HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 287 
and the capacity of the cells to respond to pressure and contact 
stimuli are not fundamental properties present in full degree in 
the ancestral types of the group but that these characters have 
increased and become specialized with the gradual development 
of the highly modified and lobed form of the cells. Simple ad- 
hesion of the cells in a palmelloid mass may have been the initial 
stage in colony formation. Light reactions may have favored the 
development of the plate-like expanded form though this is 
achieved now by the polar differentiation and reactions of the 
swarm-spores quite independently of the direction of the light. 
These reactions to pressure and contact and the resulting form 
determinations are typical examples of biogenetic processes in 
ertwig's sense. It is quite possible that such reactions may be 
the determining factors in the root behavior which led Noll (200) to 
assume morphaesthesia as a fundamental phase of morphogenetic 
behavior. Morphaesthesia is for Noll the expression of the 
capacity of lateral roots to regain a radial direction of growth 
after they have been forced out of it by an interposed obstacle— 
radial not to the point of origin of the root from the main axis 
but radial to the axis from the point at which the root becomes 
free from the obstacle. The capacity to regain such a generalized 
relation as that of the radius from any point of the axis opposite 
to which the root happens to be certainly implies a response to 
form-determining stimuli of the most delicate sort. Noll was 
inclined to regard it as a sort of direct reaction to the form of the 
whole organism by each of its parts. The only physical stimuli 
which seem to be involved are the pressure and contact interrela- 
tions involving weight relations, tensions due to bending, etc., 
between the cells themselves. 
Whether or not such reactions are adequate to account for the 
radial growth of lateral roots with their much greater complexity 
of structure, there can be no question, it seems to me, that the 
assumption of a fundamental capacity to achieve symmetry is 
the natural suggestion from a study of the delicately balanced 
interrelations of the cells in such types as the sixteen-celled colonies 
of P. clathratum and the eight-celled ring-shaped colonies of P. 
simplex. Direct action of surface tension on the plastic though 
anomogenous cell bodies may account, as noted, for the final 
