652 Blackman .— The Primitive Algae 
one. In the simplest types the cells so formed separate from 
one another, and so no advance in organization follows. In 
higher forms the sister-cells are united by mucilage, by 
remains of the mother-cell-wall, or in other ways, so that 
colonies arise. 
The Tetrasporine plants are always non-motile in their 
vegetative condition, and some of the most primitive, as Chlo - 
rangium , are little more than sessile attached Chlamydo- 
monads. which develop by dividing vegetatively into new cells, 
that remain attached to the mother-cell-wall and build up 
thus a little colony. Before this reaches any size, however, 
the building-up stops, and the cells—as zoospores—revert to 
the ancestral motile type and swarm off to new spots. 
Like the Volvocine tendency then, the Tetrasporine has its 
origin in the motile unicellular Chlamydomonads. The one 
of these Tetrasporine aggregations that has been most suc¬ 
cessful from the biological standpoint is the filament, and as 
transition forms we find filaments ( Stichococcus , Bumilleria , 
Hormidium ) which are not yet possessed of much solidarity, 
and which easily separate into single cells or short lengths. 
Numerous other forms of colonial cell-unions are found 
in these families, but nothing higher appears to have been 
evolved from any of them. These may perhaps be looked 
upon as experimental types of aggregation which have failed 
in the struggle for existence. The filament, however, has 
been eminently a success, and the large group of filamentous 
and membraneous thalloid forms constituting the Confer- 
voideae bears witness to this. 
From the Confervoideae, as is universally admitted, has 
been evolved the whole series of higher plants, Bryophyta, 
Pteridophyta, and Phanerogamia, so that the tendency seen 
in the Tetrasporaceae to form, by septate cell-division, thalloid 
aggregates of increasing solidarity is the one responsible for 
the structural possibilities of the higher plants. 
A line of separation between the Protococcoideae and the 
Confervoideae, that is the separation of unicellular organisms 
from filamentous organisms, is found to be untenable now that 
