244 



FOUNDATIONS OF BOTANY 



however, to retard the occurrence of conjugation by leaving the 

 Spirogyra filaments in very cold water over night, and in this way 

 the successive steps of the conjugating process may be studied ^by 

 daylight. In such ways the series of phenomena shown in Fig. 

 178, II, has been accurately followed. If the student cannot follow 

 these operations under the microscope, he may, at least, by looking 

 over the yellower portions of a mass of Spirogyra find threads con- 

 taining fully formed zygospores, like those shown in B, Fig. 178. 



THE STUDY OF PLEUROCOCCUS 



277. Occurrence. Pleurococcus may be found on old fences, 

 roofs, and many similar places, particularly on the bark of the north 

 side of trees. The individual plants cannot be detected by the naked 

 eye, but when grouped in masses they form a powdery green covering 

 over indefinite areas of bark. Plenty are seen where it is moist. 



278. Microscopical Examination of Pleurococcus. Scrape a minute 

 quantity of Pleurococcus from a specimen on bark, place it in a drop 

 of water on a slide, distributing it slightly in the water, lay on it 



a cover-glass and ex- 

 amine with a power of 

 200 or more diameters. 

 Sketch with the cam- 

 era lucida one of the 

 largest cells, some of 

 intermediate size, and 

 one of the smallest, 

 beside several divisions 

 of the stage microm- 

 eter. 



Note the clearly de- 

 fined cell-wall of cel- 

 lulose, enclosing the 

 protoplasmic contents, 

 usually green through- 

 out. Do any cells show a nucleus like that in Fig. 179, A ? 



FIG. 179. Two Cells of Protococcus. 

 (Greatly magnified.) 



A, a spherical cell of the still form ; B, a motile cell 

 with its protoplasm enclosed in a loose cell-wall and 

 provided with two cilia. 



Test the cells with iodine solution for starch. 



