harper: nature of types in pediastrum 97 



The three angles about the points of Intersection of the contact 

 surfaces of each group of three cells tend to equal 120° each, but 

 with marked deviations owing to the number and inherited form 

 of the cells. For example, the interior angles of the central cell 

 at points g^ and g^ are smaller than the adjacent interior angles 

 of cells 3, 4, and 5. This is due to the tendency in the central 

 cell to assume a two-lobed form and for the same reason the 

 interior angles of cell 4 at these same points g^ and g^ are respec- 

 tively larger than the adjacent angles of cells 3 and 5. The exact 

 value of such differences cannot be determined without a knowl- 

 edge of the strength of the tendency in the cells toward the bi- 

 lobed form. I have made the values of the three angles 87.2°, 

 134° and 138° respectively^ by taking the average of the corre- 

 sponding pairs as the bilateral symmetry of the colony requires. 



Correlated with this tendency of the cells to become bi-lobed the 

 interior angles of cells 2, 3, 4, 5, and 6 at the points d, d}, d'-, etc., 

 are smaller than the adjacent angles of cells 7, 8, 9, 10, etc., the 

 differences here being about 20°. 



In the diagram the lobed form of the outer surfaces of the 

 cells is merely sketched in free-hand. These cells represent 

 most nearly the inherited form tendencies of the individual since 

 from their peripheral position they have the largest proportion 

 of free surface and the fullest opportunity to bring to expression 

 those inherited tendencies which are not dependent upon contact 

 with or pressure from adjacent cells. The lobes of these peri- 

 pheral cells are longer but even in P. Boryanum the cells of series 2 

 and the central cell show clearly enough the tendency to the bi- 

 lobed form. The influence of internal environment in modifying 

 inherited form is especially well illustrated in such a case as this. 



The dimensions of the cells and their interior angles in the photo- 

 graph and in the diagram are given in the following table. In each 

 case the sides and angles are taken in order beginning on the axis 

 of the colony. The most marked asymmetry in this colony is in 

 the region of cell 4, especially about the point e^ and I have given 

 arbitrary values to certain angles such as will conform to the re- 

 quirements of the general radial symmetry of the colony. The 

 four cases in which these arbitrary values have been assigned are 

 indicated by asterisks in the table. The area of cell 5 in optical 

 section as shown in the photograph is greater than that of cell 4, 



