Killing, Fixing, and Storing Plant Tissues 79 



The addition of chromic acid and urea to Bouin's Ihud makes 

 what is known as the Allen-Bouin formula. For cytological work 

 use the original formula, as given in the reference manuals, or one 

 of the formulas (lacking urea) given in Table 3.2. For further trials 

 vary the glacial acetic acid equivalent from 1 to 4% by volume. The 

 formaldehyde should be added immediately before using. Tests have 

 shown that tissues may be left in these solutions for several months. 

 It is probable that hardening of the material reaches a maximum in 

 less than a week. Dehydration and subsequent processing are carried 

 out as with Craf. 



Farmer's fluid and Carnoy's fluid have limited uses in histology. 

 These fluids kill protoplasm by rapid and probably violent dehydra- 

 tion. Because of their ability to penetrate very rapidly, these fluids 

 have some value for processing extremely downy, resinous, or im- 

 permeable structures that must be preserved entire. The fluid may 

 be used alone, followed in 1 hr. or less by the subsequent operations 

 of the paraffin process. An alternative method consists of first im- 

 mersing the materials in a Carnoy or Farmer formula (the time 

 ranging from an instantaneous dip to 10 min.) and then treating in 

 one of the more critical fluids. Two widely used formulas are as 

 follows: 



1 . Farmer's fluid 



Anhydrous ethyl alcohol 75 cc. 



Glacial acetic acid 25 cc. 



2. Carnoy's fluid 



Anhydrous ethyl alcohol 60 cc. 



Glacial acetic acid 10 cc. 



Chloroform 30 cc. 



The fluids given thus far in this chapter produce an acid fixation 

 image, preserving particularly well the chromosomes, nucleoli, and 

 the spindle mechanism. Nucleoplasm and mitochondria are dissolved; 

 cytoplasm is rendered in fibrillar or alveolar form. This type of image 

 is preferred for most studies of plant structure. 



In certain cytological studies it is desirable to preserve mitochon- 

 dria and allied cytoplasmic structures. In such cases a fixing fluid that 

 produces a basic fixation image is used. Such fltuds preserve mito- 

 chondria, nucleoplasm, and in some instances nucleoli and vacuoles. 

 Chromatin and the spindle mechanism are dissolved. For serious 

 studies in this field of cytology each worker must work out specific 

 techniques based on an extensive literature. However, it is possible 



