394 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
be about equally differentiated. Differentiation beyond its most ele¬ 
mentary stages evidently depends also, and more largely, upon an increas¬ 
ing susceptibility of cells to relatively slight differences in stimuli; the 
increased susceptibility brings about the expression of noticeably different 
potentialities by cells differently situated. Probably also quite early in 
evolutionary development—very certainly on a large scale at a later stage— 
there is an increase in the range of potentialities at the disposal of the cell, 
and hence also in the variety of possible life histories. 
The secretion of a persistent partition wall between sister cells makes 
a colony possible, but of course does not determine the form of the colony. 
The form, whether unbranched or branched, filamentous or plate-like, 
depends in part upon the expression of various potentialities concerned 
with the relative growth of each cell in its respective axes and with the 
plane or planes in which division occurs. Some of these potentialities are 
among the new evolutionary developments: and, since species differ in 
these potentialities, they differ in their characteristic forms. 
V 
When, in evolutionary course, the potentialities appeared whose ex¬ 
pression resulted in the development of a massive plant—one with cell 
divisions in more than two planes—the possibility of differentiated cellular 
development led to the formation of tissues. The internal cells of a mas¬ 
sive plant are shielded from the immediate influence of conditions outside 
the plant, although indirectly, of course, still much affected by those con¬ 
ditions. The internal cells are subjected to an environment, a very import¬ 
ant and immediately influential part of which consists of the surrounding 
cells. The variety of stimuli, including pressures and tensions and elec¬ 
trical and chemical changes, which act upon different cells of the plant is 
thus greatly increased, and there is made possible an increase in the variety 
of cellular differentiations. It is to be expected, then, that, quite apart 
from the acquisition of new potentialities, the arrangement of potentialities 
into alternative life histories should be carried further than in a filamentous 
or plate-shaped plant whose cells, while by no means uninfluenced by stimuli 
proceeding from neighboring cells, are all alike in touch with the world 
outside. Any increase in the size and complexity of the plant magnifies 
the importance of the interaction of its cells. In a large measure, as Good¬ 
rich has expressed it, the higher organisms have gradually substituted 
internal for external stimuli. 
Every newly formed cell of a massive plant is embryonic, in the sense 
that it is capable of division. In general, an embryonic cell has certain 
structural characters; but the presence or absence of these characters does 
not affect its essentially embryonic nature. By the division of the embry¬ 
onic cells in a primary meristem, followed by later divisions of some of 
their still embryonic derivatives, all the cells are provided which are to 
