SUMMARY OF THE GREEN ALG^E 203 



SUMMAEY OF THE GREEN Al/LE 



231. Summary of the green algae. The green algse comprise 

 a number of well-defined groups which are evidently widely 

 separated from one another. The most conspicuous of these are 

 the Volvocacece, the desmids and pond scums, the diatoms, the 

 siphon algse, and the stone worts. They constitute independent 

 evolutionary lines of varying importance, but each one clearly 

 developing in ways peculiar to itself and quite apart from the 

 theoretical main line of ascent to the liverworts and mosses 

 (Bryophyta}. A discussion of the origin of these groups and 

 their possible relationships to one another would be much too 

 complicated for the present account. The forms of the green 

 algse which seem to be nearest to the main line of ascent are in 

 certain related families ( Ulothricacece, Chcetophoracece, and Coleo- 

 chcetacece), but it is very doubtful if any of them are directly 

 on the main line, and there are no living algse known which 

 show clearly the origin of the bryophytes. 



Almost all of the green algse at some stage in their life 

 history form zoospores or motile gametes (the sperm being 

 motile in heterogamous forms). These ciliate cells point clearly 

 to an ancestry of the green algse from groups comprising one- 

 celled motile organisms something like the flagellates (Sec. 204) 

 and lower forms of the Volvocacece (Sec. 215). The formation 

 of the zoospore and the motile gamete is considered to be a 

 return in the life history, for a short time, to the primitive 

 one-celled conditions from which the various lines of the green 

 algse are believed to have arisen. The motile conditions which 

 occupy practically all of the life history of the flagellates and 

 Volvocacece become reduced to a short reproductive period in 

 most of the green algse. The most important forward steps in 

 the evolution of the green algse came with the introduction of 

 long vegetative periods in the life histories when the proto- 

 plasts remained quiet and formed many-celled plant bodies 

 (coenocytic siphon algse excepted) of various structure. All the 



