CHLOROPHYCEAE 
183 
proceed by the Venetian turpentine method. When ready for 
mounting, the diatoms can be scraped off from the algae or other 
substratum. Other stains may be used. 
When the material is in gelatinous masses it may be fixed in 
chromo-acetic acid, with or without a little osmic acid, and imbedded 
in paraffin. There will, of course, be some difficulty in cutting, 
but the knife often breaks the frustules very cleanly, so that good 
sections may be secured. It might be worth while to try a weak 
solution of hydrofluoric acid to dissolve the silicious shells. 
Desmids.—The desmids are unicellular, free-floating or sus¬ 
pended algae. They are not found in salt water and are more 
abundant in soft water than in hard. Deep pools, quiet ponds, and 
quiet margins of small lakes are good collecting-grounds. Collections 
of other fresh-water algae often contain some desmids. It frequently 
happens that a single desirable desmid appears during examination 
of field collections. In such a case, remove it with a fine pipette, 
and get it into a drop of water on a clean slide, invert it over a bottle 
of 1 per cent osmic acid for 2 minutes, leave the slide exposed to the 
air until almost all the water has evaporated, and then add a drop 
of 10 per cent glycerin. In a few hours (6 to 24) put on a cover and 
seal. It requires more time, care, and patience than it is worth to 
attempt staining in such a case. 
Sometimes desmids occur in great abundance. They may then 
be treated like the filamentous algae, except that more care must 
be taken not to lose them when changing fluids. Four or five drops of 
1 per cent osmic acid to 50 c.c. of water fixes well, and material from 
this solution may be placed directly into 10 per cent glycerin and 
mounted by the Venetian turpentine method. It looks almost as 
if stained in iron-alum haematoxylin. The iodine solution used in 
testing for starch gives good results and may be followed by any 
stains. The larger desmids stain beautifully in iron-alum haema¬ 
toxylin. 
The Venetian turpentine method, with Magdala red and anilin 
blue, will give beautiful preparations. A deep stain with Magdala 
red and a rather light stain with anilin blue is better for the pyrenoids 
and nucleus, while a light stain in the red and a deep stain in 
blue is better for the chromatophores. When the material is suffi¬ 
ciently abundant, paraffin sections may be made as directed for 
Volvox. 
