236 BOTANICAL MICROTECHNIQUE. 



precipitates may be obtained, by means of a feebly alkaline 

 silver solution, in thin microtome sections. 



16. The Reactions of the Various Cell-constituents. 



422. In many investigations it is of interest to determine 

 what reaction the various constituents of the cell, especially 

 the cytoplasm and the cell-sap, have in the living cell. The 

 reactions of the sap pressed from large pieces of tissue 

 give very uncertain results in this respect, in view of the 

 extensive division of labor within the cell-organism ; and I 

 therefore refrain from discussing further observations made 

 in this way. 



423. So far as the cell-sap is concerned, its reaction may 

 be directly determined in those cases in which it contains 

 a coloring matter in solution which changes its color with 

 the reaction. Such a coloring matter is the so-called antho- 

 cyanin ( 184), which appears red when the reaction is acid, 

 blue when feebly alkaline, and green to yellow when strongly 

 alkaline. The cell-sap is generally alkaline or neutral in 

 blue parts of flowers, but must be acid in red parts. In the 

 same cell, in the course of its development, a change in reac- 

 tion may occur, as in the flowers of Ptihnonaria officinalis, 

 which are first red, and then blue. In the last-named plant, 

 as PfefTer has shown (V, 140), a blue color of the red parts 

 may be produced at any time by traces of ammonia. 



424. In cells with colorless cell-sap, one may reach conclu- 

 sions as to its reaction by the method, proposed by PfefTer, 

 of introducing into it an artificial coloring matter which 

 gives different colors, according to the reaction. According 

 to Pfeffer (II, 266), methyl orange is especially suited to this 

 purpose, as its orange-yellow color is not changed by dilute 

 alkalies. Pfeffer used in his experiments a .Olfo solution. 

 Pfeffer has (II, 259 and 267) also experimented with cyanin, 

 tropaeolin and corallin ; but these dyes proved less useful. 



425. It is, in most cases, more difficult to determine the 



