OFFICE OF THE ASH CONSTITUENTS. 255 



merits, that it is required for the transfer of starch. Associating 

 this view with what is known regarding the office of potassium, 

 it is easy to see why potassic chloride should be so useful a 

 salt. 1 



681. Sulphur is absorbed by plants in the form of the soluble 

 sulphates. These are believed to undergo immediate decompo- 

 sition in the plant ; for example, calcic sulphate is decomposed 

 at once by oxalic acid, and calcic oxalate is formed. The sul- 

 phuric acid thus set free is reduced, the sulphur entering into 

 the constitution of the albuminoids' 2 (see 884). 



682. Sodium salts cannot wholly replace potassium salts in 

 the plant ; nevertheless, for a portion of the potassium needed 

 by the plant an equivalent amount of sodium can in some cases 

 be substituted. It has been found possible to cultivate success- 

 fully some maritime plants which normally contain a certain 

 amount of sodium salts, when potassium has replaced sodium 

 in the water furnished to the plant. 



683. Rarer constituents. Besides the ash constituents always 

 detected in plants, there are certain elements which are only 

 occasionally met with in greater or less amount, and these will 

 be next considered. 



684. Silicium. This element is so abundant in the ash of 

 many grasses, Equisetaceae, etc., that it almost claims a place 

 in the list of indispensable elements ; but experiments have 

 shown abundantly that in grasses at least, the proportion of it 

 present can be reduced to a very low point without materially 

 affecting the vigor of the plant or the strength of the culms. 

 Thus Sachs 3 showed, in 1862, that the amount of silicic acid in 

 the ash of Indian corn could be reduced from 18 per cent to .7 

 per cent, without injurious effect on the plant. 



685. Zinc has been detected in man}' plants grown on soil 

 containing it in considerable amounts ; for instance, that at 

 Altenberg 4 (near Aix). Frej'tag 5 found that all plants experi- 

 mented upon were able to absorb more or less zinc when it 



1 Bretfeld : Das Versuchswesen, 1884, p. 134. 



2 Holzner : Flora, 1867. An interesting paper by Hilgers (Pringsh. Jahrb., 

 vi., 1867, p. 285) gives an account of the formation of crystals of calcic oxalate 

 in various parts of plants, and presents certain speculations as to their origin. 



3 Flora, 1862, p. 53. Further experiments are recorded by Knop (Yer- 

 suchs-Stationen, iv., 1862, p. 176), Rantenberg and Kiihn (Yersuchs-Stationen, 

 vi., 1864, p. 359), Birner and Lucanus ( Yersuchs-Stationen, viii., 1866, p. 141). 



4 Sachs : Handbuuh der Experimental-physiologic, 1865, p. 153. 

 6 Chemisches Central-blatt, 1870, p. 517. 



