CHLOROPHYCEAE 221 



sure to be caught and it may accentuate some markings; but, in gener- 

 al, 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-haematoxyhn and proceed 

 by the Venetian turpentine method. When ready for mounting, the 

 diatoms can be scraped off from the algae or other substratum. Saf- 

 ranin gives a beautiful stain, bringing out the radiations and the pecu- 

 liar centrosomes. This stain has been used very effectively in studying 

 auxospores. 



When material is in gelatinous masses or is clinging firmly to some 

 easily cut substratum, it may be fixed and imbedded in paraffin. The 

 knife often breaks the frustules as cleanly as if they were cut. 



Desmids. — The desmids are unicellular, free-floating, or suspended 

 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 collec- 

 tions. 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 8 seconds, 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 (6-24) 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. We have found Mi- 

 crasterias so loosely attached to Chara that a quart bottle full could 

 be squeezed off in a few minutes. A watering-trough yielded Cosma- 

 rium in almost equal 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 5 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 haema- 

 toxyhn. The iodine solution used in testing for starch gives good re- 

 sults and may be followed by any stains. The larger desmids stain 

 beautifully in iron-alum haematoxylin. 



The Venetian turpentine method, with phloxine and anihn blue, 



