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OSMOTIC PRESSURE. 5
if we find that A for a gram molecular solution of NaCl is 3.40° then we have
according to the above 1 : (l+[n — l]o) =1.86 : 3.40. In the dissociation of
NaCl two ions are formed, therefore n = 2, and from the above equation the degree
of dissociation can be calculated, a =0.83. The degree of dissociation can also
be calculated from the electrical conductivity. Only the ions take part in the con-
duction of electricity, and the molecular conductivity (
= —
i
i— —
—
:
:
—
}
* V molecular concent nit ion/
is proportional to the degree of dissociation. The dissociation increases with the
dilution and at infinite dilution all molecules are dissociated (a = l). If we desig-
nate with n*> the limit value which the molecular conductivity approaches in
infinite dilution and with y.v the molecular conductivity at some definite dilution
v, then the degree of dissociation at this dilution is a = —
.
The positively charged ions are called cations, and the negatively
charged ones anions. Common for all acids are the positively charged
H-ions while the negatively charged OH-ions are common for all bases.
Osmotic Experiments with Plant Cells. We often meet the
word osmosis in literature without understanding exactly what is meant
thereby. As a rule diffusion streams are meant, w-hich are modified
by means of the permeability conditions of an inclosing membrane.
We now know that the driving force, namely, the streaming, is brought
about by the differences in concentration, i.e., by difference in the osmotic
pressure on the two sides of the membrane.
After Nageli found that certain plant cells, when they were treated
with a sufficiently concentrated solution of certain substances, changed
their appearance so that the protoplasm retracted, 1
de Vries studied
this phenomenon further.2
He called it plasmolysis. The most important
substances for bringing about plasmolysis are the salts’ of the alkalies and
alkaline earths, varieties of sugars, polyatomic alcohols, and neutral amino-
acids. An indispensable condition for bringing about plasmolysis is that
the solution must not have any destructive action upon the cells. Xageli
gave the correct interpretation of plasmolysis, which is that those bodies
which plasmolyze plant cells pass through the cell membrane of the cell, but
not through the protoplasmic layer which follows. Instead of this the sub-
stance attracts water from the inner parts of the cell. The cell contents
surrounded by protoplasm therefore diminish in volume and the protoplasm
recedes more or less from the cell membrane. From this it follows that
only those solutions whose power of attracting water is greater than that
of the cell contents can bring about plasmolysis. As the ability to attract
water (or the osmotic pressure) increases with concentration, there must
be a limit solution for every substance above which all higher concentra-
tions plasmolyze. The limit solution is called isotonic with the cells;
1
Pflanzenphysiol. Untersuch., 1855.
2
Eine Analyse der Turgorkraft, Jahresber. f. Wissensch. Botanik, 14, 427 (1884).
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