AND INHERITANCE IN PEDIASTRUM. 433 



cesses by which the plate-shaped colony of one cell layer in thick- 

 ness and with the longer spines of the cell directed radially out- 

 ward is formed. 



We are confronted here as in practically all higher plant cells 

 with polar differentiation in the cells themselves. The question as 

 to whether this polarity is developed, as is true in so many cases, 

 under the influence of light or gravitation acting during develop- 

 ment would seem to be answered in the negative for Pediastrum. 

 So far as my observations go, the young colonies are formed in the 

 mother cells at all angles to the direction of the light falling on them 

 and in their orientation to gravitational stimuli. In those cases of 

 rather weak swarming which I have described as forming the ob- 

 long instead of circular colonies the shape of the mother cell un- 

 doubtedly exerts an influence on the form of the colony. But there 

 is no evidence that the flattened form of the mother cell prevents 

 the swarmspores from making a group of two layers in thickness 

 or an oval mass. The observation of the swarming masses as I 

 have described them suggests most forcibly that, as noted above, 

 the cells actively seek out a position in which they are in equal pres- 

 sure and contact relations with the adjacent cells right and left of 

 them and with the side which is to develop the large spines or spine 

 radially outward and not vertically upward or downward, involving 

 again in the case of the interior cells contacts with a cell or cells 

 on their basal and peripheral surfaces. No other hypothesis than 

 that of at least a biaxial polarity in the internal organization of the 

 swarmspores seems adequate to account for this definite orientation 

 of the cells in the colony. The existence of the cell axis radial to 

 the colony as a whole is of course most conspicuous owing to the 

 difference in development of the peripheral and basal lobes, but the 

 assumption of a polar differentiation in the tangential axis is equally 

 necessary. The existence of physical polarities or polar differentia- 

 tions in such complex organisms as the swarmspores with their 

 nuclei, plastids, cilia, etc., is, of course, to be expected. A purely 

 physical factor which may have had phylogenetic significance in the 

 development and fixation of radial polarity is present in the diff'er- 

 ing lateral pressures to which the basal and peripheral regions of the 

 cells in such a plate-shaped group are subjected. Such differences 



