

GENERAL 99 



growth is produced in tendrils by contact-stimulation, and presumably it 

 would not be shown if all shock-effect was avoided by allowing the contact- 

 stimulation to increase gradually from a sub-minimal to an optimal intensity. 



Sudden changes in the external conditions probably leave no organism 

 entirely unaffected, although no disturbance of growth or other pronounced 

 reaction may be perceptible. Sudden variations of temperature and of 

 illumination do, however, appear in general to excite feeble transitory 

 disturbances of growth, and these have become especially pronounced in 

 certain cases as the result of biological adaptation, so that, more especially 

 in photonastic plants, a transitory acceleration of growth is produced even 

 by a comparatively slow diminution in the illumination. All plants have 

 not the same power of response, and there are even organs which appear to 

 experience no shock-effect, although they change their position in response 

 to alterations of temperature or illumination. It is even possible that in 

 some cases a sudden change may produce a temporary depression of growth, 

 just as a shock-stimulus causes a transitory fall of turgor in one-half of the 

 pulvinus of Mimosa pudica, or a transitory cessation of streaming in a cell 

 of Chara or Nitella. 



The constant daily repetition of the sleep-movements of photonastic 

 leaves induces a periodic rhythm which gradually disappears in darkness or 

 under constant illumination. Under natural conditions the movements in 

 the morning and evening result from the co-operation of the photonastic 

 reaction with the after-effect of the previous ones, the photonastic rhythm 

 being induced and not hereditary. 



Aitionastic reactions do not always exert appreciable after-effects, for 

 these are absent from the thigmonastic movements of bilateral tendrils and of 

 the tentacles of Drosera. The same appears to apply to thermonastic move- 

 ments, such as the opening and closing of the flowers of Crocus and Tulipa. 

 Since, further, the daily variations of temperature are much more irregular 

 than the daily changes of illumination, it is not surprising that the daily 

 opening and closing of thermonastic flowers should be more irregular than 

 the periodic movements of photonastic organs. 



Photonastic, thermonastic, and hydronastic movements are often associated 

 together under natural conditions, and may also be coupled with tropic responses due 

 to unilateral stimulation. It is naturally necessary at first to determine the nature of 

 each form of response before studying conjoint actions. Granted that an organ 

 possesses definite properties, the character of its response can largely be predicted 

 from what is known as to the general influence of the external conditions on growth. 

 For instance, an opening movement may be converted into a closing one by an 

 additional rise of temperature, if the temperature optimum for the previously epinastic 

 side is lower than for the opposite more slowly-growing one. 



Under constant external conditions only autogenic movements are performed, 

 such as the expansion of the foliage and the opening of the foliage and flower-buds. 



H 2 



