20 PHYSIOLOGICAL KOLE OF MINERAL NUTRIENTS. 



Whether hypophosphites, phosphites, or hypophosphates can ever 

 be of physiological value is a question which must be determined by 

 further studies. Knop showed in an experiment with maize in 1881 

 that phosphoric acid can not be replaced by hypophosphoric acid. 

 The writer has observed that algse are at least not injured b}^ a 1 per 

 mille solution of sodium hypophosphite, phosphite, or pyrophosphate 

 and metaphosphate, and the two last-mentioned salts can even be well 

 utilized by mold fungi/* The assertion once made that phosphoric 

 acid in algae may be replaced by arsenic acid is absurd, and moreover 

 Molisch had shown it to be impossible. 



THE PHYSIOLOGICAL ROLE OF SILICA. 



Ritthausen, as well as Wolff (1880), arrived at the conclusion that 

 the role of silica in the leaves consists in causing the d}nng off of the 

 leaves during the ripening of the fruits, since from these dying- 

 leaves phosphoric acid and other mineral nutrients are transported to 

 the ripening seeds. Experiments carried on for fourteen years have 

 shown Wolff that plants yielded empty grains (taube Korner) when a 

 certain excess of phosphoric acid was not offered. But in the presence 

 of silica this excess was not necessary, since it could be drawn from the 

 leaves when dying off sooner under the influence of silica. Hence, 

 silica can be considered a means to economize the phosphoric acid in 

 the soil. 



In some barks ( Carpinus, Acer) so much silica b was found that a 

 certain value seems probable. Sometimes also the fibrous materials 

 are relatively very rich in silica; the ash of linen liber was found to 

 contain fully 28.2 per cent. 



In the case of animals silica seems to play a role in the formation of 

 hairs and feathers, since these contain an organic silicium compound 

 (Drechsel). Various organs of animals contain small amounts of 

 silica, but young animals contain in the same tissues more Si0 2 than 

 older ones (Schulz, 1902). The pancreas is especially rich in silica, 

 .12.3 per cent of its ash being Si0 2 (Faulhaber, 1899). Human hair 

 contains from 0.10 to 0.23 per cent Si0 2 (Kail). 



«Bot. Centralbl., 1895. 



& It has been asserted that the siliceous deposit in the bark of Fagus and Acer, 

 and in the leaves of various other plants, forms a protection against parasitic fungi. 

 (Forstlich-naturwissenschaftliche Zeitschrift, 1893, p. 224.) 



