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 BS, Fig. 178. 
THE STUDY OF PLEUROCOCCUS 
277. Occurrence. —Plewrococcus 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- 
Fiac. 179. —Two Cells of Protococcus. fined cell-wall of cel- 
(Greatly magnified.) 
A, aspherical cell of the stillform; B, a motile cell lulose, — the 
with its protoplasm enclosed in a loose cell-wall and protoplasmic contents, 
‘ded with ‘lia. 
provided with two cilia usually green through- 
out. Do any cells show a nucleus like that in Fig. 179, A? 
Test the cells with iodine solution for starch. 
