138 The Structure of Protoplasm 



rather than toward one of the sides. In each of the experiments the 

 centrifuging was started 1 minute after the pressure was estabUshed 

 in the experimental section of the bomb, and a uniform force, 810 X 

 gravity, was used. In view of the fact that protoplasmic streaming, 

 and consequently a redistribution of the sedimented chloroplasts, 

 would begin very shortly after the centrifuging was stopped, it was 

 necessary to fix the material in order to obtain an accurate measure- 

 ment of the degree of sedimentation. Heat fixation, obtained by 

 placing the bomb in boiling water for 45 seconds, gave excellent 

 results. This method, which avoids chemical contamination of the 

 chamber, was used in all the experiments. 

 (3) The plasmagel system in plant cells 



Although it has not been usual to speak of a "plasmasol" and 

 "plasmagel" with reference to the protoplasm of plant cells, these 

 terms do not seem inapt, at least in plant cells which display proto- 

 plasmic streaming. A close observation of a cell of Elodea during 

 active cyclosis shows that, except under special circumstances,^' 

 only a relatively small portion of the protoplasm is involved in the 

 actual stream. Usually the flowing part is restricted to a fairly nar- 

 row channel which follows the side and end walls completely around 

 the cell. The protoplasm which lies above and below this channel, 

 i. e., along the upper and lower margins of the walls, and over the 

 "roof" and "floor" of the cell, does not flow. In these parts of the 

 protoplasm, the chloroplasts, even those which border directly upon 

 the flowing part, may maintain a fixed position for many minutes. 

 Thus the nonflowing portion of the protoplasm resembles the plas- 

 magel of an amoeboid cell, whereas the actively streaming part may 

 be regarded as plasmasol. 



The gelation percentages, which in Figure 2 have been plotted 

 as a function of pressure, undoubtedly deal with the properties of the 

 plasmagel, rather than of the plasmasol portions of the protoplasm. 

 As in the case of the Amoeba, the present measurements are based 

 upon the assumption that the relative degree of gelation is in propor- 

 tion to the minimum time of centrifugation^^ required for the 

 formation of a standard zoning at each of the indicated pressures. 



C. CELL DIVISION 



The effects of pressure upon cell division as it occurs in the eggs 

 of various marine animals have now been reported by several 

 workers (Brown, '34c; Marsland, '36, '38, and '39a; Pease and Mars- 

 land, '39; and Pease, '40a), and there is general agreement that 



