ORGANIZATION, REPRODUCTION AND INHERITANCE 

 IN PEDIASTRUM. 



(Plates V. and VI.) 



By R. a. harper. 

 (Read April ig, 1918.) 



I have discussed in previous papers ('08, '12) the problems of 

 organization as seen in strictly coenobic plants in which the col- 

 ony shows little or practically no differentiation in either the struc- 

 ture or the functions of its cells. In Pediastrmn we have a type in 

 which at least the incipient steps in differentiation can be recognized. 



The margins of the flat plate-shaped cell colonies are in some 

 species quite entire, in others more or less lobed or toothed, and 

 present the problems of the development and inheritance of specific 

 and differentiated form in plants at a relatively critical stage. We 

 have here the first beginnings of cell, and, we may say, tissue differ- 

 entiation. In such coenobes as most species of Spirogyra, Hydro- 

 die f yon, Goniiim and others, though the colonies have definite and 

 probably adaptive structure, the cells are all alike in form and func- 

 tion, but in certain species of Pcdiastnim the lobed peripheral cells 

 differ markedly from the interior cells of the colony. In other 

 species the lobing is almost equally developed in all the cells. The 

 genus thus presents us with the processes of differentiation in vary- 

 ing degrees of expression in what are plainly rather closely related 

 species. 



I have also discussed elsewhere ('16) the interrelations of the 

 cells in the eight- and sixteen-celled colonies of Pediastrmn Borya- 

 num as giving the basis for a definite conception of plant types and 

 the comparison of this species with the other members of the group 

 brings out still more clearly the idea of biological form types as I 

 have discussed it there. The group is also well adapted to illus- 

 trate the relations of heredity and environment in morphogenetic 



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