CYTOPLASM 



189 



once and for all refute the idea that cytoplasm is a liquid with freely 



moving particles. 



Protoplasmic flow in Atmeha and Physarum seems to consist in the 



forcing of liquid cytoplasm through capillaries or other channels by 



a contracting gel; but this view cannot be generalized. In plant cells, 



such as in the leaves of Elodea, the whole 



protoplasm rotates along the cell wall 



(cyclosis) ; or in Uving hairs cytoplasmic 



strands even circulate across the central 



vacuole. In these cases the impelling force 



must be sought in the flowing strand 



itself. If we admit that local contractions 



are again involved, we may postulate the 



following to account for the flow (Frey- 



Wyssling, 1947). A submicroscopic part 



of the strand gelates and contracts for a 



short time ; relaxation follows and an ad- 

 joining spot contracts, etc. When such 



waves of contraction move periodically 



along the protoplasmic strand in one 



direction, there is flow either in a perist- 

 altic manner by transverse contraction 



of the surface layers, or in pulling the 



highly viscous strand by longitudinal 



contraction (Fig. no). 



In the last case the flow is opposite to the direction of the moving 

 contraction and the relaxed part of the strand must be expanded by 

 another contraction centre situated at some distance. Since in the 

 same microscopic strand, flow may proceed simultaneously in oppo- 

 site directions, diff"erent waves of contraction with opposite polarity 

 must be admitted. Loewy (1949) stresses the fact that this system 

 necessitates a solid substratum (cell wall, ectoplasm) on which the 

 gelating centres of the flowing strand can be temporarily anchored 

 by some kind of junctions. 



In any case a contraction of submicroscopic elements can only 

 produce a microscopically visible eff"ect, if the system is temporarily 

 solidified by junctions. This is evident from Fig. in. To the left of 

 this figure linear submicroscopic particles contract individually; the 



Fig. 1 10. Movement of a protein 

 strand by a contraction wave. The 

 strand streams in the opposite 

 direction to the advancement of 

 the wave {a, b, c) (from Frey- 

 Wyssling, 1947). 



