harper: nature of types in pediastrum 99 



Cells 7-16. 



Angles: 



d'eo = 136.5° + oV^2 = 144-5° + 

 0V08 = 132° = 413° -^ 3 = 137° 



oVo^ = 132° + o^e'o^ = 149 = 281° -^ 2 = 140°* 133° 



odr = 125° + o^i/'i = 127° + o'^d'r'' = 127° + 

 o7^7;.7 = 134° + oS^V« = 125° + o'dh' = 

 122° = 760 ^ 6 = 126° 



pdr = 126° + p'd^r^ = 120° + /j^^^V^ = 131° + 



pi ^7 ^7 _ 129° + pWr^ = 128° + ;^9C?V9 = 



129° = 763° H- 6 = 127° 



p^d^r^ — 122° + pH^r*^ = 125° = 247° -^ 2 = 123° 



^3^3^3 = J38° + o8(iV« = 123° = 261° ^ 2 = 130° 



o4^4^ = 120° + o'^d^r'' = 126° = 246° H- 2 = 123°* 



131° 

 p^d^r* = 136° + p^d^'r^ = 120° = 256 -i- 2 = 128° 



Sides: 



r = g mm. + ''^ = 9-5 mm. -\- r'^ = g mm. + 

 1^ = 8.5 mm. + ?''' = 8.5 mm. + r^ = 9.5 mm. + 

 r^ = 8.5 mm. + r^ = 7 mm. + r^ = 9 mm. + 

 r^ = g mm. = 87.5 4- 10 => 8.7 mm. 



Similarly, figure 2b may be considered as representing the type 

 of cell configuration found most commonly in the eight-celled 

 colonies of P. Boryanum (fig. 2a). The eight-celled, like the six- 

 teen-celled group, Is in unstable equilibrium, owing to the number 

 of Its cells (eight Instead of seven). A group of seven spheres, six 

 around a central one, gives a much higher degree of compactness 

 and a more stable Intercellular equilibrium than can be achieved 

 In a plate made of eight spheres. There is a considerable tendency 

 for one of the cells to slip out of the concentric system. In the 

 colony represented In figure 2c, two of the cells are so displaced. 



The grouping of the cells In such an eight-celled colony shows a 

 fine series of adjustments between mutual pressure, adhesion, 

 surface tension and the Inherited form tendencies of the cells in a 

 system which Is numerically disharmonic from the standpoint of 

 the principle of least surfaces. 



In the more typical colonies, such as the one shown In figure 2a, 

 the outer series of six cells forms In reality three pairs of spatially 



