238 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
niceties of adjustment, but the grouping as first achieved by the 
free-swimming swarm-spores must be admitted to be a matter of 
cellular interactions and the major stimuli in such a series of 
adjustments must be contact and pressure. That chemotropism 
could play a róle is hardly conceivable in view of the violent 
movements of the swarm-spores in the narrow confines of the 
mother vesicle. That in the last stages of swarming equilibrium 
should be reached in a situation of as nearly equal pressure and 
contact from all sides as is possible may seem to some to be 
merely a matter of physical necessity operating on what, from 
the conditions in the adult colony, might seem to be inert 
gelatinous four-lobed or one-spined masses, but in the fact that 
this equilibrium position is achieved by a group of freely swimming 
organisms, each with inherited cell-form tendencies which are 
certain to come to expression in greater or less degree, no matter 
how the cell is finally placed in the group, we find the proof that 
nothing less than the assumption of a capacity to respond to and 
maintain conditions of equilibrium when once achieved can ade- 
quately account for the symmetry of the typical colonies as we 
find them. 
That the symmetrical spatial interrelations of the cells is no 
mere expression of the direct action of the physical principles of 
surface tension, adhesion, mutual pressure, etc., is further shown 
by the endless number of variations from the type configuration. 
There is good evidence here of trial and error by complex organisms 
with every type of error as well as degree of approximation to the 
typical fixed in the endless variations in detail which can be found 
in the configuration of the adult colonies. It is difficult to give 
an adequate picture of what one sees in watching the free-swim- 
ming swarm-spores darting here and there around and through 
the mass and the gradual appearance of order out of confusion 
with the coming to rest first of a peripheral series and then of the 
interior cells, but that cellular interactions of sensitive tropic 
units determine the symmetrical final configuration rather than 
the direct operations of surface tension, adhesion, etc., on the one 
hand or any mysterious controlling and adaptive principle of 
behavior seems to me the obvious suggestion from the facts. The 
evidence seems to me adequate for assuming a high degree of 
