THE MECHANICS OF AMOEBOID MOVEMENT 283 



of contraction in muscle l . The exact part played by this internal surface-tension 

 energy in ciliary, amoeboid, and muscular movement is not yet fully established, but 

 in any case the minute subdivision into fibrillar chambers containing liquid sarco- 

 plasm gives muscle the properties of a soft solid, and not of a liquid, as Jensen 

 supposes 2 . 



In the present uncertainty it is impossible to say whether the changes of 

 surface-tension are produced by the excretion of metabolic products or in other ways. 

 Verworn 3 assumed that amoeboid movements were due to the combination of oxygen 

 with the superficial biogens lowering the surface-tension, and that the use or 

 dissociation of the oxygen caused the surface-tension to be raised again. 

 Jensen 4 supposed that the increase in the size of the superficial molecules produced 

 by assimilation lowered the surface-tension, while the diminution in the size of the 

 molecules produced by dissimilation raised it. Neither hypothesis is, however, 

 capable of proof, although the fact that amoeboid movement and protoplasmic 

 streaming often continue for a long time in the absence of oxygen, and the existence 

 of motile ciliate anaerobic bacteria show that Verworn's hypothesis cannot possibly be 

 of general application. These theories also assume the predominant importance 

 of surface-tension and neglect the part often played by changes of consistency and 

 cohesion. The fact that external agencies when intense usually produce retraction 

 affords no conclusive evidence, and merely shows that under these circumstances 

 the conditions for expansion are suppressed. 



SECTION 62. Protoplasmic Streaming. 



The protoplasm of many dermatoplasts exhibits streaming movements, 

 which may either be confined to the layer enclosing the central vacuole 

 (rotation), or may also follow more or less irregular paths up and down the 

 bridles of protoplasm crossing the latter (circulation). These two types 

 are, however, merely the direct result of the protoplasmic configuration, 

 for no cell in which protoplasmic strands cross the vacuole shows regular 

 rotation. In addition, when the application of external stimuli causes the 

 protoplasmic strands to be retracted so that a single uninterrupted central 

 vacuole is present^ the previous circulatory streaming passes into rotation 5 . 

 At the same time the velocity of streaming increases owing to the 

 diminished internal friction, possibly aided by an increased liberation of 

 propulsive energy 6 . In the adult leaf-cells of Vallisneria and Elodea and 

 in the internodal cells of Chara and Nitella the direction of streaming 



1 Cf. Bernstein, Pfliiger's Archiv fur Physiol., 1901, Bd. LXXXV, p. 305 ; Naturwiss. Rundschau, 

 1901, Bd. xvi, Nos. 33-5. 



* Jensen, Pfliiger's Archiv fur Physiol., 1900, Bd. LXXX, p. 227. 

 1 Verworn, Allgem. Physiol., 1901, 3. ed., p. 595. 



* Jensen, Die Protoplasmabewegung, 1902, p. 29. (Reprint from Ergebnisse d. Physiologic, 

 Vol. I.) 



5 Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Bd. xxiv, p. 193. 



6 Ewart, Protoplasmic Streaming in Plants, 1903, pp. 29, 35. 



