§ 1] UPON THE RATE OF GROWTH 321 



Its absence from the water in wliicli echinoid larvae are 

 developing produces dwarfs. In vertebrates, owing to the 

 need of this metal for the skeleton, large quantities of calcium 

 are taken in by the growing organism. In normal growth, 

 then, the food of animals and phanerogams must contain 

 calcium. 



• Strontium, although closely allied to calcium, is rather rarely 

 found in considerable amount in organisms. In certain plants, 

 as e.g. Fucus, it is constantly present. It can be stored up in 

 the animal organism when supplied abundantly in the food, but 

 in general is believed to be of little importance for growth. 



Manganese, likewise, is not of general importance, although 

 it is found abundantly in certain plants, e.g. Trapa natans, 

 Quercus robur, and Castanea vesca, and in the excretory organ 

 of the mollusc Pinna squamosa (Keukenbbeg, '78). 



Iron. — This most abundant of the heavy metals occurs so 

 frequently in organic substances, especially those related to the 

 compounds found in protoplasm, that it is little wonder that 

 iron has been found to be an essential ingredient of all proto- 

 plasm, hardly less important than oxygen itself. Although its 

 occurrence has been demonstrated, especially by Schneider 

 ('89), in all the large groups of animals, its amount in any 

 individual or organ is always very small. Iron oxide (FegOg) 

 forms between 1% and 2% of the ash of muscle, about 5% of 

 the ash of blood, and rarely rises to 5% in plants. 



It is found in the body, for the most part, as in yolk 

 (BuNGE, '85), in organic union. It occurs thus in the chro- 

 matin of the nucleus of all cells (^Macalltjm, '92, '94 ; 

 Schneider, '95). 



That chromatin contains iron has been demonstrated by Macallum 

 ('91) by means of a microohemical method whose general validity has never, 

 so far as I know, been questioned. It was shown by Bunge, in 1885, that 

 when tissue is put into ammonium sulphide the iron, even in an organic 

 molecule, is separated from its compound, and uniting with the sulphur 

 forms ferrous sulphide (FeS). This ferrous sulphide appears in the proto- 

 plasm as green granules (black in large quantity) ; and the fact that these 

 granules appear abundantly in the nucleus shows that iron is especially 

 abundant there. Additional evidence is given if after several weeks the 

 chromatin loses its stained appearance and becomes rusty. This result is 

 interpreted as due to the formation of ferric oxide, FCjOg; for when the 



