24 MORPHOLOGY 
the zygospore does not produce a net diiectly, but gives rise to other 
cells which do. 
Conclusions. Taking the whole assemblage of forms included here 
under Protococcales, the range of form is from a solitary cell to a com- 
plex colony, Hydrodictyon holding the same highly specialized position 
in this series that Volvox does among the Volvocales. A marked feature 
of difference, however, is that the vegetative cells are not motile. Col- 
ony reproduction occurs as among the Volvocales, the zoospores pro- 
duced by a mother cell being held together or retained until they are 
organized into a new colony. The lowest forms of Protococcales ex- 
hibit no sexual reproduction, but among the higher forms isogamy is 
attained ; while among Volvocales heterogamy is reached. 
(c) Confervales 
General character. Under this name what may be regarded as the 
representative green algae are assembled. The assemblage is a very 
artificial one, and it has been broken up and scattered by the special 
students of algae, but for our purpose it is convenient to consider these 
forms together. They are nearly all aquatic, usually filamentous, and 
always septate (with cross walls). Under certain conditions these fila- 
mentous bodies may break up into isolated cells and remain in this 
condition for a time, resembling the forms of Protococcales with isolated 
cells, as Pleurococcus. All of them produce zoospores, and it is prob- 
able that all of them have some form of sexual reproduction. In 
considering certain representative forms, two categories are convenient; 
namely, the isogamous forms and the heterogamous forms. 
(a) Isogamous forms 
Ulothrix. This very common alga may be taken to stand for a 
representative isogamous conferva form. It is a simple filament, whose 
basal cell is modified to form a holdfast that anchors the body and 
permits a permanent position in moving water (fig. 55). The ordinary 
vegetative cells are short, and each one contains a nucleus and a large 
chloroplast which is peripherally placed and has the form of a thick 
hollow cylinder. Any of the cells may produce zoospores, which may 
range in number from one to thirty-two even in the same filament (fig. 
56), the size of the zoospore depending upon the number of divisions. 
A large zoospore bears four cilia (fig. 59), and resembles a vegetative 
