LOCOMOTORY DIRECTIVE MOVEMENTS 551 



that swarmspores illuminated on all sides by light of uniform intensity can have 

 any knowledge of the fact that there exists a region at a certain distance from 

 where they happen to be at the moment where the intensity of the light is more 

 adapted to their requirements ; it is only hy chance that they reach that situa- 

 tion, where they remain on account of apobatic phototaxis. It appears to us 

 in the highest degree probable that Volvox also is apobatically phototactic, 

 although at the same time we are not prepared to deny that it is also strophically 

 so. Further investigations must be carried out upon the subject so as to deter- 

 mine in how far light intensity and the direction of the rays affect phototactic 

 movements. 



From what we have learnt in regard to heliotropism we have the right to 

 expect that all wave lengths have not the same value in phototaxis ; in fact, 

 experiments designed for this very purpose have shown that the more re- 

 frangible rays have obviously much greater phototactic influence than the less 

 refrangible. 



As to the primary physical or chemical effect of light leading to perception 

 nothing at all is known ; nor is the region where light perception occurs suf- 

 ficiently accurately determined. As in the case of chemotaxis so in the case 

 of strophic phototaxis we must assume that the organism reacts to differential 

 illumination of the anterior and posterior ends, but that in apobatic taxis locali- 

 zation of light perception is possible at the anterior end. It is known that in 

 many swarmspores, &c., there appears in the otherwise colourless anterior ends, 

 a red spot which has been termed the 'eye-spot,' and to which sensitiveness to 

 light has been attributed, but there are certain phototactic swarmspores which 

 possess no such spot, and hence it would appear very improbable that it has 

 any significance in relation to light perception. 



There are also certain free-moving organisms which exhibit a directive 

 response to the galvanic current (galvanotaxis) ; this has been shown to be true 

 especially of Amoebae and Infusoria, although there is evidence of it also in the 

 higher animals (Verworn, 1901, 476) ; it is probable that similar phenomena 

 wiU be discovered in typical plants. Amoebae and Infusoria, general speaking 

 place themselves so that their long axes lie in the direction of the current, creep- 

 ing or swimming towards the negative pole ; certain Flagellata behave exactly 

 in the contrary way, aggregating round the positive pole. It is very probable 

 that galvanotaxis is not due to a special sensitiveness on the part of the organism 

 to the electric current itself, but rather results from the chemical decomposi- 

 tion which the current gives rise to. According to Loeb and Budgett (1897) 

 free alkali arises at the anode end of the organism, and this induces negative 

 chemotactic movements towards the kathode. How it is that some Flagellata 

 move towards the anode remains yet unexplained. 



Finally, we have still to speak of geotaxis, which has been demonstrated as 

 occurring in Bacteria, Flagellata, &c., by Schwarz (1884), Aderhold (1888), 

 and Massart (1891b). Many organisms, when other attractive forces are 

 excluded, move upwards or, in other words, are negatively geotactic. Massart 

 found in the case of two species of Spirillum which were equally sensitive tono- 

 tactically and aerotactically, that the one was positively and the other nega- 

 tively geotactic. Whether, in the case of geotaxis, we have to do with some 

 sensitivity which may be associated with geotropism appears to us very proble- 

 matical ; for geotactic upward movement cannot bring the organism under 

 conditions where the influence of gravity is different from what it was before, 

 while a phototactic movement is capable of placing the organism in other light 

 intensities, just as a chemotactic or osmotactic movement brings the organism 

 into liquids of different concentration. 



Still it must be extremely useful to an organism which lives under definite 

 vital conditions to be able to reach more superficial or deeper layers of a fluid 



