20 Methods in Plant Histology 



e) Formalin Alcohol. The most satisfactory of the alcohol 

 combinations is formalin alcohol. Various proportions are used by 

 different workers. Professor Lynds Jones, who first brought this 

 combination to my notice, added 2 c.c. of commercial formalin to 

 100 c.c. of 70 per cent alcohol. We have used a larger proportion of 

 formalin, often as much as 6 c.c. to 100 c.c. of 70 per cent alcohol. 

 Results which seem equally good have been secured by adding 4 to 

 6 c.c. of formalin to 100 c.c. of 50 per cent alcohol. 



This fixing agent is ideally convenient, for material is simply 

 placed in it and left until wanted for use, an advantage which can be 

 fully appreciated only by those who make extensive collecting 

 expeditions at great distances from the conveniences of the labora- 

 tory. For anatomical work, this combination is unsurpassed, and 

 even for delicate material, like liverworts and fern prothallia, the 

 results are satisfactory. The fixing is rapid, as is shown by the 

 abundance of mitotic figures. However, for a critical stud} 7 of 

 mitotic figures, other fixing agents are preferable. 



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 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 

 structural 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 likely to cause swelling 

 rather than shrinking, thus counteracting the tendency of chromic 

 acid to cause plasmolysis. The swelling is as bad as shrinking. 

 Solutions containing more than 1 per cent of acetic acid are to be 

 regarded with suspicion. However, if the purpose is to show the 

 topography of structures like the egg and synergids in the embryo- 



