AND INHERITANCE IN PEDIASTRUM. 397 



ture of any given colony of Pediastrum conforms to its type in so 

 far as the environment permits. The multiplicity of divergences 

 from the type form for the species is an index of the varying de- 

 grees of favorableness in the surroundings of the parent colony and 

 mother cells. The relations of unstable equilibrium between the 

 units of a group of sixteen or thirty-two, as compared with a group 

 of nineteen or thirty-seven, give unusual opportunity for the play 

 of such environmental influences. 



Fluctuating Variations in the Intercellular Relations in 

 THE Colonies of P. asterum and P. Boryanum. 



The evidence from the above account of asexual reproduction is 

 clear that the colony of Pediastrum is formed by the interaction of 

 a group of free-swarming zoospores without the possibility of any 

 predetermination of its form as such in the arrangement of the parts 

 of the mother cell. A cell can apparently fill any place in the group 

 which forms the young daughter colony. A colony under favorable 

 conditions may attain the rounded outline of a typical least-surface 

 configuration for such a group of cells or under less favorable con- 

 ditions it may conform more or less wholly to the outline of the 

 mother cell even to the extent of remaining two-layered. I have 

 also described above the general arrangement of the cells and inter- 

 cellular spaces in a typical adult colony of P. aspcnim, and we may 

 now turn to the question as to the kind and degree of variability 

 which the colonies as they are found in nature exhibit. 



In the continuous disk of the typical sixteen-celled colony of P. 

 Boryanum I have shown ('i6) that the angles of intersection of the 

 cells walls vary considerably in dififerent parts of the colony, the 

 correspondingly placed angles right and left of the axis of the col- 

 ony tending to be equal. The angles of contact in any fairly typical 

 sixteen-celled colony of P. asperum (Fig. 6) are, so far as I am able 

 to determine, quite constantly 120°. The difficulties of measure- 

 ment here are much greater than in P. Boryanum, since the presence 

 of intercellular spaces reduces the surfaces of the cells in contact and 

 the length of the lines by which the angles are measured. Small 

 variations are no doubt quite regularly present, but they are within 



