266 I50TANY PAIIT i 



when the part taken by the swarm-spore in the life of an Alga is considered. In 

 order to provide for the future nutrition of the stationary Alga into which it 

 afterwards develops, it must seek the light. If a point with suitable (that is, not 

 too intense and not too weak) illumination be attained, then the swarm-spore must 

 attach itself by the end which carries the cilia : to do this it must turn itself from 

 the light towards a dark object. On the other hand, as the swarm-spores do not 

 come to rest at all in absolute darkness, but swim continuously until thoroughly 

 exhausted, the possibility of their attaching themselves in a spot devoid of light 

 where the new plant could not assimilate is excluded. 



The swarm-spores of aquatic Fungi and spermatozoids, according to PFKFFEK'S 

 investigations, are chiefly influenced in their movements by the unequal distribu- 

 tion of dissolved substances in their environment (topotaxis). The crowding 

 together of Bacteria appears, from ROTHEUT'S experiments, not to be due to move- 

 ment towards an attracting stimulus but to their inability to leave a point of 

 optimal concentration reached by accident ; the decline in this concentration 

 arrests their movement and prevents their dispersal (phobotaxis). Minute traces 

 of free oxygen can be recognised by the influence exerted on the movement of 

 certain Bacteria ; ENGELMAXN'S bacterial method of demonstrating assimilation 

 depends on this fact. According to their momentary requirements and their 

 sensitiveness to stimuli, such small organisms move either towards or away from 

 the points of highest concentration ; they are sensitive to the quantity as well 

 as the quality of the dissolved substances, and also to the osmotic effects of the 

 latter (osmotaxis) ( 73 ). 



As the result of similar CHEMOTACTIC MOVEMENTS spermatozoids approach the 

 female sexual organs. PFEFFER has demonstrated that the spermatozoids of 

 Ferns are enticed into the long necks of the archegonia by means of malic acid : 

 while the archegonia of the Mosses attract the spermatozoids by a solution of cane- 

 sugar, and those of the Marchantiaceae by proteid substances. In such cases an 

 extremely small quantity of dissolved substance is often a sufficient stimulus to 

 call forth active chemotactic movements ; a O'OOl per cent solution of malic acid 

 suffices for the attraction of Fern spermatozoids. The movements of amoebae and 

 plasmodia are similarly induced by external influences. These naked protoplasts 

 live not only in water but also in moist substrata (plasmodia, amoebae), and seem 

 to possess the power of seeking out situations with more moisture, or of avoiding 

 them (before the formation of spores) : their movements are also influenced by the 

 direction of currents in the water (rheotaxis). The term thigmotaxis or stereo- 

 taxis has been applied to the exceptional property of certain swarming cells, which 

 are stimulated by mechanical contact to remain attached to the object around which 

 they are swarming. This has been observed in spermatozoids of Fucaceae and in 

 Chromatium wcissii, a sulphur Bacterium. The fact that algal swarm-spores on a 

 hard rough substratum form stronger attaching organs than on a smooth surface is 

 related' in some degree to this. In cases where cells enclosed by cell walls (e.g. 

 Haematococcus phnnalis) swim freely about by means 'of cilia, the cilia spring 

 from the protoplasm and pierce the cell walls. 



Diatoms and Desmids exhibit quite a different class of movements. The 

 Diatoms glide along, usually in a line with their longitudinal axes, and change 

 the direction of their movements by oscillatory motions. From the manner in 

 which small particles in their neighbourhood are set in motion, it was concluded 

 that special organs of motion probably protrude, like pseudopodia, through 

 openings in their hard silicified shell. Recently these motile organs have been 

 seen in some forms. According to O. MULLEU, the movement is set up by a 



