114 METHODS IN PLANT HISTOLOGY 



in a botany can and brought into the laboratory. Before fixing, it 

 should be placed in water for half an hour. Such forms suffer more from 

 lack of air when placed in a bottle or a can than from lack of water 

 when wrapped in wet newspaper. Branches with developing buds may 

 be brought in and kept in water. Cones of the cycad, Ceratozamia, sent 

 from Jalapa, Mexico, have arrived in Chicago with cell division still 

 going on at a rapid rate. But such cases are extremes; as a rule, take 

 the killing and fixing fluids into the field. 



Always have the material in very small pieces, in order that the 

 reagents may act quickly on all parts of the specimens. Pieces larger 

 than cubes of 1 cm. should be avoided whenever possible. While one 

 sometimes needs sections 2 or even 3 cm. long, it is not hkely to be 

 necessary to fix pieces more than 4 or 5 mm. in thickness. For very 

 fine work no part of the specimens should require the reagent to pene- 

 trate more than 1 or 2 mm. 



For fixing agents of the chromic-acid series, the volume of the re- 

 agent should be about 50 times that of the material. 



Fixing agents with alcohol as an ingredient will fix a larger propor- 

 tion of material. It must be remembered that the water, which is al- 

 ways present in living tissues, weakens the fixing agent. 



The time required for fixing varies with the reagent, the character 

 of the tissue, and the size of the piece. About 24 hours is a commonly 

 recommended period for chromic-acid solutions, but 2 or even 3 days 



will do no harm. 



Directions for making and using the various fixing agents are given 

 in the chapters on "Reagents" (chaps, ii, xxxi). 



WASHING 



Nearly all fixing agents, except the alcohols, must be washed out 

 from the material as completely as possible before any further steps 

 are taken, because some reagents leave annoying precipitates which 

 must be removed, and others interfere with subsequent processes. 

 Aqueous fixing agents with chromic acid as their principal ingredient 

 are washed out with water; aqueous solutions of corrosive subhmate 

 are also washed out with water. Use running water whenever possible 

 and, whenever running water is not available, change the water fre- 

 quently. With both methods, the tubes with bolting silk on each end, 

 or the tea filters from the five-and-ten-cent store will facilitate the 

 washing. Very effective "bottles" for washing can be made of wire 



