparent, 



