HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 227 
delicacy in the contact and pressure responses of the swarming 
zoospores is certainly necessary to achieve it. It is the most 
highly specialized configuration I have yet observed in any of the 
species of the genus, though whether greater delicacy of response 
is necessary for its production than for that of the ring-shaped 
eight-celled colony of P. simplex is not easy to say. | 
'The colonies are very commonly irregular and indicate very 
clearly that the normal contact relations are not necessary for 
the development of the typical cell form, as is illustrated by the 
interior cell with one free spine shown in FIGURE 18, and by De 
Wildeman's (оо, f. 18, p. 104). I have been able in several cases 
to observe the reproduction of the species. The cells become 
much swollen but still show a very deeply lobed form as compared 
with P. asperum at the corresponding stage. FIGURE 19 shows a 
thirty-two-celled colony which is about ready for reproduction 
and FIGURES 20 and 21 show two very irregular young colonies, 
Fics. 20 and 21. Young, irregular colonies of Pediastrum clathratum А. Br., X 
about 400. 
whose birth I observed in a sealed preparation. They are only a 
few hours old but the cells show the contours characteristic of the 
species. It is of interest to compare these figures with FIGURE 14 
as to the forms of the cells. 
The whole colony is relatively fragile and is ordinarily bent 
and curved so as to make a good photograph impossible. The 
bristle-like projections from the ends of its cell lobes are extremely 
well developed and are brought out faintly in some of my photo- 
graphs. The colonies are very sensitive to currents in the water 
and seem almost self-motile at times. It is very difficult to find 
one quiet enough for photographing and the varying position 
of the apical bristles, now all close together in a parallel pencil 
