MECHANICS AND MECHANICAL MODELS 



FIG. a. Streaming cell of Elodea 

 fragmented by plasmolysis. The 

 largest thread on each ball has just 

 broken. 



around it, which drags out behind like a tail. At first this looks as if 

 the chloroplastid had an active power of move- 

 ment of its own, but the explanation probably 

 lies in the greater momentum of the chloro- 

 plastid, or in the fact that in a fluid flowing 

 around a curve the velocity is greater on the 

 convex side than on the concave one which is 

 the shorter path, the internal friction being 

 thereby reduced to a minimum. At the point B, 

 the path of the current is shortened, at A it is 

 lengthened, and in the latter case, owing to the 

 shape of the curvature, the pressure due to 

 surface-tension is added to the internal osmotic 

 pressure instead of acting against it. The finer 

 floating particles have a slightly greater velocity 

 at B than at A, that at B being slightly above, 

 and that at A slightly below the mean over the 

 rest of the cell. If, however, the ectoplasm is 

 only just separated from the cell-wall no such 

 differences are perceptible. Hence the mere 



separation of the cell-wall and protoplasm does not in itself retard the 

 velocity of streaming, arid direct contact between 

 the ectoplasm and cell-wall is not necessary for the 

 maintenance of rotation. No movement is shown 

 in ectoplasmic layers unsupported by a cell-wall, 

 nor is any translatory movement perceptible in the 

 liquid lying between the ectoplasm and cell-wall. 



O. Muller 1 has shown that the forward move- 

 ments of certain diatoms are due to the presence 

 of peripheral strands of plasma streaming in the 

 opposite direction, and returning by a central path 

 in the body of the diatom. The friction of these 

 bands against the water causes the diatom as a 

 whole to move in the opposite direction, and Muller 

 has calculated that in Nitzschia sigmoidea the 

 rapidity of streaming in the external protoplasmic 

 bands must be approximately 50 //, per sec. to 

 give the forward velocity of 17 p. per sec. usually 

 shown by this form. We are, however, here dealing 

 with an exceptional condition of things, for in or- 

 dinary plant cells covered by cell-walls no such direct 

 interaction with an external medium is possible. 



1 Ber. d. D. Bot. Ges., 1896, Bd. xiv, p. 117. 



FIG. 3. Partially plasmolyzed 

 streaming cell. 



