CIRCULATION IN CELLS : ENDOSMOSE. 33 



of green grains, into revolution. The circulation is more active in 

 the subjacent than in the superficial layer of cells, although occasion- 

 ally conspicuous in the latter : it is stopped or retarded by lower- 

 ing, and accelerated by raising the temperature. The motion often 

 appeal's to be quite rapid ; but it should be remembered that this is 

 magnified as well as the object. Mohl states it to be very slow, not 

 more than the -^^jj of a line per second in the hairs of Tradescan- 

 tia. But in Vallisneria the gi'een grains sometimes complete the 

 circuit of a cell of the ordinary size in less than twenty seconds ; 

 and in the bristles on the fruit of Circfca, which are half a line 

 long, Mr. H. J. Clark has seen the revolution completed in about a 

 minute. The circulation in one cell is totally independent of that 

 in the adjacent ones. The current is commonly seen to flow in 

 opposite directions on the two sides of a partition, or to move on 

 one side when quiescent on the other. Cyclosis, whatever its 

 nature may be, evidently has nothing to do with the 



40. Transference of Fluid from Cell to Cell. All cells, at least when 



young and living, have perfectly closed walls. There is no passage 

 from one to another through visible openings or pores, although 

 such openings may be formed in older parts. Nevertheless fluids 

 do j)erraeate cell-walls, as they do all organic membranes. And in 

 this way water, along with other matters which the roots absorb, is 

 carried up into the leaves even of the topmost bough of a tree, 

 passing in its course through many milMons of apparently water- 

 tight partitions. However governed by forces inherent in the plant, 

 the actual transference of fluids from one cell to another takes place 

 in obedience to a jihysical law, i. e. by the process which has been 

 named Endosmose or Endosmosis* and which operates in dead parts 



* Endosmose and exosmose are names given by Dutrochet (a French physi- 

 ologist) to a physical process of pei-meation and interchange whicli takes 

 place in fluids, according to the following law, briefly stated. When two 

 liquids of unequal density are separated by a permeable membrane, the 

 lighter liquid or the weaker solution will flow into the denser or stronger, 

 with a force proportioned to the difference in density (endosmosis) ; but at the 

 same time, a smaller portion of the denser liquid will flow out into the weaker 

 (exosmosis). Thus, if the lower end of an open tube, closed with a thin mem- 

 brane, such as a piece of moistened bladder, be introduced into a vessel of pure 

 water, and a solution of sugar in water be poured into the tube, the water from 

 the vessel will shortly be found to pass into the tube, so that the column of 

 liquid it contains will increase in height to an extent proportionate to the 

 strength of the solution. At the same time, the water in the vessel will become 



