REAGENTS 21 



cohol. We have used a larger proportion of formalin, often as much 

 as 10 c.c. to 100 c.c. of 70 per cent alcohol. Results which seem 

 equally good have been secured by adding from 4 to 10 c.c. of formalin 

 to 100 c.c. of 50 per cent alcohol. Material in this fixing agent may be 

 left until needed for use. 



/) Formalin acetic alcohol. — This combination, for general anatomi- 

 ical work, might almost be called a universal fixing agent. About 5 c.c. 

 of glacial acetic acid and 5 c.c. of commercial formalin to 90 c.c. of 50 

 per cent or 70 per cent alcohol is generally satisfactory. If the proto- 

 plasm shrinks away from the cell wall, increase the proportion of acetic 

 acid to 7 per cent or even to 10 per cent. We should not recommend 

 more than 10 per cent of acetic acid in any fixing agent. 



When one is on a long trip, moving frequently from place to place, 

 with httle opportunity to make the numerous changes which are neces- 

 sary when using the chromic formulas, this is the best fixing agent we 

 have found. It will fix and preserve an amount of material equal to its 

 own weight, and the material may be left in the solution for months. 

 The reagent is good for almost any material, except the unicellular 

 and filamentous algae and fungi, which are more satisfactory in media 

 containing no alcohol. 



THE CHROMIC-ACID GROUP 



Chromic acid, or solutions with chromic acid as a foundation, are 

 the most generally useful killing and fixing agents yet known to the 

 botanist. A 1 per cent solution of chromic acid in water gives good 

 results, but it is better to use the chromic acid in connection with other 

 ingredients, such as acetic acid, formic acid, osmic acid, etc. Chromic 

 acid does not penetrate well, and this is one reason why it is seldom 

 used alone. Unfortunately it precipitates some liquid albuminoids in 

 the form of filaments and networks, which may be mistaken for struc- 

 tural elements. In botanical work, acetic acid is nearly always mixed 

 with chromic acid. The pickles of the dinner table show that acetic 

 acid is a good preservative, and that it causes little or no shrinking. It 

 penetrates rapidly, and is hkely to cause swelling rather than shrink- 

 ing, thus counteracting the tendency of chromic acid to cause plas- 

 molysis. The swelhng is as bad as shrinking. If the proportion of 

 acetic acid is too high, material may even break up; but 2 per cent, or 

 even 6 per cent, may be used to show the topography of an embryo sac 

 of an angiosperm, or the free nuclear stage of the endosperm of a gym- 



