§1] UPON THE RATE OF GROWTH 323 



results in medical practice, Bunge ('85) concluded that only- 

 organic iron compounds are assimilable. The studies of 

 KxJNKEL ('91 and '95) and Woltering ('95) have, however, 

 shown in the clearest manner that inorganic iron is assimilated, 

 stored in the liver, and made use of in the construction of such 

 organic compounds as the haemoglobin of the blood. At the 

 same time, as SociN ('91) and others have shown, iron may 

 be gained from organic compounds. Apparently, iron com- 

 pounds of any sort may be made use of by the organism. 



Magnesium. — This metal is closely associated with calcium, 

 the two usually occurring together in organisms just as they 

 do in the inorganic world. As the table on page 297 shows, 

 magnesium is of constant occurrence among organisms, although 

 never present in great quantity. The magnesium is gained by 

 green plants at the same time with the calcium from mineral 

 salts (chiefly magnesium carbonate and sulphate) derived from 

 disintegrating rocks. Fungi can also make direct use of the 

 salts of magnesium (excepting always the chloride) ; and ani- 

 mals, although no doubt chiefly gaining their magnesium from 

 plants or the waters in which they live, may make use of min- 

 eral salts, especially in the construction of the mineral parts of 

 various formed substances, such as those of bone. 



So constant an element is presumably necessary to the organ- 

 ism, and numerous observations make it quite certain that this 

 is true for green plants in general. Indeed, the fact that mag- 

 nesium occurs in chlorophyllan of chlorophyll makes it prob- 

 able that it functions in assimilation. Concerning its indis- 

 pensableness for fungi there can likewise be no doubt, since 

 Benecke's ('95, p. 519) experiments show it to be replaceable 

 neither by calcium, barium, or strontium. While some investi- 

 gators, like Hoppe-Seyler, believed it to have little signifi- 

 cance for the animal body, — -to be an accidental accompaniment 

 of calcium, — later study has shown that it is of importance 

 for regeneration of hydroids (Loeb, '92), and, according to 

 Herb^t ('97), a constituent of the sea water which is necessary 

 to the normal growth of various marine larvae. Also its occur- 

 rence, although slight, in milk, as well as its very abundant 

 occurrence iri seeds, indicate that it plays an important, if an 

 unknown, part in growth. 



