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 
alge, 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 Coleochete. So far as is known at 
present, this is the highest type the plant body assumes 
among the Chlorophycee or green alge except in the 
case of the Characez, whose affinities with the other 
alge are doubtful. From some forms probably not 
unlike Coleochete, 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 alge have 
both sexual and non-sexual reproductive cells, the latter 
most commonly being motile zodspores. 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 
