24 MORPHOLOGY 



t he zygospore does not produce a net directly, but gives rise to other 

 cel ls 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 featur e 

 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 



