100 



MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



similar figures, 6 and 3, 5 and 8, 4 and 7, but with their sides in 

 the same order. They can be conformably superimposed if they are 

 first rotated through 180° on the median axis of the colony, which 

 is a line de through the surfaces of contact between cells 6 and 7 

 and 3 and 4, and then again rotated through 180° on an axis at 

 right angles to this median axis of the colony. 



Fig. 2a. Pediastrum Boryamim, eight-celled colony, X about 750. Original print 

 enlarged about one half in reproduction. 



h. Diagram showing the symmetry relations in such a colony. The dimensions of 

 the cells were measured on a photographic enlargement about twice the diameter of 

 figure a, and the average was taken of the corresponding sides and angles about the 

 axes de and fg. The diagram has been reduced about one fourth in reproduction. 



c. Irregular eight-celled colony of the same species. Two cells not in concentric 

 order. X about 750. 



If the colony is split along its median axis and one half rotated 

 through 180° on a median line, cutting the axis at right angles, 

 it will be bilaterally symmetrical, so far as its outer series of cells 

 is concerned, though its general radial symmetry will have been 

 much disturbed and the two inner cells are di\'ided unequally. 

 These two central cells are in their outlines cciual pohgons and 



