ON THE NATURE OF TYPES IN PEDIASTRUM 



R. A. Harper 



Colnmhia University 



The Pediastrum group has been perhaps as much as any other 

 in the plant kingdom the subject of divergent judgments as to 

 the number and characterization of its species. Its sharp de- 

 Hmitation from all other genera by the form and life history of 

 the colony and the obvious subdivisions which are suggested by 

 the lobing of the cells were early recognized, but the wide range 

 of variation in cell proportions, number, and arrangement in the 

 colonies, has led to the most diverse opinions as to the number 

 of species which should be recognized. Nitardy's (6) summary of 

 the literature brings out very adequately these divergent views 

 as to what should constitute specific characters in the group. 



Pediastrum is also interesting as a type in which the beginnings 

 of cell differentiation in a coenobic colony are clearly shown. In 

 the common species, P. Boryanum, the cells of the outer series are 

 each provided with a pair of conspicuous spine-like lobes. In 

 some other species the difference between the cells is less marked. 

 We have thus before us incipient stages of differentiation in 

 structure and an approach to a typical metaphytic habit. 



As Meyen (3) observed, the specific organization of the colonies 

 of Pediastrum, as shown in the arrangement of its cells, is most 

 mathematically definite in general plan but shows an almost 

 infinite degree of fluctuating variation in its details. As noted 

 by Naegeli, (4 and 5), Al. Braun (1) and others, the most common 

 form of the i6-celled colony of P. Boryanum shows one cell in the 

 center with five cells around it and ten in the outer circle. The 

 cells are really as nearly as possible in groups of three, which 

 is the most compact arrangement they can assume. At all 

 points of contact, we find three walls converging to a point as in 

 a typical foam structure with one layer of vesicles. Less com- 

 monly, as Naegeli and Braun also observed, there is a central 

 group of 4, 5 or 6 cells with 12, 11 or 10 cells respectively in the 



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