SEPTEMBER 8, 1899. ] 
generally made that turgidity is a prereq- 
uisite for growth and regulates it, there 
are some strong reasons for thinking that 
the relation is rather the reverse, and that 
growth regulates turgor. 
Pathological changes may also be brought 
about by abnormally high osmotic pres- 
sure, a notable instance being furnished by 
cedema of various organs, especially leaves. 
In such a case, turgor seems to distend the 
cell walls extraordinarily, and to act as a 
stimulus on growth, causing a local hyper- 
trophy characterized by bladdery tissues. 
For interpreting all these processes, most 
fundamental for nutrition and growth, the 
new knowledge of solutions furnishes inval- 
uable aid. This theory, developed mainly 
within the last decade by the labors of Pfef- 
fer, Van t’ Hoff, Arrhenius, Ostwald, Raoult, 
and others, looks uponasubstancein solution 
in water as essentially a gas. Its molecules 
are freer to move than they are in the solid 
state because of their relations to the mole- 
cules of water. These, at the same time 
that they make mobility possible, obstruct 
the movements of the solute, so that the 
molecules of the latter are not nearly so 
free to move as the molecules of a gas. 
Thus enormous pressures are necessary to 
move the solute through the solvent or to 
remove its molecules from it. Many demon- 
strations establish firmly the fact that the 
molecules of solutes exhibit the well-known 
laws of gases. This general applicability of 
the fundamental laws of gases to solutes has 
made evident the proper basis of comparison 
between solutions of different compounds. 
For many years, and for some years after a 
proper knowledge of physical chemistry 
would have led to their abandonment as 
not comparable, physiologists were compar- 
ing the physiological action of percentage 
solutions or solutions of definite specific 
gravity, in ignorance that this was like 
comparing the action of one gas at atmos- 
pheric pressure with that of another at 10 
SCIENCE. 
319 
atmospheres pressure. Henceforth, we must 
deal with equi-molecular solutions if a com- 
parative knowledge of physiological action 
is sought. s 
A further study of the behavior of solu- 
tions has made us acquainted with the fact 
that when water solutions which conduct 
electricity, i. e., electrolytes, are of less than 
a certain concentration, the solute under- 
goes partial dissociation, no longer existing 
alone as a definite chemical compound. A 
certain amount, depending on the concen- 
tration of the solution, is broken up into 
electrically charged part molecules or ions, 
which behave osmotically as molecules and 
increase the osmotic pressure of the solute. 
Moreover these ions exert a very marked 
physiological effect upon the protoplasm. 
Certain ions are extremely injurious, in- 
hibiting the activity of the protoplasm and 
resulting in death. Poisons, so called, pro- 
duce a similar result. It is possible that 
by a study of ionic action we may obtain a 
more accurate idea of what actually hap- 
pens when living matter dies by ‘ poison.’ 
It would be surprising were there nota 
considerable diversity in the actual effects 
of various ‘ poisonous’ agents. 
Again, certain ions have a less marked 
physiological action, which is designated 
as stimulation, calling forth corresponding 
change in the activity of the protoplasm. 
Unquestionably many of the peculiarities of 
growth and development of an organism are 
responses to the action of ionic stimuli, but 
of these practically nothing is yet known. 
Certain human sensations have already 
been shown by Kahlenberg in his investi- 
gations on taste to be due solely to the ac- 
tion of definite H and OH ions. In no or- 
ganisms is there so good an opportunity as 
among plants to determine precisely how 
these factors, always acting in complex com- 
binations, effect the modifications of form 
and function that constitute adaptation to 
external conditions. 
