68 A LABORATORY MANUAL OF BOTANY 
from each other before they in turn begin to divide? How 
do you account for the grouping of the plants previously 
noticed? Drawings should show one plant in the earliest 
stage at which the process of reproduction may be detected, 
one when the dividing wall is fairly well established, one as 
the new individuals are about separated, and other stages 
illustrating any other important points observed. 
General questions—Why are masses of Pleurococcus 
brighter green in wet than in dry weather? Why do they 
not grow.on the sunny side of trees? Why on isolated trees 
do they not grow high up in the air? Are the grayish or 
bright-green masses forming new plants most rapidly? 
Why? Note the plants late in autumn and early in the 
spring to see whether growth is inhibited by cold weather. 
In your notes give a full description of Pleurococcus, 
telling where it grows, its general structure, and how it re- 
produces itself, referring by number to the drawings as they 
illustrate the features mentioned. 
LESSON XXXV 
Nostoc and Oscillaria 
Materials——Specimens of Nostoc and Oscillaria. Glceo- 
capsa, Anabeena, or Rivularia may be used instead of those 
mentioned. If laboratory periods are short, this should be 
divided into two exercises. 
Nostoc 
General study.—In a dish in which there are some of the 
Nostoc balls, note the general appearance, color, form, and 
texture of the masses. These are composed of many Nostoc 
plants. The jelly-like material that holds them together 
is formed by the breaking down of the outermost parts of 
the walls. 
Vegetative structures.—Remove, mount, and examine a 
small piece of the jelly-like material. Throughout the al- 
