22 METHODS IN PLANT HISTOLOGY 



nosperm; and for filamentous algae, which are to be mounted whole, 3 

 per cent is very effective. 



It is convenient to have in the laboratory the following stock solu- 

 tion of chromo-acetic acid from which various solutions can be made 

 as they are needed 



Chromic acid crystals 10 g. 



Glacial acetic acid 10 c.c. 



Water 1,000 c.c. 



Keep the stock solution in a glass-stoppered bottle because the 

 acetic acid evaporates very rapidly. 



To make a solution containing 0.5 g. of chromic acid and 2 c.c. of 

 glacial acetic acid to 100 c.c. of water, add 50 c.c. of water to 50 c.c. of 

 the stock solution, and then add to the weakened solution 1.5 c.c. of 

 glacial acetic acid. Any desired proportions can be secured in a similar 

 way. Weighing the crystals for every new proportion is more tedious. 

 The proportions of the various ingredients, for the present at least, 

 must be determined by experiment. With favorable objects Uke fern 

 prothallia, Spirogyra, and other things which can be watched while the 

 fixing is taking place, suitable proportions are rather easily determined, 

 because specimens, after being placed in the reagent, may be examined 

 at frequent intervals, and combinations which cause plasmolysis may 

 be rejected and different proportions tried until satisfactory results are 

 secured. For example, fern prothallia might be placed in the following 

 solution: chromic acid, 2 g.; acetic acid, 1 c.c; and water, 97 c.c. If 

 plasmolysis takes place, as it probably will, weaken the chromic or 

 strengthen the acetic. In general, it will be better to weaken the 

 chromic, but not to less than | per cent. If there is still some shrinking 

 after the chromic has been reduced to ^ per cent, strengthen the 

 acetic. For most fern prothallia, the stock solution, with the addition 

 of 2 c.c. of glacial acetic acid to 100 c.c. of the solution, is satisfactory 

 for material to be mounted whole, and also for sections. A combina- 

 tion may be quite satisfactory for fern prothallia and still fail to give 

 good results with Spirogijra, and a combination which is excellent for 

 Spirogyra may fail utterly with Vaucheria. For critical work the most 

 favorable proportions must be determined for the particular object 

 under observation. In observing the effect of the fixing one can deter- 

 mine whether there is any noticeable plasmolysis or distortion, but 

 whether the fixing is thorough can be determined only by noting how 



