56 PHYSIOLOGICAL ROLE OF MINERAL NUTRIENTS. 



In studying the questions as to what the nourishing function of mag- 

 nesium salts is and why they can not be replaced physiologically by 

 calcium salts, the probable answer is found in the well-known property 

 of the magnesium salts to undergo dissociation easily, as the writer 

 pointed out some y ears ago. a Magnesium salts are easily hy droryzed , as 

 is shown in the preparation of chloridor carbonate of magnesium b} 7 the 

 ordinary method, whereby a part of the acid is easily liberated. More- 

 over, in boiling with water the secondary magnesium phosphate is 

 decomposed into tertiary phosphate and free phosphoric acid. The 

 inference suggests itself that this easy dissociation is of great value 

 to the cells, since in the assimilation of nitrogen from nitrates, sulphur 

 from sulphates, and phosphoric acid from phosphates the dissociation 

 of these salts would immediately precede assimilation; hence the easier 

 these acids are separated from the base the easier their assimilation will 

 be accomplished. However, this deduction relates more to the assimi- 

 lation of phosphoric acid than to that of sulphur and nitrogen. This 

 latter assimilation must be possible also, to a certain degree, from 

 other sulphates and nitrates besides those of magnesia. 5 According to 

 this view the formation of nucleoproteids depends upon the presence 

 of magnesium salts. As a matter of fact, it is found that magnesia 

 alwa} T s increases where rapid development is taking place. In accord- 

 ance with this view also, small quantities of magnesium salts can be 

 used for a great deal of work, since the same amount of base can serve 

 over and over again as the vehicle for assimilation of phosphoric acid. 

 This may also explain the fact, pointed out long- ago by Adolph Mayer, 

 that "magnesia is more movable in the plant than lime is," and that 

 " magnesia, like the phosphates, follows the proteids." 



The fact that comparative!} 7 little magnesia can serve for extended 

 plrysiological operations may be noticed in fungi in culture solutions 

 devoid of lime, and also when seeds are left to develop in culture solu- 

 tions free from magnesia and with only a moderate amount of lime in 

 proportion to phosphoric acid. For example, beans may reach even 1 

 meter in height in such solutions, the reserve magnesia sufficing for 

 this result. 



Besides the easy dissociation the solubility of the secondary magne- 

 sium phosphate in water is also of value. This solubility is much 

 greater than that of the secondary calcium phosphate. When 100 cc. 

 of a 0.2 per cent solution of disodium phosphate are mixed with 2 to 

 3 cc. of a 10 per cent solution of magnesium nitrate at the ordinary 

 temperature no precipitate is formed, while with an equivalent amount 

 of calcium nitrate there will be a considerable precipitate. It may be 

 inferred, therefore, that the secondary magnesium phosphate is more 



a Flora, 1892, p. 286. 



h It is important to remember that nitrates and sulphates are reduced in their 

 assimilation while phosphates are not. 



