724 



PHYSIOLOGY. 



Fig. 1098. 



analogous process (diffusion of gases), that the cells on the surface 

 of plants are enabled to absorb and transmit gaseous matters. 



Endosmose may be explained as follows : — Whenever two 

 fluids of different densities are separated by a permeable mem- 

 brane which is capable of transmitting them, there will always 

 be a tendency to equilibrium of density between the two, from the 

 formation of a double current, — one passing from the thinner to 

 the denser liquid, and the other, from the denser to the thinner. 

 This endosmotic action may be easily observed, by filling abladder 

 with a colom-ed syrup, attaching to its open end a glass tube, 

 and then immersing it in a vessel containing water (Jig. 1098). 

 Under such circimistances, the volume 

 of the denser fluid in the interior of 

 the bladder becomes increased (as may 

 be at once seen by its rise in the tube), 

 by the passage through the membrane 

 of the thinner fluid ; whilst, at the same 

 time, a portion of the thicker liquid 

 passes out into the water or thinner 

 fluid, as maybe proved by the sweet taste 

 and colour which it gradually acquires. 

 This double current will continue as 

 long as there is any material difference 

 of density in the two liquids. The 

 stronger in-going current is termed en- 

 dosmose, and the weaker out-going cur- 

 rent, exosmose. If the position of the 

 liquids be reversed, the currents will be 

 reversed in like manner, the preponde- 

 rating current, in almost all cases, being 

 that which sets from the thinner to 

 the denser liquid. This rapidity, how- 

 ever, is modified to some extent by the 

 nature of the membrane ; for if two dif- 

 ferent liquids are separated by a mem- 

 brane, that will pass through most rea- 

 dily which is most freely imbibed by 

 the membrane. 



The absorption and transmission of 

 liquid matters through cells is now 

 very easy to explain, for as the fluid contents of the cells of 

 the roots of plants are denser than the water contained in 

 the media in which they grow, they will continually absorb 

 by endosmose (see Absorption)-, and as the changes which are 

 going on in the cells by assimilation and other processes on the 

 surface of plants tend to thicken their contained Uquids, there 

 will also be a constant passage of the absorbed fluids from cell 

 to cell towards those parts where such processes arc taking place. 



Fig. 1098. Apparatus to show 

 erulosmose and exosmose. It 

 consists of a bladder filled 

 with syrup, to the open end 

 of which a tube is attached, 

 and the whole placed iu a 

 vessel containing water. 



