GELL-COXTENTS AND FORMS OF CELLS. 205 



which colors the cell-sap blue, afterward precipitating the tannin. 

 This reagent has the advantage that when used in very dilute 

 solution (i part methylene blue to 500,000 of water) it does not 

 injure the protoplasm of the living cells, so that the cut end o<f a 

 twig may be placed in the solution for I to 24 hours and sections 

 examined from time to time. Another reagent that is very 

 satisfactory in the examination of living material is a solution of 

 ammonium carbonate, which causes a precipitation of the tannin 

 in the cells in the form of very small globules or rods. This solu- 

 tion may be used either directly upon sections or by placing freshly 

 cut stems in dilute solutions ( I part ammonium carbonate and 

 200 parts water). Ammonium carbonate does not precipitate 

 gallic acid and therefore may be advantageously used in the study 

 of the development of tannin and related substances, as in galls. 

 The following reagents also give distinct reactions for tannin. 

 Copper acetate in concentrated aqueous solutions is one of the 

 very best reagents for the localization of tannin cells. It is em- 

 ployed by allowing the leaves or twigs to remain in the solution 

 for some days, when the tannin forms a reddish-brown precipitate 

 in the cells. Ferric chloride and ferric acetate also precipitate 

 tannin. Moeller has suggested the use of a solution of iron 

 chloride in anhydrous ether, the cut pieces of the stems and leaves 

 being placed directly in this reagent. Potassium bichromate and 

 chromic acid in dilute solutions give yellowish-brown or blackish- 

 brown precipitate with tannin. 



DISTRIBUTION OF TANNIN. There are very few plants in 

 which tannin does not occur in some of the parts or at least in cer- 

 tain cells during some period in their development. This is fre- 

 quently noted in making sections of plant material with a razor ; the 

 liberated cell-sap is colored a dark blue. It is found in the form of 

 highly refracting globules in the Zygnemaceas and other Algae. It 

 occurs in relatively large amounts in some of the ferns, and, with 

 the exception of the Monocotyledons, is widely distributed in the 

 Spermophytes. As tannin is widely used in the making of leather 

 and as a mordant in dyeing, etc., it is extracted from various 

 plants and is an article of commerce. The following are some 

 of the important tannin-yielding plants: The bark of hemlock 

 (Tsuga canadcnsis, Fam. Pinaceas) yields nearly 14 per cent, of 



