Killing, Fixing, and Storing Plant Tissues 75 



is necessary after some killing fluids, is discussed in connection with 

 specific formulas. 



One of the most useful types of killing and preserving fluid, 

 kno^vn as FAA, is represented by the following formula: 



Ethyl alcohol (95%) 50 cr. 



Glacial acetic acid 5 cc. 



Formaldehyde (37-40%) 10 cc. 



^Vater . . . ! 35 cc. 



Propionic acid may also be used, the formula is then designated 

 FPA. 



Several modifications may be found in the literature. This fluid 

 is stable, has good hardening action, and material may be stored in 

 it for years. These properties make this formida suitable for large 

 or impervious objects such as woody twigs, tough herbaceous stems, 

 and old roots. The high concentration of alcohol is likely to produce 

 shrinkage of succulent materials, although it is possible to develop a 

 formula for some apparently tender subjects and even for filamentous 

 algae. A balanced formula can be worked out by varying the acetic 

 acid, which has a swelling action on protoplasm, from 2 to 6% by 

 volume. The formaldehyde and alcohol, which have a shrinking 

 action, should be held at the indicated concentrations. When making 

 trials of variations from the fundamental formulas, kill a trial lot 

 or batch of material in the formula to be tested, and a check lot in 

 a standard formula, and carry the batches through identical process- 

 ing simultaneously, so that differences in cellular detail will be the 

 result of variations of formula. 



Pieces of thin leaf are killed and hardened in 12 hr. The actual 

 killing of the protoplasm probably occurs in much less time. Thick 

 leaves or pieces of small stem require at least 24 hr. Woody twigs 

 should be kept in FAA at least a week before continuing the process- 

 ing for embeddino. Materials do not need to be washed after FAA. 

 The ingredients of this fluid are soluble in the dehydrating agents 

 and are thus removed before infiltration is begun. 



An extensively used formula consists of FAA containing bichloride 

 of mercury (HgCL) to saturation. This fluid penetrates and hardens 

 tissues rapidly. It preserves bacterial zoogloea in plant tissues, thus 

 being useful in pathological studies. The alcohol may be increased 

 to 70%. Prolonged storage in fluids containing bichloride is undesir- 

 able. The tissues should be transferred after 48 hr., or at most a 

 week, to a fresh solution of the original formula which does not 



