182 
METHODS IN PLANT HISTOLOGY 
make a mount, smear the slide very slightly with Mayer’s albumen 
fixative, add a little of the material, and heat just enough to coagulate 
the albumen. When perfectly dry, add a drop of balsam and a cover. 
Or, after coagulating the fixative, dip in absolute alcohol and then in 
xylol before mounting in balsam. Without the alcohol and xylol, 
some air is sure to be caught and it may accentuate some markings; 
but, in general, we prefer to use the alcohol and xylol. 
To show the cell contents, diatoms must be fixed and stained. 
If they are clinging to filamentous algae, the algae with the diatoms 
attached should be put into chromo-acetic acid (24 hours) and then 
washed in water for 24 hours. Stain in iron-haematoxylin and 
Fig. 38. —Diatoms: diatomaceous earth from Cherryfield, Maine, a Pleistocene deposit, 
showing the great variety of forms usually found in such material; photomicrograph by Miss 
Ethel Thomas from a preparation by Rev. E. L. Little. X400. 
