426 HARPER— ORGANIZATION, REPRODUCTION 



General Discussion. 



Ceil Form. — There are plainly two sets of form-determining in- 

 fluences operating- on the cells of Pediastnim. First, the cell 

 heredity which, given free play, as in the case of cells largely out of 

 contact with other cells of the colony, develops what we may call 

 the typical cell form. And, second, the environmental pressure and 

 contact relations which exist between the cells as ordinarily placed in 

 the colony. This may result in the extreme difference which we 

 find between the internal and peripheral cells of P. Boryanum or 

 the very slight differences which we find between these same cells 

 in P. dathratum (Fig. 3). The familiar antithesis between heredity 

 and environment in the determination of adult form and structure 

 is fully in evidence in Pediastrum, but under such relatively simple 

 conditions as to permit of an attempt at analysis. It is obvious 

 here, as has been held in general by students of heredity, that 

 inheritance through cell division may perpetuate the type form and 

 structure, while many at least of the fluctuating variations in the 

 type are directly traceable to environmental conditions of interac- 

 tion between the cells and favorable or unfavorable outside condi- 

 tions at the time the colony is formed and during its growth. 



The evidence is clear, as I have shown from the cases of acci- 

 dentally misplaced cells, that all the various cell forms found in the 

 genus are transmitted from one colony to the next by inheritance 

 in some fashion or other. I shall discuss the method of transmis- 

 sion below. The cell form also obviously determines the character 

 of the colony as a whole. The form and character of. the colony 

 as a whole may be said also in turn to influence the form of the cells, 

 but the modifications so produced are of the nature of environmental 

 limitations on the complete development of the cells, as, for example, 

 the shortening of the lobes on the interior cells as compared with the 

 marginal cells of the colony. The position of the cell in the colony 

 influences its form only in minor, though perfectly obvious and 

 definite degrees, but the structural and organic characters of the 

 colonies which are the basis for their classification into subgenera 

 and species are the direct expression of the inherited characters of 

 the cells. 



