HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 221 
be represented by the form I have shown in FIGURES 8, 9, and Io. 
In my opinion, these are merely colonies of P. simplex approaching 
the reproductive stage, but I have not so far observed swarm-spore 
formation in the species. Nitardy (714, p. 178) regards a warty 
surface, pentagonal peripheral cells, and the non-tapering form 
of the spine as important characters of Р. Sturmii. 
FIGURE 10 gives quite convincing evidence on this point. 
In this colony five of the cells are well advanced toward reproduc- 
tion while the remaining eleven have remained immature. The 
mature cells have the form characteristic of P. Sturmii; the 
immature cells are like those of the ordinary colonies of P. simplex 
which have not yet reached the reproductive stage, but it is to 
be noted that the cells in this colony are much smaller than in 
the other two. The change in form of the cells as they approach 
the period of reproduction is very marked. Their rounding up 
leads to the at least partial withdrawal of material from the 
lobes and a narrowing of their bases. Failure to recognize these 
changes may be responsible for confusion as to the real character 
of P. Sturmii. Professor B. M. Davis has kindly shown me, and 
permits me to refer to, as yet unpublished figures showing the 
reproduction of P. simplex which seem to me not inconsistent with 
this view. 
In FIGURE 11 I have been able to bring out faintly, in my 
original prints, the curious bristle-like appendages at the ends of 
the spines, which like the similar structures in plankton diatoms 
tend to keep the organism afloat (see Petersen, ’11, and Zacharias, 
'o3). As shown here, it is clear that the individual setae may be 
widely divergent or almost parallel. The suggestion that they are 
movable on their bases or points of insertion in the spines is very 
obvious. The nodular or pear-shaped swellings at their bases are 
also very conspicuous in the case of the widely spread group from 
the spine of cell a and may very well be contractile droplets of 
cytoplasm functioning as motile organs. 
This colony is further interesting from the fact that the cell a 
seems relatively large as compared with the cells next to it. The 
whole colony also shows only fifteen instead of sixteen cells. 
Аз Nitardy and others have emphasized, the law of bipartition 
and the resultant 4-, 8-, 16-, 32-, etc., cell numbers are very firmly 
