436 HARPER— ORGANIZATION, REPRODUCTION 



Relations of Heredity and Environment to Morphogenesis. — The 

 organization of the colony is determined by the number of the cells 

 resulting always from bipartition, their viscosity, surface tension, 

 mutual attraction and adhesion, their inherited form, whether with 

 one or more spines, and their mutually differentiated polarities by 

 virtue of which they can achieve a definitely oriented relation to 

 each other and thus to the group as a whole. We have here an 

 exhibition of cell qualities influencing morphogenetic processes in 

 such a fashion as to produce a very definite and probably adaptive 

 structural result without any adequate evidence that the characters 

 of the colony as a whole are directly represented in any way in the 

 cells. The form of the colony is achieved in ontogeny as a result 

 of the interactions of the cells. 



The characters of the cells are of two classes. First, metiden- 

 tical as illustrated in the green color, which is transmitted directly 

 by the division of the plastids in which the pigment is borne and, 

 second, characters depending on the organization of the cell as a 

 whole, such as its form, due to its anomogenous consistency from 

 the standpoint of surface tension, its polarities, viscosity, adhesive- 

 ness, etc. These cell characters are transmitted in heredity and can 

 be achieved independently of the position of the cell in the colony or 

 any interrelations with other cells. The internal environment of the 

 cell in the colony may modify its form more or less according to the 

 species hut is not necessary for its typical development. The close 

 approximation to their normal shape achieved by cells in such ir- 

 regular colonies as those shown in figures 26, 27, and 28 and in the 

 malformed young colonies (Figs. 21-24) is striking evidence of the 

 fixity of the form characters of the cells as contrasted with the form 

 characters of the colony as a whole. The two types of characters 

 are in different categories. 



The influence of the external environment on the form char- 

 acters of the colony as a whole is most strikingly shown in the dif- 

 ferences between colonies in which the swarming has been long con- 

 tinued and vigorous and those in which it has been reduced or has 

 disappeared entirely. There can be no question that in general the 

 colonies of Pediastrum are typical in form in direct proportion to 

 the vigor displayed by the swarmspores at the time the colony is or- 



