STRUCTURE.] ACTION OF IODINE. 127 



composed of primitive membrane, and in the intercellular 

 substance that unites the cells. 



In examining the horny albumen of Endogens, several 

 interesting remarks were made. The cells of which it is 

 composed are generally very thick-sided, perfectly colourless, 

 and are readily distended with water. When a slice of such 

 albumen, previously softened in water, is exposed to the 

 action of a concentrated tincture of iodine, the cellular mem- 

 branes presently acquire colour ; but it is not easy to describe 

 in what way the changes of colour take place, because at first 

 the iodine does not produce the colour which eventually 

 results from its prolonged action. In general it produces at 

 first a yellow, which, by the intense action of iodine, becomes 

 brown ; moreover this agent produces, in most cases, if it acts 

 long enough, a blue colour. This blue is, however, never of 

 the clear indigo tint that is observed, for instance, in the 

 shields of Lichens, but it is always reddish, and of all 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 ab- 

 sorbed ; 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 be communicated 

 to cellular 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 varia- 

 tions 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 organi- 

 sation 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 



