AND INHERITANCE IN PEDIASTRUM. 



413 



pairs in a series of colonies of similar cell arrangement with those 

 derived by averaging the values of the corresponding pairs of angles 

 in a colony selected for its obvious symmetry. 



I have emphasized the difficulty of measuring the angles in pho- 

 tographs of such small objects with the lack of sharpness in the cell 

 walls, especially at points of intersections, etc. Only approxima- 

 tions can be achieved with the technique I have used. The fact 

 that the colonies are not absolutely fiat figures so that the plane of 

 the photograph cuts the colony at different levels and the further 

 fact that the walls are not absolutely vertical to the plane of the 

 colony present difficulties inherent in the nature of the material. 



The average values of the angles measured from the center of 

 the colony which the corresponding cells in the series of seven colo- 

 nies subtend are given in Table VI. The exact determination of 

 the center of the colony is not easy in view of the irregularities of 

 its boundaries. The point taken as the center in each colony was 

 midway between the point of intersection of the major axes of the 

 colony and the point of intersection of the major axes of the central 

 cell. The values obtained from averaging the series like those for 

 the single selected colony No. 55 and those of the corresponding 

 angles in P. asperum (Table II.) are fairly close to 36°, the angle 

 for a strictly surface tension configuration. There can be little 

 doubt that these angles tend to be equal as would be expected were 

 surface tension and adhesion alone operative. 



TABLE VI. 



Dimensions of Cells 7-16 in a Series of Seven Colonies of P. Boryanum 



AS Indicated by their Bases Measured as Arcs of a Circle 



About the Center of the Colony. 



