HARPER: NATURE OF TYPES IN PEDIASTRUM 95 



figure as nearly as possible like the more stable least surface 

 configuration which can be formed with nineteen such units. 

 The adhesion between the walls of the different cells acts in the 

 case of the whole colony quite as the molecular forces are sup- 

 posed to operate in producing surface tension. 



We may take the i6-celled colony of P. Boryanum, which is 

 commonly figured in textbooks, to illustrate the principles of 

 organization common to the whole group. For convenience we 

 may number the series of cells in the colony from the center 

 outward. The central region of the colony may, as noted, show a 

 central group of three or four cells or a single cell. We may 

 number this central group or single cell i, the next outer series 2, 

 etc. We may also number the cells beginning with the central 

 one, as shown in figure ib. As there are in this case twice as 

 many cells in the outer series as in the next inner series (5), and 

 the cells are equal in size, it follows that two of the outer cells 

 subtend in general the same arc as one cell of the inner series, that 

 is, 72°. Five of the outer third series stand radially outward 

 from the five cells of the second or inner series and five cells are 

 inserted with their middle points opposite the points of contact 

 between the cell walls of the second series. 



In the common type of this species three cell walls meet at a 

 point throughout the whole colony. The alternate cells of the 

 outer series 3 are in contact with three or four cells respectively. 

 The cells of series 2 are each surrounded by six cells and the 

 central pentagonal cell by five cells. 



Each cell of series 2 shows a slight re-entering angle on its 

 peripheral surface, indicating its tendency to become bilobed 

 like the peripheral cells. The central cell also shows such a re- 

 entering angle whose vertex is on a straight line through the 

 center of the colony and the surface of contact of cells 2 and 6. 

 It is obvious that the colony is bilaterally rather than radially 

 symmetrical with this line for its axis of symmetry. 



If the cells were spherical in shape, and were nineteen instead 

 of sixteen in number, we should get the simple configuration of a 

 least surface group which represents the maximum of compactness 

 and stability. In a colony of sixteen instead of nineteen cells 

 which as in Pediastrum arises as a group of free-swimming swarm- 

 spores it is obvious that only the most favorable conditions with 



