28 



Nageli ascribed the diflerences in the physiological capacities of 

 potassium and sodium salts to their different affinities for water. So- 

 dium salts bind the water of crystallization, but corresponding- potassium 

 salts do not, and thus being free from such a dense sphere of water 

 the latter are better qualified for katalytic work. The dense laj^er of 

 water around the molecules of sodium salts would not only prevent the 

 salt itself from coming into immediate contact with other molecules, 

 but it also would impede an effectual transmission of vibrations. On 

 this basis also Niigeli tries to explain the fact that the soil absorbs 

 potassium salts better than it does sodium salts, claiming that the lat- 

 ter are prevented by their water mantle from following the attracting 

 forces.' However, objections can be easily raised against this view, 

 the most serious one being that by no means has every sodium salt 

 the water of crystallization. 



THE PHYSIOLOGICAL 



ROLE OF CALCIUM AND MAGNESIUM 

 SALTS. 



DISTKIBUTION OF LIME AND MAGNESIA IN PLANTS. 



It has long been known that calcium and magnesium salts can not 

 physiologically replace each other, and the question as to the functions 

 of these salts has until recently been a matter of conjecture. The 

 striking regularity with which the leaves of plants show a relative 

 increase in lime, while the seeds show an increase in magnesia, has fur- 

 nished a clue to the mystery of the action of these salts. A number 

 of cases will serve to illustrate that different parts of the same plant 

 contain quite different proportions of lime and magnesia. Let us first 

 consider the leaves of the Gramineie, since in them the absence of cal- 

 cium oxalate excludes a misleading factor. The following data are 

 taken from tables in Liebig's work:^ 



Per cents of lime and magnesia in the ash of the {/rain and the stratv of Graminew. 



A better basis for a comparative estimate will be obtained if the aver- 

 age of these figures is taken and compared with the relative number of 

 molecules instead of the absolute weight. The seeds of Gramineai will 



1 Sitzungsber. d. Bayr. Akad. Wiss, 1879, p. 348. 



^Die Chemie in ihrer Anwendung auf Agrikiiltur und Pliysiologie, 7 ed., Part I. 

 The analyses were made by Way and Ogsten, Weber, and others. 



