HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 213 
asperum and further with those in representatives of the Anomo- 
pedium group, P. integrum, the Monactinia, the Triactinia, the 
Tetractinia, etc. I have not been able to study the reproduction 
and colony formation in these species and can only compare 
their adult forms with those I have more fully studied. 
A. Anomopedium.— Pediastrum integrum Nag. 
I have seen only a few specimens of P. integrum. The indi- 
vidual shown in FIGURE I ('18) has but seven cells, the missing 
cell perhaps lies at some point above or below the other cells of 
the colony though it can not be made out in the photograph nor 
was it discovered in the specimen before photographing. The 
species is figured by Nageli (49) as occurring in 4-, 8-, 16-, 32-, etc., 
celled colonies, all of whose marginal cells have two very rudi- 
mentary spines. АП the colonies figured by Nägeli, except three 
of those with four or eight cells, are very irregular and he states 
that the regularly concentric arrangement of the cells which is 
common in other species of Pediastrum is exceptional in P. 
integrum. Не also states that it is common to find the cells in 
two layers. Braun (755) does not figure the species. 
Nägeli gives the habitat of the species as wet cliffs. Nitardy 
(14) reports that he has seen but one specimen. His figure shows 
Fic. І. Pediastrum integrum Nag. Perhaps the same as P. muticum Borge. 
Spines more prominent than in the eight-celled colony figured in '18, fig. 1. X 
about 300 
that it was quite irregular and he notes that it was plainly two- 
layered. He explains his failure to find more specimens as due to 
his having been concerned especially with material from lakes, pools, 
etc., and notes the agreement of most authors that P. integrum is 
found on rocks over which water trickles rather than in the deeper 
waters of pools, etc. The question at once arises whether the ir- 
regular form of the colonies in two layers is not due to environmen- 
