310 
A TEXTBOOK OF GENERAL BOTANY 
alow are generally believed to be the most primitive, while the 
other three groups are usually considered to be descended from 
a class of unicellular 

Fig. 311. Forms of blue-green alge 
Left, Rivularia ; upper center, Apha- 
nothece; lower center, Merismopedia ; 
right, Oscillatoria 
alge, which include the green, 
organisms that are known as flagellates. 
Blue-green alge are the most 
simply organized of chlorophyll- 
bearing plants. The individuals 
consist either of single cells, 
rows of cells, or colonies of 
cells (Figs. 311, 312). In this 
group no method of sexual re- 
production is known; propa- 
gation results from the division 
of cells. The class as a whole 
is characterized by the absence 
of a definite nucleus and mi- 
totic division, although in the 
higher members there is a cen- 
tral body, frequently called an 
incipient nucleus, whose struc- 
ture approaches that of a nu- 
cleus. Also, the chlorophyll is 
not aggregated into plastids 
but is diffused in the periph- 
eral portion of the protoplasm. 
While the blue-green alge are 
the most simply organized plants 
with chlorophyll, there is great 
uncertainty as to how they may 
be related to other chlorophyll- 
bearing plants. 
The flagellates are believed 
to be the ancestors of the true 
the brown, and the red alge. 
The flagellates (Fig. 317) are single-celled organisms which 
move by means of long, slender, protoplasmic projections called 
flagella. The green flagellates contain chloroplastids and a 
