Compounds in Chloroplasts oj Green Cells of Plants. 563 



certain Confervae and Cladophora) deposit around them a layer of yellow to 

 rust-red colour consisting of mixed ferrous and ferric oxides ; this is often 

 actively secreted from waters containing only traces of iron. 



A considerable number of lichens also secrete incrustations of the mixed 

 oxides of iron to such an extent as to change their appearance to an iron- 

 oxide or ochre colour, so that they have been termed by systematic botanists 

 " formae oxydatae, ochraceae " or " iron lichens." The iron-oxide forms a fine 

 incrustation usually on the mycelium of the fungus. No association of this 

 iron-oxide with a photo-synthetic function has ever been suggested, but in 

 view of our present knowledge of the photo-synthetic activity of iron salts 

 some investigation in this direction is highly desirable. It is an interesting 

 observation of Molisch, from our point of view, that these " iron lichens " 

 flourish exclusively on the oldest primitive rock-formations (" Urgestein "). 

 They are never found upon chalk formations, but grow upon granite, gneiss, 

 syenite, and porphyry. Molisch was unable to find inorganic iron in the 

 other lichens, but this doubtless arose from the less delicate methods he had 

 at his command at that time, and from the fact that the fatty bodies con- 

 tained in the green cells of the alga of the lichen had not been removed. 

 When the lichen is extracted with alcohol and Macallum's hematoxylin test 

 then applied, the algal cells rapidly stain a deep blue-black, showing the 

 presence of inorganic iron, while the hyphae of the fungus only take on a 

 brownish tinge during the same time, and only give a faint positive reaction 

 at the end of some days or weeks. 



It is somewhat remarkable that the presence of iron in the chloroplast 

 should for so long have escaped discovery. The explanation probably lies in 

 the fact that little attention has been given to the application to the green 

 cell of the histo-chemical tests for iron since the discovery by Macallum of 

 the more delicate haematoxylin iron test, as also to the delicacy of the 

 chloroplasts to the more drastic earlier method used by Molisch, and to these 

 factors may be added the difficulty with which some of the chemical reagents 

 for iron penetrate the green cell, and the presence in the chloroplast itself of 

 fatty and lipoidal substances which prevent the ingress of the water-soluble 

 stains. 



Macallum* in 1894 before his discovery of unmordaunted haematoxylin as 

 a reagent for iron, and using then ammonium sulphide in glycerine as a 

 reagent, states that bacteria gave no evidence, of an organic iron compound, 

 but in the Cyanophyceae the chromophilous portions of the " central 

 substance " contain iron, and iron may be also demonstrated in the peripheral 

 granules containing the so-called cyanophycin. At this period, Macallum 

 * 1 Roy. Soc. Proc.,' vol. 57, p. 261 (1894). 



