No. 466.] STUDIES ON PLANT CELL.— Vill. 71I 
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 Rhodophycez as in Nemalion and Batracho- 
spermum whose cells hold a single large chromatophore while 
most of the more highly organized red alge have numerous 
plastids. A beautiful series of stages illustrating the evolu- 
tionary principles outlined above might be worked out in the 
Phaophyce=. 
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- 
