io8 THEORETICAL AND GENERAL 



chloroplastids of Vallisneria, Elodea, Lemna> Fern prothalli, &c., though 

 they may remain living and capable of CO 2 -assimilation for several hours, 

 never show any directive movements in response to light however intense, 

 however they may be placed, and in whatever media they may be immersed. 

 Hence, apparently, the chloroplastid can only move when imbedded in 

 living plasma, and whether the movement is due to a reaction between the 

 two or is due to the cytoplasm alone, still remains an open question. 



SECTION 49. Theories of Streaming. 



Briicke and also Hanstein x have suggested that the moving proto- 

 plasm circulates in closed tubes, which are either themselves at rest, or 

 which rhythmically dilate and contract. This is, however, highly improb- 

 able and devoid of proof either by experiment or observation. The mere 

 fact that relatively large chloroplastids commonly swim in the stream when 

 the movement is active, would necessarily postulate tubes of corresponding 

 width, whose walls would be easily distinguishable if they possessed the 

 necessary physical and vital properties. It is indeed sufficient to follow 

 a floating chloroplastid a few times round a cell to convince one's self of 

 the impossibility of the existence of any such tubes. The theory that 

 streaming is a phenomenon of contractility is no longer tenable, since it 

 postulates the existence of a fixed contractile framework, or of firm elastic 

 boundaries to the moving streams. Frequently the indifferent line between 

 two opposed streams has no perceptible breadth. Yet the streams do not 

 interfere or commingle in the least and behave as if separated by a stationary 

 invisible membrane. This may be a surface-tension film of oily or other 

 substance which possibly is not wetted by either stream, and which 

 possesses sufficient elasticity to repel slowly moving solid particles which 

 collide obliquely against it. 



Hofmeister and Sachs 2 have suggested that changes in the power of 

 imbibition of the protoplasmic particles might cause some to temporarily 

 extrude water, others to absorb it. Waves of such action and reaction 

 passing around the cell would cause the protoplasm to move in a particular 

 direction. The free water of the protoplasm must, however, move in the 

 opposite direction with a velocity corresponding to the relative masses of 

 it and of the protoplasm. This mode of propulsion involves a very high 

 internal friction, and a correspondingly large consumption of energy, 

 whereas the actual consumption of energy has been shown to be relatively 

 small (Sect. 10). Moreover, the rhythmic imbibitory changes which should 



1 Briicke, Unters. iiber das Protoplasma und die Contractilitat ; Hanstein, Protoplasma, Heidel- 

 berg, 1880. 



* Hofmeister, Die Lehre von der Pflanzenzelle, p. 63 ; Sachs, Physiologic, 1865, p. 451. 



