Autotropism, automorphogenesis, and gravity.

Segments of organs that have undergone gravitropic curvature later straighten during the course of gravitropism or after the g-vector becomes randomized on a clinostat. Little is known about the mechanism underlying these and perhaps related phenomena which have been described with various overlapping terms such as autotropism, autotropic straightening, automorphosis, automorphogenesis, automorphic curvature, and gravitropic straightening. The types of phenomena that historically have been named by the above terms are reviewed critically with respect to an interaction with gravitropism. We suggest that the term "autotropism" should not be applied to the phenomenon of organ straightening that occurs during the course of gravitropism, since this straightening is part of a complex series of local growth adjustments overall through time, and since this phenomenon is not itself a tropistic response to a directional exogenous stimulus. It is suggested that the term autotropism should be used only for the phenomenon of organ straightening that occurs after the g-vector is randomized on a clinostat or withdrawn in the microgravity conditions of spaceflight. Usage of the term automorphogenesis is most appropriate for describing curvatures or orientations that result from morphological relationships such as in nastic curvatures.