322 



EFFECT OF CHEMICAL AGENTS 



[Ch. XI 



nuclei are subjected to hydrochloric acid and potassium ferrioyanide they 

 immediately assume a deep azure-blue color, clearly due to the formation 

 ■of "Prussian blue" or Fe4(Fe €(,^5)3. This series of reactions can only be 

 •explained on the ground that there is iron in the nucleus. 



Iron has shown itself to be essential to the growth of all 

 organisms. Its importance for growth is indicated by its rela- 

 tive abundance in the yolk of birds' eggs, and by the fact of 

 its occurrence in larger proportion in a mammal just born than 



NoFe 



Fig. 89. — Kchinus larvae from parallel cultures, all Bj days old. a, reared in a solu- 

 tion containing all salts, including iron as FeCla ; 6 and c, from solutions contain- 

 ing all salts excepting iron. (From Herest, '97.) 



in later stages. When excluded from a nutritive solution 

 which is otherwise complete, growth is imperfect (Fig. 89). 

 This may be associated with the facts that in plants the chloro- 

 phyll granules are not developed in its absence, that in the 

 higher animals hsemoglobin cannot be formed, and that the 

 chromatic substance of all cells requires it. 



The question in what form iron is absorbed b}'' the organism 

 has been the subject of an extensive discussion. Although 

 ■doses of metallic iron have long been used with favorable 



