546 TRANSFORMATION OF ENERGY 



This difference in sensitivity between apobatic and strophic organisms, 

 discovered by Rothert, does not surprise us, since we have already met with 

 similar phenomena in studying the movements of organs in fixed plants. Ob- 

 viously, the sensitivity of strophically reacting organisms corresponds exactly 

 to that associated with tropisms (in the true sense of the word), while apobatic 

 reactions resemble nastic movements (heat curvatures of tendrils, sleep move- 

 ments, &c.). 



It is to be hoped that Rothert' s observations on the difference between 

 apobatic and strophic chemotaxis may soon lead to detailed experimental 

 investigations on the subject ; the field for such inquiry is a wide one, and there 

 are still many related problems which are very obscure and of which we can 

 present no explanation here. There is only one point to which we may draw 

 attention, viz. the difference between positive and negative taxis, for in this 

 respect also strophically and apobatically reacting organisms are not alike. So 

 far as strophic organisms are concerned the difference between positive and nega- 

 tive taxis lies only in the reaction, the motive cause of the stimulus is the same 

 in both cases and is due to the diffusion of the stimulant into the water. The 

 case is different with apobatic organisms, where the reaction is always the same 

 (a retreating movement) ; whether a positive or negative taxis takes place, 

 depends only on the motive cause, being positive when the concentration 

 of the stimulant decreases and negative when it increases. The position of the 

 optimum of the stimulant probably determines whether the decrease or the 

 increase in concentration will induce stimulation. Strophic organisms actively 

 seek this optimum, for they turn their bodies sometimes in one direction, some- 

 times in another, towards it ; apobatic organisms, on the other hand, are sensi- 

 tive, not to the approach to but only to the withdrawal from the optimum and, 

 in the latter case, retreat from it. It is not impossible that the same organism 

 may exnibit both types of taxis at the same time. [This has been shown to 

 be the case in the sperms of Isoetes by Shibata (1905 b), and it must be true 

 of fern antherozoids also.] 



The existence of the optimum is especially strikingly manifested if the 

 concentrations in any one preparation are greatly diversified, for then the 

 motile organisms congregate at a definite place where the optimum concentra- 

 tion prevails. Aggregations such as these are shown by certain Bacteria {Spi- 

 rillum : Engelmann, 1881 ; Beijerinck, 1893 ; Beggiatoa : Winogradsky, 

 1887), which seek regions where there is low oxygen tension and which are 

 negatively aerotactic to high tensions and positively aerotactic to low tensions. 

 Many instances of aggregation of motile organisms at definite distances from 

 the mouth of the capillary tube have been demonstrated by Pfeffer and others 

 (Rothert, 1901, p. 402). Pfeffer draws special attention to the case of Spi- 

 rillum undula, which may frequently show both positive and negative response 

 to the same stimulant, and, in so far as the action of this stimulant is purely 

 chemical, the difference in the results can be due naturally only to differences in 

 concentration. If, however, a definite concentration can act at the same time 

 both attractively and repulsively, negative osmotaxis must be allowed to be 

 always operating in addition to positive chemotaxis. The fact that not every 

 stimulant induces positive and negative chemotaxis leads us to conclude that the 

 optimum for many substances is close to zero, while in the case of others it is very 

 high ; in the former case we observe that the stimulant has always a repulsive 

 influence, but, in the latter case, the organisms are always attracted or are 

 indifferent. 



We have hitherto spoken of chemotaxis in general and considered the 

 directive influence both of substances in solution and of gases. The question 

 we have now to answer is whether a gaseous stimulus operates in the same 

 way as a solution of a solid body and whether the sensitivity to different gases 



