84 SMITH : TETRADESMUS, A NEW COENOBIC ALGA 
DISCUSSION 
Tetradesmus belongs to the family Coelastraceae, recently estab- 
lished by Wille (19). Tetradesmus shows a close relationship to 
Scenedesmus. The method of reproduction is essentially the 
same as that which occurs in Scenedesmus, the chief difference 
being that at the time of liberation from the mother cell wall the 
cells forming the coenobe in Scenedesmus become arranged in the 
form of a plate, while in Tetradesmus they remain rolled up. It 
seems probable that one form has been derived from the other, 
but there is little evidence as to the direction that the phylo- 
genetic development has taken. There is also the possibility 
that these two forms have been derived from a common ancestor. 
Tetradesmus may have been derived from Scenedesmus through a 
loss of the tendency for the cells of the coenobe to unroll and spread 
out into a linear series after their liberation from the mother cell 
wall; or some colony of Tetradesmus may have developed this 
character of unrolling the cells in order to obtain an arrangement 
of the individual cells better adapted for photosynthesis. 
Cytological study shows that these forms (Tetradesmus and 
Scenedesmus) are more closely related to Hydrodictyon and Pedias- 
trum than has been usually supposed. In the last-named forms 
reproduction takes place by the formation of a more or less definite 
number of zcéspores, which then become arranged in the form of 
the mature colony while they are still inside the old mother cell 
wall or inside a gelatinous vesicle. In Tetradesmus and Scenedes- 
mus there is a successive division into a definite number of uni- 
nucleate masses of protoplasm. These correspond to the zodspores 
of Hydrodictyon and Pediastrum in every way except for the fact 
that they have no power of movement. We may easily think of 
these daughter cells in Tetradesmus and Scenedesmus as zodspores 
which have lost the power of movement. The loss of power of 
motility of the zodspores inside the mother cell wall is probably 
a step in advance of the condition in Hydrodictyon. There is 
also the possibility that this loss of movement of the zoospores 
is connected with the smaller number of spores produced and the 
greater ease with which they get into their permanent positions. 
pepeinetion in Tetradesmus is completed by a movement of the 
zc6spores”’ into the position which they will occupy in the 
