212 PART II. — VEGETABLE HISTOLOGY. 



Picric Acid. Besides its usefulness in staining certain structures, particu- 

 larly in combination with carmine or certain of the aniline dyes, it is of value 

 in fixing the protoplasmic contents of cells, and preparing them for study. For 

 ordinary tissues a saturated watery solution is to be preferred. In this the fresh 

 tissues, first cut into pieces of small size, so that the liquid will readily pene- 

 trate them, are soaked for twenty- four hours. They may then be thoroughly 

 washed, stained and examined. But if not immediately required for use they 

 should first be well washed in clean water, then transferred to 50 per cent, 

 alcohol, then, after a few hours, to stronger alcohol, and, finally, to 98 percent, 

 alcohol, in which they may be kept until required for use. 



A saturated solution of picric acid in a mixture of equal parts of alcohol 

 and water is a very serviceable hardening and fixing agent, particularly for the 

 study of algae. 



Carbolic Acid, when dissolved in water or in alcohol, is of value as a 

 clearing agent, often producing excellent results, particularly in the study of 

 pollen grains. 



Chloral Hydrate, in watery solution, is also a good clearing agent for the 

 study of pollen grains, and when used in the proportion of three parts of the 

 chloral to two of water it is excellent for removing the coloring matter from 

 chlorophyll-bodies. If afterwards a little iodine solution be added to the pre- 

 paration the starch grains enclosed by the chlorophyll-bodies will be stained 

 blue and rendered distinctly visible. 



Diphenylamine. This is used as a reagent for the detection of nitrates or 

 nitrites in tissues. A solution is prepared by dissolving one part of diphenyla- 

 mine in 360 parts of pure sulphuric acid. If a drop of this be applied to a 

 section containing either nitrates or nitrites a deep blue coloration, due to the 

 formation of aniline blue, will be produced in the parts of the section which 

 contain these salts. 



STAINING FLUIDS. 



The staining fluids of most value to the student are the following : 

 Grenadier's Alum Carmine. Prepare a two per cent, solution of 

 ammonia alum in distilled water, and in this boil a little powdered carmine for 

 about twenty minutes, so as to produce a deep red solution. After the solution 

 has cooled filter it and add a trace of carbolic acid to preserve' it. It stains 

 cellulose a fine red color, but less readily strongly lignified tissues and suberized 

 tissues not at all. When allowed to act for some time, say from twelve to 

 twenty-four hours, upon cells containing protoplasm, the latter is stained, and 

 the nucleus more strongly than the rest. The stain works best upon tissues 

 which have lain for some time in alcohol and then have been washed with 

 water. By a judicious use of the ^ per cent, solution of hydrochloric acid in 

 70 per cent, alcohol the stain may be removed from the cell walls, leaving only 

 that in the protoplasm and nucleus. 



Ammonia Carmine. Carmine is dissolved in strong ammonia water until 

 the latter is saturated. It is then evaporated over a water-bath to dryness and 

 the solid carminate of ammonia thus obtained is dissolved in distilled water in 



