GLIDING MOVEMENTS 273 



to a waltzing movement would be shown when, as frequently happens, 

 the protoplasm streams in opposite directions on the two valve sides. The 

 moment of the couple is not, however, great enough to produce rotation 

 around the short axis in forms lying on the substratum, the opposed forces 

 mutually antagonizing so that the movement ceases l . All Diatoms do not 

 possess an investing layer of mucilage, which is, therefore, not essential to 

 movement 2 . When present it is either set in motion by the streaming 

 protoplasm or the latter by friction against it gives the organism an 

 onward movement. In both cases a trail of mucilage is often left behind, 

 and this has in some cases given rise to the idea that Diatoms possessed 

 a motory flagellum, while the same appearance probably gave rise to the 

 theory of propulsion by a backwardly directed water-jet. In any case the 

 resulting trail, as in the case of Oscillarias, serves to indicate the path of 

 movement 3 , but whether the movement may also be induced or aided by 

 the extrusion of masses of mucilage must remain at present an open 

 question 4 . The existence of attached Diatoms serves to indicate that all 

 the members of this group do not necessarily behave similarly or develop 

 the power of independent locomotion. 



Oscillariaceae. The existence of a power of movement in these plants 

 has been known since the time of Adanson (i767) 5 , and the threads are 

 usually covered by a gelatinous sheath in which or with which they move 6 . 

 Continual secretion keeps the apex covered with the mucilage, in spite of 

 that which is left behind along the path of movement. The locomotion is 

 not connected with any power of curving, since it is also shown when the 

 filaments remain perfectly straight, but no sure proof has as yet been 

 brought forward of the existence of extracellular protoplasm 7 . It is in 

 fact uncertain whether the locomotion results from the exudation and 

 swelling of mucilage or from an appropriate development and utilization of 

 surface-tension energy. Hansgirg supposed that the ejection of water 

 produced the movement, but all the weight of evidence is against this 

 assumption, for if such action existed perceptible signs of it would be 

 detected upon minute neighbouring suspended particles. Cilia do not appear 



1 M. Schultze, Archiv f. mikr. Anat., 1865, Bd. I, p. 386. Cf. Benecke, I.e., p. 553. 

 3 O. Miiller, I.e., 1897, p. 81. 



3 M. Schultze, 1. c., p. 399 ; O. Miiller, 1. c. ; Lauterborn, 1. c., &c. 



4 Cf. Schiitt, Jahrb. f. wiss. Bot., 1899, Bd. xxxin, pp. 645, 656 ; Ber. d. hot. Ges., 1902, p. 202 ; 

 O. Miiller, Ber. d. hot. Ges., 1899, p. 445 ; 1900, p. 481 ; 1901, p. 195. 



5 Meyen, Pflanzenphysiol., 1839, Bd. in, p. 563; Mohl, Vegetabilische Zelle, 1851, p. 136; 

 Nageli, Beitrage z. wiss. Bot., 1860, Heft 2, p. 89; Correns, Ber. d. bot. Ges., 1897, p. 141 ; Kolk- 

 witz, Ber. d. bot. Ges., 1897, p. 460; Hansgirg, Bot. Ztg., 1883, p. 831 ; Physiol. u. Phycopbytol. 

 Unters., 1893, p. 207; Brand, Beihefte z. bot. Centralbl., 1903, Bd. XV, p. 53. 



6 Cf. Hunger, Biol. Centralbl., 1899, Bd. XIX, p. 385 ; Schroder, Verhandl. d. naturh. -medic. 

 Vereins zu Heidelberg, 1902, Bd. vn, Heft 2, p. 187. 



7 Cf. Engelmann, Bot. Ztg., 1879, p. 54. 



PFEFFER. Ill T 



