40 THE MAIN LINE OF DESCENT THROUGH THE GREEN ALG. 
of some primitive Chlamydomonas, but it must be noted that 
tetraspora-like forms have probably yielded as their highest 
expression the marine genera Ulva and Enteromorpha and 
the freshwater Monostroma. One of the simpler forms of the 
Palmellacezee—Palmodactylon (fig. 4)—has irregularly scattered 
cells in the containing mucilage, but a single row of cells is’ 
not uncommon, and probably from some such form was evolved 
the first filament by the formation of distinct walls between 
the cells. The simple filament is well illustrated by the 
familiar Ulothrix. Closely allied to Ulothrix is a form that 
has been observed from a few stations in the west of Scotland— 
Hormospora—in which there is no definite wall between the 
cells, which may be quite separated from each other (figs. 6 
and 7). This might be taken as an intermediate step between 
the palmelloid stage and Ulothrix, or, at anyrate, might be 
taken as indicating the line along which development has 
proceeded. That the Ulothrix filament (fig. 8) is derivable from 
Chlamydomonas through the Palmellaceze seems indisputable 
—it is hardly necessary to refer to the close similarity be- 
tween its reproductive organs (zoospores and gametes) and 
the primitive unicell from which we started—and once granted 
this step there is no difficulty in leading to the higher forms. 
It is worth noting in passing that we have here a remarkable 
extension of the resting period overshadowing the essential 
motile period which must be resorted to for reproduction. In 
fact the history of the whole line is “a history of the inter- 
calation of a vegetative phase between two successive motile 
(Chlamydomonadine) phases” (1). The derivation of a branched 
thallus from the simple filamentous forms presents no difficulty, 
and there is no doubt that the Chetophoracee are descended 
from an ulotrichaceous ancestor (fig.9). Certain forms along this 
line have tended towards a terrestrial existence, as in Pleuro- 
coccus, which forms a green covering on stones, tree-trunks, 
etc., and seems to be a reduced member of the Cheetophoracee ; 
and in Trentepohlia (Chroolepus), which is common on damp 
and somewhat shaded walls and rock faces, our species being 
readily recognisable by their brownish-red colour. 
Another interesting group forming the genus Coleochete 
has evidently arisen directly from branched Chetophorales ; 
the simplest forms have a simple branched thallus (fig. 10), 
but our commonest species, C. scutata (fig. 11), occurring 
