SECT, ii PHYSIOLOGY 287 



but the growth of all will he 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 he 

 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 GEOTROPIO 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 remarkable corroboration 

 of the result of KNIGHT'S experiments, and is 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 autouomic movements are those resulting in epinastic and 

 hyponastic curvatures (p. 272), and the retrogression of recently formed paratonic 

 curvatures through longitudinal extension (autotropism) or by curvature in the 

 opposite direction ( 90 ). 



Such autonomic movements should not be confused witli 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. 281) 

 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 this, curva- 

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

 they were for a long time actually considered as such ( 91 ). When stem-climbers 

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

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

 nutations ( 92 ). 



E. Curvatures induced by Contact Stimuli 



The protoplasm of plants, like that of animals, exhibits an irrit- 

 ability to contact, whether momentary or continuous. This is 

 apparent, not only from the behaviour of naked protoplasmic bodies, 

 but also from the reactions manifested by walled cells and by whole 

 organs, the functions of which may be so disturbed by the action of 

 mechanical stimuli that death ensues. The almost universal irritability 

 of vegetable protoplasm to mechanical stimulation is utilised by a 

 number of plants for the production of movements which lead to their 

 ultimate attachment to the irritating body. The mechanical stimulus 

 is frequently perceived by means of special sense organs, the protoplasts 

 of which are altered in shape owing to the contact. Tendril-climbers, 

 in particular, have developed this irritability to contact stimuli as a 

 means of attaching themselves to supports (cf. p. 67, Fig. 69); and 



