ACTIVE TRANSPORT AND MEMBRANE EXPANSION-CONTRACTION CYCLES 635 



of cytoplasm of thickness d^. This has an area of i jd-^^. Similarly, when the 

 thickness increases to d^, the area decreases to i /V/3, and the area contracts 

 to dijd^ of its original area. Measurements of the ratio d-^^jd^ varied from 

 1/3 to I /id in different cells. The thickness do of cytoplasm flowing away 

 from the junction was also usually greater than that flowing in, and its 

 speed consequently less. Measurements of the ratio of the speeds were 

 approximately equal to the measured ratio of the thicknesses, and changed 

 from time to time in a given cell, and varied from cell to cell. 



The cause of this contraction seems to be as follows : the cytoplasm 

 moves fastest near the cell membrane and slowest near the vacuolar 

 membrane as particles on the outside of the stream can be seen continually 

 overtaking particles on the inside (see also [8]). A tangential force appears 

 to be acting on the cytoplasm in contact with the membrane. This force 

 also appears to be partly associated with the chloroplasts in this kind of 

 cell, for {a) if some of the chloroplasts are removed from their rows by 

 micromanipulation, streaming stops at the gap, and {h) detached chloro- 

 plasts can often be seen spinning in the cytoplasm at about 23 revolutions 

 per second — even in stationary cytoplasm squeezed out of the cell under 

 oil — indicating a tangential force between chloroplast and cytoplasm (see 

 also [9]). 



Chloroplasts are absent from the wall joining two cells, so that the cyto- 

 plasm would tend to accumulate here, being driven into an inactive region. 



Chloroplasts, however, cannot be the motive force, or the sole motive 

 force, since the rhizoids have no chloroplasts and the cytoplasmic move- 

 ment seems very similar, though the thickness of the moving cytoplasmic 

 layer is in the region of 1-2 microns compared with 10-70 microns in the 

 internodal cell. 



3. Rhizoids 



Several active regions in the rhizoid cells where membrane contraction 

 occurred were found, and these regions were, in addition to increase in 

 thickness, characterized by conspicuous wrinkling and vacuole formation. 



(a) rhizoid tip (fig. I, a) 



The thickness of the cytoplasm at the tip of the rhizoid cells is about 

 100 times greater than the thickness of the rapidly moving layer on the 

 rest of the cell. As the rapid thin stream feeds into this deep layer, it slows 

 greatly and masses of wrinkles appear at the point of entrv. 



(b) jun'ction between rhizoid cells (fig. I, c) 



The peculiar discontinuity in the shape of rhizoid cells where thev join 

 one another (rather like a hand-clasp) gives rise to a thick pool of more 

 slowly moving, wrinkling, cytoplasm. 



