July, 1923 ] 
ALLEN — POTENTIALITIES OF A CELL 
395 
constitute the tissues of the plant. The individual destiny of the cells 
formed in the meristem is evidently in no sense predetermined. Whether 
any such cell is to become a highly differentiated element of the xylem, 
phloem, or epidermis, or is to retain its embryonic character as an element 
of the meristem, cambium, pericvcle, or cortex, depends upon its position, 
and therefore upon the stimuli which affect it. The same statement holds 
for the cells formed in the cambium or in any other embryonic region. 
A material alteration of conditions, as by a wound, by the incursion of a 
parasitic fungus, or even by a change in the position of the plant, may 
result in radically changing the course of development of a cell, even after 
it is well started upon the processes of differentiation. The life history 
of the cell, under the changed conditions, is shifted to a course quite dif¬ 
ferent from that which a cell in its position would have followed under 
ordinary circumstances. The facts of development under both ordinary 
and exceptional conditions thus indicate that all the cells of a complex 
as of a simpler plant are, at least while embryonic, equipotent and toti¬ 
potent. 
A special case of a shift in the life history of a cell or of a group of cells 
as a result of changed conditions is seen when new organs or new plants 
are produced by regeneration. So extensive is the power of regeneration 
in bryophytes that one is almost tempted to say that any cell, at any time 
during its active existence, can give rise to a complete plant. In the vas¬ 
cular plants the power of regeneration is widespread also; witness the 
varied forms of vegetative multiplication, regeneration from leaves, the 
production of adventitious shoots and roots. In some cases regeneration 
involves the dedifferentiation of cells already well advanced in their de¬ 
velopmental history. But it seems clear—though perhaps not yet rigidly 
demonstrated—that in a vascular plant each type of differentiating cell may 
reach a point in the course of its development beyond which, although it 
is still alive and functional, dedifferentiation and division are impossible. 
It is evident that such a state has been attained by “mature”—that is, 
dead—tracheids and vessels, as well as by the still living but enucleate 
cells of sieve tubes; but it seems to be true that in these and in other tissue 
cells there are conditions—among others, much thickened walls-—which 
effectively inhibit division even before differentiation has run its full course. 
The statement of the primary totipotence of all the cells of a plant 
must be modified by the admission that occasional irregularities in the 
working of the mitotic mechanism may result in an unequal distribution 
of potentialities. There is both cytological and genetic evidence that 
these mitotic accidents occur, perhaps not infrequently. But the possibility 
of such disturbances does not affect the validity of the general law of equi- 
and totipotence. 
Among the metazoa an unequal distribution of potentialities is perhaps 
regularly affected in some cases; for example, by a discarding of part of 
