48 MICRO-CHEMICAL REAGENTS. 



COLORING AGENTS. 



Lately various dyestuffs have been employed for 

 the differentiation of the tissue systems, as well as 

 for the recognition of some of the cell contents ; 

 of these only the most important will be mentioned. 



TINCTURE OF ALCANNA. 



A red dye, extracted by alcohol from the root of 

 Alcanna tinctoria. is employed for coloring resins, 

 which have a special avidity for this substance. 

 Protoplasm is also colored pale red by it. 1 Prepa- 

 rations colored by alcanna do not endure drying. 



COCHINEAL. 



An aqueous extract of cochineal is employed 

 with acetic acid or alum, for staining the prosen- 

 chyma (bast cells) of the phloem in fibro-vascular 

 bundles. After lying in the dye for some time, 

 these cells assume an intense red color, while the 

 other elements are not affected, or but slightly 

 colored. Yet there are certain kinds of wood 

 which imbibe the cochineal extract, but after-treat- 



1 N. J. C. Miiller : Untersuchungen liber die Vertheilung der Harze. 

 Jahrbiicher fiir wiss. Bot., V., p. 387. Hanstein: Organe der Harz- und 

 Schleimabsonderung. Bot. Zeitung, 1869, pp. 707, 708. 



