CHLOROPHYCEAE 



227 



The form is so small that in living material little more than the 

 general form can be distinguished. Excellent mounts are easily and 

 quickly made. Smear a very thin layer of albumen fixative upon the 

 slide, and add a drop of water containing the Scenedesmus. The drop 

 may be inverted for 4 or 5 seconds over the fumes of 1 per cent osmic 

 acid. No washing is necessary, and good mounts may be made without 

 any fixing whatever. Allow the drop to dry completely. It is better 

 to leave it for 24 hours before proceed- 

 ing. The usual difficulty with this form, 

 and with many others, is that the back- 

 ground stains and so makes the mounts 

 untidy. The following method by Yam- 

 anouchi will produce beautiful prepara- 

 tions (Fig. 50) : 



1. Dry on the slide, 24 hours. 



2. Ten per cent alcohol overnight to re- 

 move chlorophyll. 



3. Safranin (alcoholic), 4 days. 



4. Water, 5 minutes. 



5. Aqueous gentian violet, 2 days. 



6. Water, a few seconds. 



7. Orange G, aqueous, 3 minutes. 



8. Ninety-five per cent alcohol, a few 

 seconds. 



9. Absolute alcohol, 1 minute. 

 10. Clove oil, until the stain is satis- 

 factory. Different collections of 

 Scenedesmus stain very differently, 

 but the time in clove oil is likely to 

 be long, even as long as 6 hours. 



11. Xylol, 5 minutes. 



12. Mount in balsam. 



Fig. 51. — Pediastrum: A, colony 

 with 16 cells; B, colony with 8 cells. 

 Both are beginning to form new colo- 

 nies inside each cell. X770. 



Pediastrum. — This is a beautifully regular little alga (Fig. 51). 

 Scattered specimens often occur mixed with other algae and, occasion- 

 ally, one finds it in abundance. Look in small ponds, ditches, and bog 

 pools, where it is associated with other water plants. If it is at all 

 abundant, centrifuging very gently for a few minutes may help you 

 get a hundred specimens in a single drop. If material is scarce, put it 

 into formahn-acetic acid — 5 c.c. formalin and 5 c.c. acetic acid to 



