192 BOTANICAL MICROTECHNIQUE 



useful, — a weak, a medium, and a strong. Any shrinkage of the ceU con- 

 tents during fixation indicates that the solution is too strong in chromic 

 acid, which has a tendency to contract the protoplast, partially compen- 

 sated by the acetic acid which is employed because of its tendency to swell 

 the cell contents. 



A. Weak chrom-acetic acid i : 



1% chromic acid 25 cc. making .25% chromic acid. 

 1% acetic acid 10 cc. making .1% acetic acid, 



distilled water 65 cc. 



100 cc. 

 This is generally the most satisfactory strength of chrom-acetic acid 

 for algse and fungi, and the more delicate structures of the liverworts 

 and mosses will be excellently fixed by it, likewise fern prothallia. 



B. Medium chrom-acetic acid : 



1% chromic acid 70 cc. making approximately .7% chromic acid, 

 glacial acetic acid .5 cc. making approximately .5% acetic acid, 

 distilled water 30 c'c. 

 100.6 cc. 

 A good fluid for most work on the histology of the pteridophytes 

 and seed plants and the firmer structures of mosses and liverworts. 



C. Strong chrom-acetic acid : 



1% chromic acid 100 cc. making approximately 1% chromic acid. 



glacial acetic acid 1 cc. making approximately 1% acetic acid. 



101 cc. 

 This solution may prove more satisfactory than medium chrom-acetic 

 aTcid when the tissue is dense or with very heavy cell walls. 

 The determinations of the proper relative strengths of chromic and acetic 

 acids become matters of experience and experiment which must be tested 

 with untried subjects, but those who use this fixing fluid are soon able to 

 judge very accurately the strength and time necessary for good results. 

 Chrom-acetic acid keeps perfectly, and costs so little that it may be made up 

 in large quantities ; it is the most useful general fixing agent in the labo- 

 ratory. It should be employed in liberal quantities, perhaps one hundred 

 times the bulk of the material, or in such amounts that the fluid is not 

 noticeably discolored by the material. 



1 The formulsB in this account are generally given in terms of 100 cc. The 

 proportions may be multiplied by tens and hundreds for larger quantities. 

 Chrom-acetic acid is so useful a reagent that a laboratory should always have a 

 stock supply. Another plan is to keep on the same shell or table a large bottle of 

 1% chromic acid, another ol 1% acetic acid, and a small bottle of glacial acetic 

 acid, together with a large and small graduate and the formulae posted on the 

 wall. Solutions may then be made up at any moment. 



