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AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
tion is from time to time profoundly modified, the transition from one phase 
to another seeming to occur on relatively slight provocation. Such plas¬ 
ticity is especially striking in view of the present very imperfect knowledge 
of most species; indeed, it is not certain that all the phases into which any 
one species may pass have yet been recognized. A comparable plasticity, 
it may be observed, characterizes other protistan groups; recent reports, 
for example, indicate the occurrence in bacteria of comparable and quite 
remarkable modifications in cell size and cell structure. The pigmented 
flagellates are selected for present mention because of their apparent phylo¬ 
genetic significance. 
In entering upon and in passing through a particular phase (such as 
the amoeboid), the cell of a flagellate race performs certain functions. 
The ability to perform each of these functions results from the chemical 
and physical make-up of the living matter or of some of its parts. Since 
all known living matter is organized into cells, the possibilities or potentiali¬ 
ties that inhere in the constitution of the living matter may be spoken of 
as the potentialities of the cell. Each function that the cell or one of its 
parts performs is the expression of a potentiality. 
The expression of certain potentialities, particularly of those concerned 
in the more fundamental processes, is common to two, or to several, or to 
all phases. But the potentialities manifested in one phase differ, as a 
group, from those manifested in another phase. Smaller differences exist 
between different stages, for example, of the amoeboid phase, as to the 
particular group of potentialities momentarily functional, or as to the 
degree or manner of expression of particular potentialities. Some poten¬ 
tialities can be expressed only in conjunction with certain others; some are 
mutually exclusive; and from the latter fact arises the possibility of dis¬ 
tinct phases in the history of the race. A list of the potentialities of any 
cell, therefore, would be in effect a list of all the functions,the capacity for 
which is inherent in the living matter of that cell. 
A distinction may be drawn between an actual change in the form and 
activities of a cell—as the transition from a flagellate to an amoeboid 
form—and the origin from the cell, by division, of cells of a different form— 
as in the production from a plasmodium or a multinucleate cyst of numerous 
flagellate offspring. In either case, the race passes from one phase to an¬ 
other; the individual cell may not. Obviously a cell can transmit to its 
offspring only the potentialities which it possesses; and hence the poten¬ 
tialities of all phases possible to the race are possessed by any cell in any 
phase; even though the cell, in a particular phase, is itself debarred from 
expressing some of its inherent potentialities. 
This fact, that a cell possesses potentialities which, having reached a 
certain phase, it can not itself express although it can transmit them, 
calls attention to the widely recognized dual constitution of living matter. 
The heritable potentialities arise from what may be called, non-commit- 
