GLIDING MOVEMENTS 271 



Diatoms and Oscillarias glide slowly over solid substrata or over moist 

 surfaces which serve as a fulcrum for movement. After a time the 

 direction of movement is usually reversed, the posterior end becoming 

 anterior. Since, however, the motion is usually along a more or less curved 

 path, the organism does not always regain its original position. If owing 

 to tropic stimulation the movement towards the light is more energetic 

 and lasts longer than that away from it, progression will on the whole be 

 made in a definite direction. 



Usually Diatoms and Oscillarias glide along with one of their longer 

 surfaces lying mainly or entirely on the substratum, but they may some- 

 times raise themselves so far as to balance on one end. Except in the case 

 of Cylindrotheca and Nitzschiella T Diatoms do not revolve during the 

 forward movement, whereas all Oscillarias show a rotation round the 

 longitudinal axis, which is genetically connected with the mode of locomotion, 

 and which is reversed when the direction of motion changes. Both in the 

 case of the rigid Diatoms and the flexible Oscillarias the movement takes 

 place without any appreciable bending of the body, although mechanical 

 curvatures are readily produced when a flexible Oscillaria comes into 

 contact with an obstacle or temporarily adheres to some fixed body. It is 

 in this way that the nodding, snaky, or jerky movements are produced 

 which are very pronouncedly shown by certain species 2 . Some forms may, 

 however, be found to perform active autonomic curvatures due to heter- 

 auxesis, as do various species of Algae. 



The locomotory energy can be shown to be developed on the outer 

 surface by the streaming movement of external protoplasm, which under- 

 goes a periodic reversal of direction. This is shown by the fact that 

 particles of sand or indigo adhering to the upper valve side of a fixed 

 Diatom are moved alternately backwards and forwards from one pole to the 

 other. We owe this observation to Siebold, and its confirmation to 

 Schultze 3 . Although the detailed mode of locomotion of Oscillaria is 

 uncertain, here also adhering particles move on the outer wall, but in 

 correspondence with the rotation shown by filaments free to move, the 

 streaming protoplasm, as evidenced by the adhering particles, travels to and 

 fro in a spiral path around the filament. 



Diatoms. Several authors 4 have considered the movement to be due to 



1 Borscow, Die Siisswasser-Bacillariaceen des siidwestlichen Russlands, 1873, p. 35. 



2 Nageli, Beitrage z. wiss. Bot, 1860, Heft ii, p. 89; Correns, Ber. d. hot. Ges., 1897, p. 141 ; 

 Kolkwitz, ibid., 1897, p. 460. 



3 Siebold, Zeitschr. f. wiss. Zool., 1849, Heft i, p. 284; Schultze, Archiv f. mikr. Anat, 1865, 

 Bd. I, p. 386. 



* Nageli, Gattungen einzelliger Algen, 1849, P- 2O > Siebold, I.e. ; Dippel, Beitiage z. Kennt- 

 niss der in den Soolwassern von Kreuznach lebenden Diatomeen, 1870, p. 332; Borscow, I.e.; 

 Mereschkowsky, Bot. Ztg., 1880, p. 529. 



