MORPHOLOGY 
the connection with the next group of green algae 
is to be sought in such forms as Chlamydomonas 
and Sphaerella. 
Flagellates. This problematical group of organisms is 
so suggestive of a relationship to Volvocales that it should 
be mentioned in this connection. They are one-celled, 
active, aquatic forms, with one or two (sometimes more) 
cilia (fig. 33). The cell is naked or with a distinct mem- 
brane, which rarely contains any cellulose. The protoplast 
shows contractile and amoeboid movements, and contains 
a nucleus, a pulsating vacuole, and in many forms distinct 
green or brown chromoplasts. Some of them are very 
animal-like in taking food, ingesting solid particles; and 
their usual multiplication by longitudinal splitting is not 
plantlike. No sexual reproduction is known, but the 
frequent formation of thick-walled " resting spores " is 
suggestive of plants. This combination of plant and 
animal characters has suggested that the flagellates may 
be regarded as the starting point for such one-celled thallo- 
phytes as the Volvocales on the one hand, and for Protozoa on the other. In 
fact, by some authors Volvocales are included among the flagellates. 
Fi- 33- Euglena: 
showing a single cilium 
(flagellum), a large nu- 
cleus, and numerous 
chloroplasts. 
(b) Protococcales 
General character. The group of forms included here under this 
name is probably a very heterogeneous assemblage, and it has been 
much broken up recently by the special students of algae, but it will 
serve our purpose. The plants occur mainly in fresh water, ranging 
from an extreme aquatic habit to occurrence in moist places, such as 
tree trunks, shaded earth, etc. Some are endophytic, living in the 
intercellular spaces of certain aquatic seed plants; others enter into the 
structure of lichens; while still others give the green color to certain 
animals, as fresh-water sponges, hydra, etc. The possible connection 
of the solitary (one-celled) forms with such solitary Volvocales as 
Chlamydomonas is apparent, the former being characterized by the 
absence of cilia on the vegetative cells; in other words, there is an occa- 
sional loss of motility by the vegetative cells of Volvocales, and this is 
the permanent condition of the vegetative cells of Protococcales. 
Pleurococcus. This is an exceedingly common green slime found 
on flower pots, damp bricks, tree trunks, etc., and it may be regarded 
as a representative one-celled green plant. If there is any connection 
between Protococcales and Volvocales, it is through such forms as 
