1 1 2 Chodat . — On the Polymorphism of the Green 
In a similar manner Sorastrum 1 may be derived from some 
unicellular P olyedrium , as I have shown ; and the same thing 
is highly probable with respect to Pediastrum , which is con- 
nected with the unicellular Polyedrium- forms through the 
interesting genus Euastropsis 2 . But in this latter small group 
curiously enough the zoospores are still present ; discharged as 
in other cases by the walls becoming gelatinous, they remain 
enclosed in the vesicle, and their evolution takes place within 
it. Finally, a new coenobium is formed. I have demonstrated 
by experiment that it is possible to induce Pediastrum to 
form the new coenobium directly within the mother-cell 
without the intermediate stage of the enclosed motile cells. 
By concentrating the nutritive solution, Pediastrum produces 
new colonies in the manner shown in Coelastrum , i. e. typical 
auto-colonies. 
In Hydrodictyon , which seems to be a very special type, an 
intermediate condition between Pediastrum and Coelastrum 
occurs. The swarming stage is attained not, as in Pediastmnn, 
outside the mother-cell, but within it. The question whether 
Hydrodictyon properly belongs to this group seems to me an 
open one. Perhaps the likeness to the Pediastreae is due to 
a convergence rather than to a common origin. There are in 
fact so many analogies between Cladophora and this genus 
in the mode of formation of the zoospores, and the constitution 
of the cell, that a new line of comparison is required. 
To sum up: we have in the Pediastreae a repetition of the 
evolution of the other Protococcoideae, and a great analogy 
with the Volvocineae in the complication of the thallus, though 
for reasons which are quite independent in both groups. 
Ulvaeeae-Chaetophoraceae. The type which is realized in 
the Ulvaceae is still essentially similar to that of the true 
Palmellaceae. In fact there is only the greater consistency 
of the walls and, consequently, the more definite form of the 
thallus. It is true that here the cells of the thallus are more 
1 See De La Rue, Sur le developpement du Sorast?'U7n, Ann. Sc. Nat., ser. V. 
t. 17, 1873, and Chodat, Evolution des Coelastrum , loc. eit., p. 275. 
2 See Lagerheim, Ueber Arktische Kryptogamen, I, Tromso-Museum, 1894. 
