50 



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 

 passes 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. 



