Light responses of phycomyces.

The various growth spurts and bends obtained in simple programs or irradiation can be interpreted, up to a point, in terms of optics, symmetry, and the cell's intrinsically regulated growth. Simple kinetic models imitate many gross features of the responses, and imply that light catalyzes some step in the formation or extension of the cell wall. This step may well be a nearly terminal one, perhaps involving slippage of chitin chains over one another. The most pervasive concept in these studies is that of adaptation, of which the steady-state level is primarily set by the light flux. But, since adaptation is tested by a growth response, every "dark" process contributing to cell enlargement is also implicated. From these sources comes the prompt negative feedback after an increase in light flux, as well as the conservation of growth seen in bending. Useof light by the plant seems to be only for operation of a crude guidance system for spore dispersal. For the investigator, light is a tool that displaces or unsteadies the mechanisms of cell extension and gives glimpses of their otherwise-concealed complex interplay.