528 THE NATURAL HISTORY OF 



between their reaction and that of starch, which also fre- 

 quently occurs in similar small globules. But that the 

 red, oil-like, or at all events, fluid substance, should be 

 actually identical with the colourless, solid starch, which 

 always presents a definite structure, can scarcely be as- 

 serted. May there not, however, in the Vegetable king- 

 dom, be a coloured fluid, exhibiting the same reaction 

 with iodine, in all respects as starch ? If this be the case, 

 the infallibility of the blue reaction with iodine, as a cri- 

 terion of the presence of starch, would become question- 

 able ; and, particularly in the case of the unicellular Algse, 

 in which large-grained starch does not occur, would this 

 observation be of importance. Nageli had noticed the 

 blue colour assumed by this orange-coloured oil on the 

 addition of iodine, especially when it was collected into 

 large blue-green drops by means of alcohol.* 



Of great importance, moreover, is the relation of the 

 green and red colouring matters to each other. For, 

 notwithstanding their different chemical and physical 

 conditions, the one passes into the other, and vice versa. 

 The observations hitherto made on this subject, indicate 

 that the red colour which normally is always formed as 

 the cells become drier, particularly in moist air, depends 

 upon a less saturation of the cells with moisture ; is the 

 attribute in fact of a lower hydrate of chlorophyll. But 

 that a deficiency of water is not the sole cause of the 

 change of colour, is proved by longer observations of the 

 vegetation of Protococcus. Nor does light either appear 

 to be the exclusive cause of this phenomenon, which 

 remains still in considerable obscurity. 



With respect to the solid constituents of the Proto- 

 coccus cell-contents, they may be distinguished into 

 chlorophyll vesicles, colourless or green granules, the 

 above-mentioned amylaceous granules, and the nucleus. 



* The yellow oil, of which there is a considerable quantity, in the winter 

 spores of Folvox, (the so-called F. aurem, Ehr.,) is coloured blue-green by 

 iodine. ED. 



