54 THE VEGETABLE CELL IN GENERAL. 



crystals for examination, their chemical composition has not yet 

 been determined with certainty in all cases. That a protoplas- 

 mic film usually envelops both solitary and aggregated crystals, 

 can be shown by the method pointed out by Payen ; ' namely, 

 by dissolving the crystal slowly in very dilute nitric acid, and 

 testing with iodine, when the film will become yellowish-brown. 

 It has also been made out be}'ond question that some crystals 

 have a considerable admixture of cellulosic matter, and that a 

 few others are covered b}' a membrane of cellulose. 2 But these 

 two substances do not obscure the chemical reactions in ordinary 

 cases, by which it has been shown that the larger number of crys- 

 tals consist of calcic oxalate, after which, in frequency of occur- 

 rence, comes the carbonate of the same metal. These two salts 

 can be easily distinguished from each other by the following 

 simple tests : 



Since these two salts may occur in the same specimen, it is best 

 to use acetic acid first ; by this agent all traces of the carbonate 

 are removed, and hydrochloric acid can then be applied in order 

 to detect the presence of oxalates. Sanio 8 and Holzner have 

 shown conclusively that man}' crystals which have been supposed 

 to be calcic carbonate consist merely of the oxalate. 



Crystals of calcic sulphate have been reported as occurring 

 in certain Musaceae, 4 in the bark of the willow, in the roots of 

 aconite, bryony, and rhubarb ; and also in the root of a young 

 bean. 8 Calcic phosphate is said to have been detected in the 



1 Payen : Mem. des savants Grangers, ix., 1846, p. 91. 



2 Rosanoff (Bot. Zeit., 1865, 1867), Crystals in pith of Ricinus and Kerria. 

 Pfitzer (Flora, 1872), crystals in the leaves of orange and the bark of many 

 trees. 



Hilgers has investigated the occurrence of crystals at different periods of 

 growth of different organs. From his results it appears, (1) that in the very 

 youngest parts no crystals are to be fonnd ; (2) they appear, however, very 

 early in most parts, and (3) speedily attain their maximum size, after which 

 they undergo no change (Pringsheim's Jahrb., vi., 1867, p. 285). 



Sanio : Monatsber. Berliner Akad., 1857. 



4 Van Tieghem : Trait de Botnnique, p. 526. 



8 Sitzungsberichte der Wiener Akad., xxxvii., 1859, p. 106. 



