286 EVOLUTION OF PLANTS 



the mother-cell after a preliminary division of the 

 nucleus into as many secondary nuclei. In the latter 

 case, the resulting cells may be non-sexual, or they 

 may exhibit the simplest form of sexual reproduction, 

 i.e. the cells may be similar gametes which unite in 

 pairs preliminary to the formation of new individuals. 

 These reproductive cells are usually motile and closely 

 resemble the ancestral Volvox cell. 



If the two cells resulting from the fission of a uni- 

 cellular organism remain together, and this is re- 

 peated, there results a cell-complex, the simplest type 

 being the cell-row found in so many of the green 

 algse, like Spirogyra or Conferva. The next step in 

 advance is the formation of filaments, like those of 

 CEdogonium, with definite base and apex, the filament 

 usually being attached by a simple holdfast. Next by 

 division in two planes is formed such a simple flat 

 thallus as that of Coleochsete. So far as is known at 

 present, this is the highest type the plant body assumes 

 among the Chlorophycese or green algte except in the 

 case of the Characese, whose affinities with the other 

 algse are doubtful. From some forms probably not 

 unlike Coleochsete, the lowest of the mosses were 

 derived. 



The increasing complexity of the plant body has 

 been accompanied by a corresponding specialization of 

 the reproductive parts. Most of the green algse have 

 both sexual and non-sexual reproductive cells, the latter 

 most commonly being motile zoospores. The lower 

 members of the series have the gametes, or sexual cells, 

 alike, but in the higher ones the female gamete, or 

 egg, loses the power of movement and is retained within 



