300 PRACTICAL COURSE IN BOTANY 
over a sheet of white paper, and examine with a hand lens; 
then place under the microscope. It will probably be found 
to contain a number of minute organisms, but the pleurococci 
can be recognized as small round bodies of a bright green 
color, some of them separate, others adhering together in 
groups of two, four, or more, with the sides that are in contact 
slightly flattened. Each of these bodies is an individual 
plant consisting of a single cell, whence they are said to be 
unicellular. Draw one of the single cells and one of the 
groups, or colonies, as they appear 
under the microscope. Try to make 
out the cell wall and the nucleus, and 
label all the parts (see 7). If you 
have any difficulty in distinguishing 
the cell wall, drop a little glycerine 
or salt water on the slide. This will 
cause the cell contents to shrink by 
osmosis (56, 59). Can you make 
_ out the structure of the cell colonies? 
ee re They have resulted from the peculiar 
se A A es ete mode of multiplication that prevails 
B, division further advanced; among this class of plants. A cell 
pleaded division, r- elongates, contracts in the middle, 
and divides into two parts, each of 
which becomes an independent plant like the mother cell. 
See if you can find one in the process of division. The 
daughter cells repeat the process, each one giving rise to two 
new individuals, and so on indefinitely. The new cells do 
not always separate immediately on their formation, but fre- 
quently adhere together for a time, in colonies, before falling 
away and beginning an independent existence. 
338. Reproduction by fission. — This kind of reproduction 
is called fission, or cell division, and marks a very primitive 
stage of development. Under stress of adverse conditions 
the cells formed by division may remain inactive for a time. 
They are then called resting spores, and when more favorable 
