No. 466.] STUDIES ON PLANT CELL VIII. 711 



various directions. The only striking exceptions to this broad 

 principle are Anthoceros, whose cells contain each a single large 

 chromatophore, and Selaginella. Selaginella is especially inter- 

 esting for, while the cells of the meristematic region and young 

 organs contain but a single chromatophore, this structure may 

 divide later in some types to form a chain of discoid plastids in 

 older cells connected with one another by delicate strands of 

 protoplasm. Thus in the life history of certain species of Sela- 

 ginella we have plainly shown the change from a single chroma- 

 tophore to a number of plastids. It seems probable that this 

 history repeats in general outline the evolutionary history of the 

 condition characterized by numerous plastids within a cell from 

 a primitive type of cell structure with but a single chromato- 

 phore. Anthoceros and Selaginella may be regarded as forms 

 whose cells still retain the primitive conditions with respect to 

 the single large chromatophore. There are somewhat similar 

 illustrations in the: Rhodophycese as in Nemalion and Batracho- 

 spermum whose ^ells hold a single large chromatophore while 

 most of the more highly organized red algse have numerous 

 plastids. A beautiful series of stages illustrating the evolu- 

 tionary principles outlined above might be worked out in the 

 Phseophycese. 



What is the fundamental principle underlying the substitution 

 of numerous plastids in a cell in place of a single chromato- 

 phore } The author believes that it must have relation to the 

 preservation within large cells of a certain balance of the meta- 

 bolic centers. The fission of a plastid is a process of constric- 

 tion and studies on Anthoceros (Davis, '99, p. 94) indicate 

 that the bounding cytoplasmic membrane exerts pressure' upon 

 the elongating structure. It seems probable that the division is 

 due to the mechanical separation of material that is too bulky 

 for the most effective results of photosynthesis which in the 

 case of a single chromatophore are centered in a particular 

 region of the cell. By the division of a chromatophore into 

 numerous plastids the photosynthetic activities are distributed 

 among several centers and a much better balance results within 

 the cell. It is very interesting that the large elaborate chroma- 

 tophores with their peculiar internal differentiations, the pyre- 



