104 Methods in Plant Histology 



Take the killing and fixing fluids into the field. If one waits 

 until the material is brought to the laboratory there may be some 

 fixing, but it will, in many cases, be too late to do much killing. 

 Material which has begun to wilt is not worth fixing. Material like 

 Spirogyra, however, may be brought from the field into the laboratory 

 before fixing, if considerable water be brought with it. 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 

 likely 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 penetrate more than 1 or 2 mm. 



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

 reagent should be about fifty times that of the material. 



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

 portion of material. It must be remembered that the water, which 

 is always 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 com- 

 monly 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, xxix). 



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 



