HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 223 
C. Diactinium. 
In the Diactinia we have the largest and most common of the 
subgenera of Pediastrum. The delimitation of species, however, 
seems in high degree difficult and uncertain. De-Toni (780) 
recognizes eight species in the group with eight varieties under 
P. Boryanum (Turp.) Menegh. and seven varieties under P. 
duplex Meyen, many of which are regarded by other authors as 
good species. Nitardy (714) believes the whole series can be best 
regarded as three species with three varieties under P. Boryanum 
(Turp.) Menegh. and three varieties under P. pertusum Kütz. 
Braun (’55) gives no figure of his P. pertusum Kiitz. var. 
clathratum, but refers to figures of Meyen (20), Hassall ('45) 
and Corda (?39, pl. 3. f. 18). Corda’s figure of P. diodon is cer- 
tainly unreliable. Meyen's figures, though poor, seem to agree 
rather better with Lagerheim's figures of his var. reticulatum 
Fic. 12. Pediastrum Boryanum (Тигр.) Menegh. Sixteen-celled colony typical 
arrangement, form with slender equal spines, Х about 300. 
Fic. 1 P. asperum, sixteen-celled colony, typical form, Х about 425 
P. duplex Meyen, var. reticulatum Брава Intermediate between 
Fic. 14. plex 
P. asperum and P. clathratum (figs. 15-21), X about 300. 
(82) than with those of De Wildeman (200, p. 104, f. 17 and 16), 
which are labelled P. duplex var. reticulatum Lagerh., and those of 
Chodat (от, p. 227 and 228) labelled P. duplex Meyen and P. 
duplex f. genuinum (A. Br.). Hassall's figure (45, pl. 02. f. 4) is 
certainly widely different from those of De Wildeman and Chodat. 
It seems doubtful whether Nageli (740), Braun, or Lagerheim had 
these 5 + 11 forms figured by De Wildeman, Chodat, and 
Nitardy (та, pl. 8. f. тт). Both types are found in this country 
