July, 1923 ] 
ALLEN — POTENTIALITIES OF A CELL 
393 
from expressing certain potentialities. A similar limitation, it has been 
seen, may be experienced by a one-celled organism when it has passed into 
particular phases. It follows that there is no difference in inherent poten¬ 
tialities as between the constituent cells of a coenobe. Harper has shown 
that in Hvdrodictyon the potentialities of the equipotent cells determine, 
with the interaction of the environment, the character of a comparatively 
large and definitely constituted plant. He has shown the same to be 
true of Pediastrum in which some differentiation of cells appears, in well 
as of so complex a plant as Dictyostelium with its extensive cellular dif¬ 
ferentiation. 
IV 
The transition from coenobes to plants with differentiated cells, like 
that from unicellular organisms to coenobes, is gradual. Differentiation 
in structure and function implies the expression by different cells of differ¬ 
ent groups of potentialities; it results directly, as is obvious in the filamen¬ 
tous algae and fungi, from differences in the conditions surrounding the 
respective cells. All the cells of the plant are still equipotent; and any 
cell possesses all the potentialities of all the cells—it is totipotent—as is 
shown by the fact that any cell, at least while young, may give rise, in one 
way or another, to a complete new plant. The potentialities common to 
all the cells are, as it were, under the influence of the environment, sorted 
out and arranged into alternative life cycles; or it would be better now to 
say life histories , since some of the alternatives may not lead to reproduc¬ 
tion and hence to a repetition of the story. But the differentiation takes 
place in the life of each cell. All the cells are, at the start and throughout 
at least most of their history, alike in their potentialities; all are, in Weis- 
mann’s phrase, potentially immortal. 
There may come a time in the history of a vegetative cell when it can 
no longer divide, and hence can under no circumstances originate a new 
plant. Obviously the power of reproduction is lost in the changes which 
precede the death of the cell; but it is possible that this loss, in some types 
of cells, anticipates the appearance of any degenerative changes. Cer¬ 
tain of the alternative histories may, therefore, lead in time to a condition 
in which the cells upon which those alternatives were forced lose their 
immortality. One-celled organisms may likewise, in some environments, 
pass into a condition in which reproduction can not occur. The possi¬ 
bility of a condition of this sort is thus not new to the constituent cells of 
a many-celled organism; but in the latter case some of the cells necessarily 
lose their reproductive power as a result of their position in the plant. 
Some degree of differentiation must result from a mere increase in size 
of the colony; for the larger the colony the more varied are the conditions 
to which its different cells are exposed. If, however, cellular differentia¬ 
tion were a function of size alone, Rhizoclonium and Draparnaldia would 
