392 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
plasticity involves, in the case of a slime mold, not so much a loss of particu¬ 
lar potentialities as the organization of the expression of potentialities into 
a more nearly rigid series. 
Ill 
The changes involved in the evolution of coenobic species may be simi¬ 
larly outlined. No sharp line exists between temporary colonies which, 
for the period of their existence, are coenobes, and the more durable colo¬ 
nies that characterize some flagellates as well as many derived forms. So 
far as concerns the potentialities upon which the formation of a coenobe 
depends, perhaps little strictly new appears at this evolutionary level, 
because the prototype of each method of colony-formation (instance those 
of Scenedesmus, Hydrodictyon, Tetraspora, and Spirogyra) is to be found 
among the flagellates. Changes have occurred which render the cell less 
likely to respond to environmental changes by separation from its fellows. 
The phases characterized by an independent existence have been subordi¬ 
nated. Especially is this true of the flagellate phase, which in some lines, 
such as the Zygnemaceae, has dropped entirely out of the ordinary life 
cycle. That a particular phase does not regularly appear in the life cycle 
is not, to be sure, proof of the total loss of the potentialities necessary to 
its appearance. This fact is shown by the ability of the cells of Tetraspora 
and Stigeoclonium under some conditions to pass into an amoeboid phase 
in which they are capable of holozoic nutrition. 
The type of coenobic colonial plant that proved best adapted to further 
evolution on this planet involves the continued close contact of sister cells 
after each of a series of divisions. The existence of the colony, in general, 
depends upon the formation of a persistent rigid layer between the sister 
cells at the time of, or shortly after, their formation. This layer may in 
some cases constitute the final thickness of the partition wall. Much 
more commonly, probably, it is supplemented by the deposition of addi¬ 
tional layers on either side; the original layer then corresponding, in his¬ 
tory, position, and function, to the middle lamellae of the larger green 
plants. The formation of a persistent partition wall depends upon the 
ability to secrete, under definite conditions, a certain substance or certain 
substances. Sometimes the original layer is easily ruptured, as in the 
common yeasts. But in the evolution of persistently colonial plants the 
middle lamella, if once fragile, has become more stable. 
In a coenobe, such as Spirogyra, every vegetative cell has, throughout 
its active life, the full range of potentialities of every other cell. Any cell 
may remain vegetative, grow, and divide, or may become a gamete or an 
azygospore. When, in a coenobe, a cell responds to particular stimuli by 
taking on the characters of a gamete or of a spore, it becomes differentiated 
from the vegetative cells; but it can still transmit its full original equip¬ 
ment of potentialities to its offspring although it may itself be debarred 
