AND INHERITANCE IN PEDIASTRUM. 431 



the difference between the marginal and interior cells of P. Bory- 

 anum. The cells all have the same inherited growth tendencies, the 

 differences between them are due to their respective positions in the 

 colony. Such differences are the expression merely of a certain 

 degree of susceptibility to contact and pressure stimuli which, for 

 example, w^e can think of as greater in P. Boryatmm than in P. as- 

 perum. It is certainly more natural to think of such degrees of 

 sensitiveness as characters dependent on the growth reactions of 

 cells as wholes rather than on the presence or absence of a particu- 

 lar chromosomal granule or region. 



The organization of the leaf of Elodea with its spinous mar- 

 ginal projections affords an interesting parallel in the higher plants. 

 It would be difficult for anyone to conceive that any cell of the leaf 

 might not form a typical spinous projection if it were properly 

 placed on the margin instead of in the interior of the leaf. 



We can distinguish then in such cases as those of Pcdiasfrum 

 three grades or degrees of directness with which the hereditary 

 transmission of characters is accomplished. First, direct transmis- 

 sion by division, associated with the division of the germ cell, of 

 the particular structural element whose possession constitutes the 

 character, as for example the transmission of green color by chloro- 

 plasts. Such a character is metidentical, that is, it is the same thing 

 in the germ cell as in the many-celled organism as a wdiole. Second, 

 the more indirect transmission of the characters of the diff'erentiated 

 adult cells, which are not visibly present as such in the germ cell. 

 Examples are the lobed form of the adult cells of Pediastrum. In 

 this case the conceptions here involved can be easily extended to the 

 higher plants. The stellate pith cells of the bulrush and the elon- 

 gated thick-walled cells of wood and bast can be thought of as in 

 the same category as to heredity and ontogeny as the lobed cells of 

 Pediastrum. Such characters express what we recognize as the or- 

 ganization of the cell as a whole including cytoplasm and nucleus, 

 and are hardly to be conceived as represented in any particular part 

 or, organ of the cell. Third, the entirely indirect transmission of 

 the characters of the many-celled organism as a whole, in Pedi- 

 astrum, such characters as the plate-shaped form of the colony, the 

 presence or absence of perforations, the arrangement of the cells, 



