782 ENDOSMOSE.—EXOSMOSE.— TURGIDITY OF CELLS. 
be easily observed, by filling a bladder with coloured syrup, 
attaching to its open end a glass tube, and then immersing it in 
Fig. 1140. 
Fig. 1140. Appa- 
ratus for illus- 
trating the ef- 
fect of stretch- 
ing or com- 
pressing a tur- 
gidcell. (After 
Sachs. ) 
a vessel containing water (jig. 1139). Under such 
circumstances the volume of the denser fluid in 
the interior of the bladder becomes increased (as 
will at once be seen by its rise in the tube), by 
the more rapid passage through the membrane of 
the thinner fluid than of the thicker, though at 
the same time a less portion of the syrup passes 
out into the water or thinner fluid, as may be 
proved by the sweet taste and colour which the 
latter gradually acquires. This double current 
will continue so long as there is any material 
difference of density between the two liquids. 
The stronger in-going current is termed endos- 
mose, and the weaker outgoing current exosmose. 
If the position of the liquids be reversed, the 
currents will be reversed in like manner, the 
preponderating current, in almost all cases, being 
that which sets from the thinner to the denser 
liquid. 
The pressure exerted by the water absorbed 
by endosmose against the walls of a cell, is spoken 
of as turgidity or turgescence. In such a cell the 
pressure exercised against the walls reacts upon 
the cell contents. If a cell is turgid, but capable 
of further extension without bursting, the changes 
which would be produced by stretching or com- 
pressing it, or otherwise altering its form, may be 
easily shown by the use of such a piece of appa- 
ratus as that represented in fig. 1140, where K isa 
wide and thick india-rubber tube, to which the 
glass rod s g acts as a stopper at one end, while 
into the other end is fitted a glass tube drawn 
out, Ro. The tube is now filled with water, 
the upper level of which is at n. It will be found 
that on stretching the tube its calibre is dimin- 
ished, but its capacity is increased, as shown by 
the fall of the water below n; while compression, 
bending, or creasing will diminish the calibre, and 
thus raise the level above n. 
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 con- 
tinually absorb the latter by endosmose ; and as the changes 
which are going on in the cells by evaporation, assimilation, 
and other processes on the surface of plants, tend to thicken 
. 
