654 Blackman.—The Primitive Algae 
This easy reversion to a unicellular form shows how the 
filamentous type may have been evolved by adhesion of the 
products of vegetative division. The unbranched primitive 
filamentous Confervoideae are no doubt polyphyletic, and it 
seems natural to associate the main forms, having zoospores 
and gametes, with the Tetrasporaceous unicellular type, and 
forms not having them with the Pleurococcaceous type, as in 
the case of Stichococcus. 
Wille (’ 90 ) and others represent the membraneous Ulvaceae 
as the root-family of the Confervoideae because the passage 
from Tetraspora to this family is so easy. The derivation of 
the filamentous Ulothrichaceae from the Ulvaceae seems to 
me a mistaken idea, when a direct origin for them from those 
Tetrasporaceae which tend in a filamentous direction is so 
much simpler. Radiofilum , Bohlin ( 5 97 ), G. S. West ( 98 ), 
suggests such a form, consisting of a string of cells imbedded 
in a mucilaginous matrix which has the shape of an un¬ 
branched filament (Fig. 14). The Ulvaceae may be regarded, 
then, as a side branch from the Tetrasporaceae, ending blindly 
in the highly differentiated form Letterstedtia. 
The case of Pseudopleurococcus , Snow (’ 98 ), is another of 
those in which by change of conditions an Alga that is nor¬ 
mally unicellular passes into a branched filamentous state. 
Here the two forms taken independently would be placed in 
the branched Chaetophoraceae and in the Pleurococcaceae 
respectively, which is a wider separation than had been made 
between the forms of Stichococcus. 
The characterization of families by external form is, in this 
group, quite unpermissible 1 , and the present subdivision into 
families, representing different grades of vegetative morpho¬ 
logical complexity, can only be regarded as temporary until 
our increasing knowledge enables us to form phyletic families. 
1 Senn (’ 99 ) has shown that by appropriate conditions the coenobia of some 
Pleurococcaceae— Coe/astrum, Scenedesmus —can be made to give rise to single 
isolated vegetative cells which lack the formal characters of the cells in the 
coenobia. Fig. 14, Coe/astrum reticulatum , a, cells united by mucilage arms as 
in the coenobium; b, armless free cells. 
