34 MOVEMENTS OF PROTOPLASM IN CELL-CAVITIES. 



in the vacuoles, and none of the granules ever pass over into the cell-sap from the 

 protoplasm. 



Larger bodies, such as the round grains of green colouring-matter or chlorophyll, 

 are in many instances not carried forward, but remain stationary, the protoplasmic 

 stream gliding over them without altering them in any way. Further, the outer- 

 most layer of the protoplast, contiguous with the cell-membrane, is not in visible 

 motion in most vegetable cells. On the other hand, occasionally the entire pro- 

 toplast undoubtedly acquires a movement of rotation, and then the larger bodies 

 imbedded in its substance, i.e. chlorophyll corpuscles, are driven along like drift- 

 wood in a mountain torrent (fig. 5^ and 5^). On these occasions a wonderful 

 circulation and undulation of the entire mass takes place: chlorophyll grains are 

 whirled along one after the other at varying speeds as if trying to overtake one 

 another; and yet another structure, the cell-nucleus presently to be discussed, is 

 dragged along, being unable to withstand the pressure, and, following the various 

 displacements of the net-work of protoplasmic strands in which it is involved, is at 

 one moment pulled alongside of the cell-wall, at another again is taken in tow by a 

 rope of central protoplasm and hauled transversely across the interior of the cell 

 (fig. 5^). 



When the rate of the current itself is estimated by the pace at which the gran- 

 ules are driven along, results which vary considerably are obtained, depending chiefly 

 on a qualitative difference in the protoplasm, but secondarily also on temperature and 

 other external conditions. A rise in temperature up to a certain point as a general 

 rule accelerates the rate of the stream. Particles of protoplasm in particularly 

 rapid motion pass over 10 m.m. in a minute; others in the same time traverse from 

 1 to 2 m.m.; and some, in still less haste, advance only about a hundredth part 

 of a millimeter. Larger bodies, especially the bigger chlorophyll grains, move 

 slowest of all. So it is often hours before chlorophyll grains lying near one side of 

 a cell are pushed through the protoplasm over to the other side, a distance only 

 equal to a small fraction of a millimeter. 



The minute granules, as well as the larger grains of chlorophyll and the cell- 

 nucleus, are entirely surrounded by protoplasm ; and the protoplasm, whether in the 

 form of bands or threads, whether a peripheral lining or an indefinite mass, must 

 be conceived as always composed of two layers, the outer "ectoplasm" being tougher 

 and denser than the inner "endoplasm," which is softer and somewhat fluid. The 

 former is homogeneous and non-granular, so that it is the more transparent and 

 has the effect of a skin clothing the inner, softer layer, which is granular and 

 turbid. It would be incorrect, however, to think of this as a very strongly-marked 

 contrast, sufficient to mark off* one layer clearly from the other. In reality there 

 are no such sharp boundaries, and the tougher ectoplasm passes gradually into the 

 softer and more mobile endoplasm. Of course the granules and corpuscles which 

 one sees drifting in streaming protoplasm are situated within the more yielding 

 endoplasm. It is true, minute particles often appear to glide from one side to the 

 other upon a delicate protoplasmic strand as if it were a tight-rope; but on closer 



