92 PHARMACEUTICAL BOTANY 
In these cells the walls adjacent to the fibers are thickened while 
the opposite walls bulge outwards. Each stegma possesses a mass 
of silica which usually fills the cell lumen. 
15. TANNins.—Tannins are amorphous substances occurring 
in plants having an astringent taste, and turning dark blue or 
green with iron salts. They occur in greatest quantity in the 
bark of exogens, and in gall formations. They are soluble in 
water, alcohol, glycerine, and a mixture of alcohol and ether. 
They are almost insoluble in absolute ether and chloroform. 
They give insoluble precipitates with organic bases such as 
alkaloids and with most of the salts of the heavy metals. 
According to their behavior with solution of iron chloride or 
other soluble iron salts, two kinds of tannic acid are recognized: 
(a) a form of tannic acid giving a bluish-black color, as that which 
is found in Rhus, Castanea, Granatum, Galla, etc.; (b) another 
tannic acid producing a dark green coloration, as that found in 
Krameria, Kino, Mangrove bark, Quercus, Catechu, etc. 
If sections are placed in a 7 per cent. solution of copper acetate for a week or 
more, then placed on a slide in 0.5 per cent. aqueous solution of ferric chloride, and 
after a while washed with water and mounted in glycerin, an insoluble brownish 
precipitate will be produced in those cells containing tannin. 
Braemer’s reagent (Sodium tungstate 1 Gm. and Sodium acetate 2 Gm. dis- 
solved in 10 cc. of water) is superior to solutions of iron salts in the detection of 
tannin, since some other plant constituents give a dark-green or bluish-black color 
with iron. It gives a yellowish-brown precipitate. 
16. ProTeins.—Proteins are complex nitrogenous substances 
forming the most important of the reserve foods of plants. ‘They 
are found in the cytoplasm, nucleus and plastids of all the living 
and many of the dead cells of plants, although most abundant in 
seeds. ‘They occur as crystalline or amorphous bodies or in 
colloidal solution. Protoplasm, itself, is composed largely 
of these substances. They all contain carbon, hydrogen, oxygen, 
nitrogen and sulfur, and many contain in addition phosphorus. 
They are formed by the addition of nitrogen, sulfur and fre- 
quently phosphorus to elements of grape sugar. The nitrogen, 
sulfur and phosphorus elements are obtained from nitrates, 
sulfates and phosphates which are dissolved in the water taken 
in through the roots. All living plant cells are capable of syn- 
thesizing proteins from carbohydrates. The process of protein 
