300 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



redistribution takes place when the protoplasm exhibits streaming move- 

 ment. Even in this case, however, a local accumulation of protoplasm 

 or chloroplastids may pass several times around a cell before being 

 broken up. 



When displacements are only slowly readjusted, the protoplast may 

 never reach a condition of stable equilibrium, owing to the continued 

 production of new dispositions before the adjustment to the older ones is 

 completed. It is, therefore, impossible to predict what the stationary 

 condition of equilibrium would be, and in fact the protoplast might 

 maintain permanently an unequal distribution of tension. In general, 

 however, the shape and arrangement of the protoplast and of its organs 

 correspond to what would be expected in a viscous emulsion. 



Since the organs of the protoplast lie in an active living medium, local 

 changes of surface-tension are likely to be of common occurrence, quite 

 apart from the changes of imbibition and swelling produced in the interior 

 of the organs affected. In addition, the various organs as well as portions 

 of the cytoplasm or nucleoplasm may acquire temporarily or permanently 

 a more solid consistency, and may then be capable of active changes of 

 shape. The preservation of their shape by the chlorophyll-bands of 

 Spirogyra demands the existence of a fair degree of consistency in them, 

 and possibly changes of cohesion play a part in the changes of shape and 

 configuration of the chromosomes during nuclear division. It is, however, 

 uncertain in most cases how the various internal movements and changes 

 of shape are produced. Even in the much studied case of protoplasmic 

 streaming an element of doubt still attaches, for the evidence in favour 

 of its surface-tension origin is for the most part indirect in character 1 . 



Although in certain cases the chloroplastids and nuclei are undoubtedly 

 passively carried by the streaming protoplasm, they may also be capable 

 of slow independent locomotion by amoeboid change of shape, or by the 

 maintenance of appropriate differences of surface-tension. In the case 

 of comparatively large plastids and nuclei embedded in viscous protoplasm 

 only very slow movements could be produced in this way ; but the smallest 

 force will produce movement, since the resistance to flow is kinetic and pro- 

 portional to the velocity. The total force available in such cases would 

 not, however, suffice to overcome the static resistance offered even by a very 

 attenuated colloid to an incipient movement. In any case, if the differences 

 of surface-tension were only produced by the metabolic activity of the 

 surrounding cytoplasm, it is doubtful whether we should be justified in 

 speaking of an active locomotion of the nucleus, although the motory 

 energy was actually liberated at the boundary of nucleus and cytoplasm. 

 Similarly, when a drop of oil comes into contact with a soap-solution the 



1 Cf. Ewart, Protoplasmic Streaming in Plants, 1903, pp. 108-19. 



