62 BOTANICAL MICROTECHNIQUE. 



An aqueous i$ solution of oxalic acid reacts in the same 

 way as a concentrated solution of ammonium oxalate. 



92. Calcium carbonate, as well as calcium oxalate, is 

 changed by sulphuric acid into gypsum. It is best in this 

 case to use a pretty strongly dilute acid. For instance, if 

 sections of the leaf of Ficus elastica are placed in \% acid r 

 large masses of gypsum needles are formed in the immediate 

 vicinity of the cystoliths, formerly incrusted with calcium 

 carbonate. 



Calcium carbonate is not changed at first by burning, but 

 is finally transformed into calcium oxide. 



c. Calcium Sulphate, CaSO 4 . 



93. Calcium sulphate has been recognized in many Des- 

 mids by A. Fischer (I), and occurs in them chiefly in the 

 form of tiny prisms and plates, which are sometimes en- 

 closed in sharply-defined vacuoles, as, for instance, in the 

 ends of the cells of Closterium sp. (cf. Fig. 22, a), or are dis- 

 tributed throughout those parts of the cell which contain 

 cell-sap (cf. Fig. 22, ^). For the microscopical recognition 

 of gypsum, A. Fischer uses the following reactions : 



Concentrated sulpJiuric acid leaves gypsum unchanged and 

 undissolved when cold ; barium chloride transforms it into 

 barium sulphate, which is insoluble in hydrochloric and nitric 



FIG, 22. a, the end of a cell of Closterium lunula, with gypsum crystals in the apical 

 vacuoles; b, median lobe of Micrasterias rotata. The gypsum crystals are all col- 

 ored black (X 675). After A. Fischer. 



acid ; burning\^^v^^ the gypsum crystals unchanged. They 

 are also insoluble in acetic acid, but dissolve slowly in cold 

 caustic-potash solution, hydrochloric or nitric acid, or at 

 once on heating. 



94. Hansen (I, 10) has observed in the leaves of various 



