THE GREAT GKOUPS OP ALQM 237 



being of this simple kind. It is evident, therefore, that the 

 group Chlorophyceas begins with forms just as simple as 

 are to be found among the Cyanophycese. 



Pleurococcus is used to represent the group of Protococ- 

 cus forms, one-celled forms which constitute one of the 

 subdivisions of the Green Algas. It should be said that 

 Pleurococcus is possibly not a Protococcus form, but may 

 be a reduced member of some higher group ; but it is so 

 common, and represents so well a typical one-celled green 

 alga, that it is used in this connection. It should be 

 known, also, that while the simplest Protococcus forms re- 

 produce only by fission, others add to this the other meth- 

 ods of reproduction. 



164. Ulothrix. — This form is very common in fresh wa- 

 ters, being recognized easily by its simple filaments com- 

 posed of short squarish cells, each cell containing a single 

 conspicuous cylindrical chloroplast (Fig. 205). 



The cells are all alike, excepting that the lowest one of 

 the filament is mostly colorless, and is elongated and more 

 or less modified to act as a holdfast, anchoring the filament 

 to some firm support. With this exception the cells are all 

 nutritive ; but any one of them has the power of organizing 

 for reproduction. This indicates that at first nutritive and 

 reproductive cells are not distinctly differentiated, but that 

 the same cell may be nutritive at one time and reproductive 

 at another. This plant uses cell division to multiply the 

 cells of a filament, and to develop new filaments from frag- 

 ments of old ones ; but it also produces asexual spores in 

 the form of zoospores, and gametes which conjugate and 

 form zygotes. Both zoospores and zygotes have the power 

 of germination — that is, the power to begin the develop- 

 ment of a new plant. In the germination of the zygote 

 a new filament is not produced directly, but there are 

 formed within it zoospores, each of which produces a 

 new filament (Fig. 205, F, 6). All three kinds of repro- 

 duction are represented, therefore, but the sexual method 



