Special Functions and Relations 183 



species of the scouring rush (Equisetum) has an ash con- 

 tent of Si0 2 , amounting to from 70 to 80 per cent. The ac- 

 cumulation is chiefly in the cell-wall, where it is doubtless 

 important in support and protection. Silicon is regarded 

 as an inessential element because development proceeds 

 in its absence ; but in the complex relations of plants in 

 the field it may determine the capacity of a plant to exist 

 in a particular habitat. Wolff regarded silicon as impor- 

 tant in furthering the migration of phosphoric acid com- 

 pounds from maturing leaves and stems to the forming 

 seeds. 



REFERENCES 



BREAZEALE, J. F. The Relation of Sodium to Potassium in 



Solution Cultures. Journ. Amer. Chem. Soc. 28 : 1013- 



1025, 1906. 

 DELEANO, N. T. Etude sur le role et la fonction des sels mine- 



raux. 48 pp., 1907. 

 LOEW, O. Ueber die physiologischen Funetionen der Calcium 



und Magnesiumsalze im Pflanzenorganismus. Flora. 76 : 



368-394, 1892. 

 The Physiological Role of Mineral Nutrients. Bur. Plant 



Ind., U. S. Dept. Agl. Bui. 46 : 70 pp., 1903. 

 REED, H. S. The Value of Certain Nutritive Elements to the 



Plant Cell. Ann. Bot. 21 : 501-543, 1907. 

 RAUMER, v. Calcium und Magnesium in der Pflanze. Landw. 



Versuchsst. 19 : 253-280, 1883. 

 WOODS, A. F The Relation of Nutrition to the Health of 



Plants. Yearbook U. S. Dept. Agl. (1901) : 155-176, 7 pis. 



Texts. Jost, Pfeffer. 



