1182 Wisconsin Academy of Sciences, Arts and Letters. 
condition is that shown in Figures 28.and 30, where the cells 
are arranged in one plane about a common center. The cells may 
all be in contact with one another at a central point, or there 
may be a small rectangular space with which they are all in 
contact (Fig. 30). In other cases a pair of cells opposite one 
another are in contact, and the other two cells are separated by 
the first pair (Fig. 28). This last described arrangement makes 
the outline of the coenobe more diamond-shaped than in the two 
former cases. Sometimes the cells are not all in the same plane 
(Fig. 29), but one cell is superimposed upon the others. In 
this the cellular arrangement is not coplanar concurrent but non¬ 
concurrent nonparaJiel noncoplanar. That these irregular col¬ 
onies are abnormal is shown by the differences in the size of 
the cells (Figs. 27 & 29). When the cells are all of the same 
size the arrangment is nearly always regular; when they are 
not of the same size, the arrangement is irregular. These ir¬ 
regularities in size and arrangement suggest that the swarm- 
spores were not all as vigorous or as healthy, as those which 
formed the colonies of regular shape, and that, as a result of 
their more or less pathological condition, they were unable to 
arrange themselves in the normal position. 
Since Pediastrum Boryanum was obtained in unialgal cul¬ 
ture, the variations of the four-celled coenobes could be easily 
studied. The variations, in the case of P. Boryanum, may be 
either in the shapes and relatives sizes of the cells, or in their 
spatial relationships. As in P. tetras, a dwarfing of certain 
cells of the colony sometimes occurs, with a consequent irregu¬ 
larity in the arrangement of the cells (Figs. 24, 25). Nothing 
brings out more strikingly the fact that the cells of a colony 
are independent of each other, at least as far as nutrition is 
concerned, as the occurrence of colonies one or more of whose 
cells are dwarfed and wholly abnormal in appearance. An¬ 
other equally striking variation is that the “horns” of the cells 
in one colony differ in length from those of the cells in another 
colony. All conditions, from that of colonies whose cells have 
quite long “horns” (Fig. 18) to that of colonies whose cells 
