166 



Methods in Plant Histology 



cake must be very thin. Fig. 32 shows that even such a delicate 

 organism as Volvox can be imbedded in paraffin without shrinking. 

 Dr. Nieuwland reports that Pandorina, Eudorina, and Gonium, 

 also members of the Volvocaceae, are commonly found in summer as 

 constituents of the green scum on wallows in fields where pigs are 

 kept. The flagellate, Euglena, is often associated with these genera. 

 Pleurococcus. -This form, which is used everywhere as a labora- 

 tory type of the unicellular green algae, is found on the bark of trees, 



where it is more abundant 

 on the north side and 

 near the ground. It is 

 also found on stones and 

 fences, and in moist situ- 

 ations generally. It is 

 easily secured in nearly 

 all localities and at all 

 seasons. 



A study of the living 

 material is sufficient for 

 any general course. The 

 bright-green cells, scraped 

 off and mounted in a 

 drop of water, show the 

 rather thick wall, the 

 chromatophores, and 

 usually the nucleus. A 

 drop of iodine will bring out the nucleus, if it does not show 

 already, and will also stain the pyrenoid, if the cell contains one. 

 A mount in Venetian turpentine, stained in Magdala red and anilin 

 blue, shows the nucleus very clearly. 



Scenedesmus. -Scenedesmus (Fig. 33) is found everywhere as a 

 regular constituent of the fresh-water plankton. It is more abundant 

 in stagnant water. It often appears iii considerable quantity in 

 laboratory cultures. It may be kept for years in a tightly closed 

 glass jar without renewing the water, the lid being removed only 

 when material is needed. 



FIG. 32. Volvox: photomicrograph of a section 

 stained in Delafield's haematoxylin ; from a prepara- 

 tion and negative by Dr. W. J. G. Land. 



