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tints from vinous to violet, so that in fact it presents all the 
tints observable in vapour of iodine of different degrees of 
density. 
From these and a great many more observations, Professor 
Mohl draws the following conclusions. 
1. Iodine causes the cellular membrane of plants to 
assume different colours, according to the quantity of it that 
is absorbed; a small quantity produces a yellow or brown 
tint, a larger quantity forms violet, and a still larger amount 
of it causes the production of blue. Iodine may commu- 
nicate to cellular membrane, when dry, a yellow or brown 
tint, provided it, the iodine, is dissolved in alcohol, or comes 
in contact with the membrane in the form of vapour; but 
the violet or blue colours are only formed when the membrane 
is saturated with water. - Blue changes to violet or red as 
the membrane dries, and returns when it is again moistened. 
Similar variations of colour are obtained with common starch, 
according as it is dry or moist. 
2. The colour that the membrane of cells assumes, under 
the action of iodine, is not dependent merely upon the quan- 
tity of iodine employed, but is also connected with the orga- 
nization of the membrane itself. Membranes which are softest, 
and most tenacious, distending most in water, assume, even 
when acted upon by only a small quantity of iodine, either a 
violet or blue tint immediately, or at first a yellow hue, which 
ses afterwards into violet or blue, even before the evapora- 
tion of the liquid. Membranes that are harder, more brittle, 
and less distensible in water, on the contrary, take on, under 
the action of iodine, a yellow or brown colour, and do not 
shew a trace of blue, after being dried and again moistened, 
unless a great quantity of iodine has acted upon them. 
3. The development of a blue colour is an attribute of 
the cellular membrane itself, and may be caused by the ab- 
sorption of a sufficient quantity of iodine. 
_ _ It is to chemists, says Professor Mohl, to say whether 
iodine colours cellular membrane by merely interposing itself 
between the particles of that membrane, or whether iodine 
and woody fibre form determinate chemical combinations, of 
which one is yellow and the other blue. Be this as it may, 
we cannot but regard these observations of the highest interest 
to all engaged in the study of Vegetable Physiology. 
