CILIARY MOVEMENT 267 



* 



to withdraw somewhat while the rotation around its own axis is reversed. 

 The normal rotation and forward movement is then resumed and may again 

 produce an impact against an obstructing plate of glass l . At the same 

 time, the orientation is usually somewhat altered, so that on the next 

 forward movement the organism has a better chance of avoiding the 

 obstacle. In other cases the organism continues the normal rotation 

 around its own axis when the glass plate prevents any forward movement. 

 According to Nageli, organisms which normally move in a straight line 

 remain pressed against the same point of the glass, but perform circles 

 on the surface of the glass when they have a natural tendency to eccentric 

 or spiral movement. In other cases, as for instance when the organism 

 glides or creeps over a solid substratum, the former locomotion continues, 

 while the rotary movement ceases 2 . The boat shape of the free-swimming 

 Bodo saltans causes the twisting movement of the cilia to produce a rocking 

 movement but no rotation 3 . 



Since both locomotion and rotation are due to ciliary activity, it is 

 not surprising that the same type of rotation should be retained so long 

 as the direction of movement is unaltered. The ciliary activity might, 

 however, easily be so modified as to reverse the rotation without producing 

 any change in the direction of locomotion, but observations pointing to 

 this conclusion must be accepted with caution *. A reversal of this kind 

 does, however, appear to be satisfactorily established in the case of Gonium 

 pectorale 6 . 



The rapidity and duration of the movement are naturally very 

 dependent upon the external conditions. Antherozoids, as well as the 

 asexual zoospores of Algae and Fungi, come to rest after a definite period 

 of activity, which may be comparatively short. It is, however, possible 

 under special nutrient conditions to keep bacteria, the swarm-spores of 

 Myxomycetes, and possibly also many Flagellatae and Volvocineae, per- 

 manently motile, and to prevent the recurrence of any resting stage 6 . 



Even under favourable conditions the most active plant zoospores 

 do not attain the speed of movement of Infusoria, and progress but slowly 

 in absolute measure. The highest velocity does not appear to exceed 

 i mm. per second, and is often not above 0-05 mm. per second 7 . A zoo- 



1 Cf. Nageli, Beitrage zur wiss. Bot., 1880, Heft 2; Biitschli, I.e., p. 854; Jennings, Centralbl. 

 f. Physiol., 1900, Ed. xiv, p. 106. 



3 See Nageli, 1. c., p. 101 ; Biitschli, 1. c., p. 853; Schiitt, Die Peridineen d. Planktonexpedition, 

 1895, p. 117 ; Jennings, American Naturalist, 1901, Vol. XXXV, p. 372. 



3 Pfeffer, Unters. a. d. hot. Inst. zu Tiibingen, 1888, Bd. II, p. 594. 



4 Cf. also Biitschli, 1. c., p. 853. 



5 Migula, Bot. Centralbl., 1890, Bd. XLIV, p. 104; Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 

 1884, Bd. I, p. 443. Cf. also Nageli, 1. c., p. 97. 



6 On the zoospores of Myxomycetes cf. Klebs, Jahrb. f. wiss. Bot., 1900, Bd. XXXV, p. 196. 



7 According to Hofmeister, Pflanzenzelle, 1867, p. 30, the zoospores of Aethalium septicunt 

 cover per second o^-o^ mm., and those of Gonium pectorale 0-046 mm. See also Nageli, I.e.; 



