28 
METHODS IN PLANT HISTOLOGY 
i ) Merkel’s Fluid.— 
Equal volumes of a 1.4 per cent solution of chromic acid and a 
1.4 per cent solution of platinic chloride. This is also an expensive 
reagent. It is recommended for mitotic phenomena, but does not 
seem to equal Flemming’s solution. 
j) Hermann’s Fluid.— 
1 per cent platinic chloride. 15 parts 
Glacial acetic acid. 1 part 
2 per cent osmic acid. 4 or 2 parts 
This is the most expensive fixing agent yet discovered, and for 
botanical purposes it does not seem to be any better than the cheaper 
chromic mixtures. It is mentioned here with chromic mixtures 
because it originated as a variation of Flemming’s fluid, the platinic 
chloride being substituted for the chromic acid. Recently, it has 
been resurrected and highly recommended for the structure of the 
chromosome. Personally, I do not believe it is equal to Flemming’s 
weaker solution; and, even in this weaker solution, the percentage 
of osmic acid may be too high. The chromo-acetic-osmic solution 
given under (< d ) fixes chromatin very well and, with 2 c.c. of osmic 
acid, seems to be equal to any of the modifications of Flemming’s 
solutions. 
PICRIC ACID 
Use a saturated solution in water or 70 per cent alcohol. One 
gram of picric acid crystals will saturate about 75 c.c. of water or 
alcohol. This reagent penetrates well and does not make the material 
brittle. It is to be recommended when difficulty is anticipated in 
the cutting. If used cold, the time varies from 1 to 24 hours, depend¬ 
ing upon the character of the tissue and size of the specimen. If 
used hot (85° C.), 5 or 10 minutes will be sufficient. Material 
should be washed in 70 or 50 per cent alcohol. Water is injurious, 
and some even go so far as to avoid aqueous stains, unless the ma¬ 
terial has been thoroughly washed. The washing should be continued 
until the material appears whitish and the alcohol no longer becomes 
tinged with yellow. Picro-carmine gives its best results after this 
reagent. Picric acid can be combined with various other fixing agents, 
and so we have picro-sulphuric acid, picro-nitric acid, picro-chromic 
acid, picro-chromic-sulphuric acid, picro-osmic acid, picro-alcohol, 
and picro-corrosive sublimate. The picric acid in all mixtures should 
be rather strong. 
