io MOVEMENT 



The rapidity of the return movement depends on the prevailing condi- 

 tions, but it is usually much slower than that induced by the original 

 stimulation, as is strikingly shown by comparing the sudden closure of 

 the leaflets of Mimosa or of the leaf-lobes of Dionaea, induced by a blow 

 or by contact, with their subsequent gradual re-expansion. 



Since autogenic factors are always in play, even the movement 

 resulting from a single external stimulus is as much the result of conjoint 

 stimuli as when two external stimuli act simultaneously. As the result 

 of the co-operation of these autogenic and aitiogenic factors and of the 

 reactions due to the movement itself, the final curvature assumed is usually 

 preceded by a series of oscillations. The movement of the mercury in the 

 gas-regulator. of a hot chamber when the temperature is raised to a new 

 level forms a suitable analogy, for here also the excessive movement excites 

 factors tending to its reduction, and to a rapid diminution in the amplitude 

 of the vibrations. Oscillations of this kind occur during tropic and nastic 

 movements, as well as during the return of stimulated leaves of Mimosa to 

 their original position. The persistent after-effects of the daily movements 

 are also the result of oscillations of this kind, although oscillations having 

 a purely internal origin may exist. 



These general remarks apply not only to the higher and lower plants but also to 

 each individual protoplast, for in each case the functionally dissimilar parts and organs 

 are variously affected by stimuli and are unequally responsive and' active. We do 

 not, however, know either the organs of perception or by what changes the latter may 

 be modified. Just as particular powers and properties may appear and disappear 

 under particular conditions, so also may the power of perception not always be 

 present. Furthermore it is possible that in many cases the perception of a stimulus 

 may involve the simultaneous awakening of different processes, and that the inca- 

 pacity for any one of these may make the organism irresponsive. 



Since the organs of the protoplast are capable of a variety of functions, it is 

 hardly to be expected that any of them should be capable of response to a single 

 stimulus only, or that special sense-organs capable only of limited excitation should 

 be developed. It is, however, possible that in particular cases the nucleus may 

 perceive the stimulus or act as a reflex centre, whereas in others it may take no part. 

 Thus in non-nucleated masses of cytoplasm functions such as streaming and ciliary 

 movement may continue and be affected by external stimuli, as is especially well 

 shown when non-nucleated fragments of Infusoria exhibit galvanotaxis. Even when 

 interaction with the nucleus is necessary for the performance of a response by the 

 cytoplasm, it does not follow that the nucleus perceives the stimulus. For instance, 

 the unicellular rhizoid of Marchantia or of a fern prothallium responds by a nega- 

 tively heliotropic curvature when the tip is exposed to light, although the nucleus is 

 at its base and is not directly exposed to the stimulus of light. 



The different parts of the cytoplasm have without doubt different and change- 

 able powers, but even when a particular stimulus is perceived by the isolated cilia 

 of a motile organism, the ectoplasmic membrane and other parts may also be 



