6 MOVEMENT 



It follows, therefore, that when two different organisms respond similarly 

 to the same stimulus the processes of perception and response may be alike 

 in both, but need not necessarily be so. We do not know, for instance, 

 whether the mode of perception of light and of other tropic stimuli is in all 

 cases the same. Even if this were the case the power of response may vary 

 indefinitely, and can never be the same in a rooted plant as it is in a free 

 swimming one. The former may respond by movements due to growth or 

 to changes of turgidity, whereas the latter is dependent entirely upon the 

 special mode of locomotion it possesses. The same stimulus may produce 

 different responses according to the properties of the responding organism, 

 and widely dissimilar stimuli produce the same type of movement in 

 a particular plant. It is only natural, for instance, that swarm-cells should 

 always use the locomotory organs they already possess in moving from one 

 place to another as the result of stimulation, and should not seek out 

 and utilize currents of water or other external mechanical agencies for 

 this purpose. Similarly, we may assume that the curvatures resulting 

 from heliotropic, geotropic, and chemotropic stimuli are in many cases, 

 at least, carried out in a similar manner in all rooted plants. 



Every organ which has the power of responding to one or more tropic 

 stimuli, singly or conjointly, must possess a special power of perceiving 

 each such stimulus, and the power of perception as regards one stimulus 

 may be lost or destroyed without the general perception being affected. 

 This remains true, in spite of the fact that plants possess no special sense- 

 organs, and that we are unable to say how it is that of two apparently 

 similar protoplasts one may temporarily or permanently possess a different 

 irritability and power of response to the other. The response is always 

 dependent upon the existent condition of tone, which again depends upon 

 the stage of development and upon the existing and previous external 

 conditions 1 . Furthermore, the resultant action of two conjoint stimuli is 

 not necessarily the arithmetical sum of their individual actions. Supposing 

 that the power of movement remains unaltered, either two separate impulses 

 may be exercised upon it, or the two stimuli may fuse during perception 

 and act as a single excitation. 



1 The terms ' tone ' and ' tonic condition ' may be used in the same sense as in animal physiology, 

 so that by ' tonic stimuli ' we may denote the conditions which render possible an irritable response. 

 Cf. Massart, Biol. Centralbl., 1902, Bd. xxxn, p. 41 ; Miehe, Jahrb. f. wiss. Bot, 1902, Bd. xxxvn, 

 p. 571. Miehe distinguishes between 'anatonic,' ' katatonic,' and ' metatonic ' stimuli, according to 

 whether the reaction is increased, decreased, or reversed by them. Engelmann's ' photokinesis ' is 

 merely an instance of light acting as a tonic stimulus, as when illumination excites the movement 

 of certain motile forms. (Engelmann, Pfliigers Archiv f. Physiol., 1882, Bd. xxx, p. 169.) On the 

 equally unnecessary term chemokinesis,' cf. Rothert, Flora, 1901, p. 374, and also Nagel, Bot. Ztg., 

 1901, Ref., p. 298. Carrey (The Effects of Ions upon the aggregation of flagellate Infusoria, 1900, 

 p. 291) has used the term 'photokinesis ' in another sense, to indicate the changes of movement pro- 

 duced by sudden alterations of illumination. 



