Department of Botany, University of Washington, Seattle, WA 98195
Abstract We have earlier published observations showing that
endogenous alterations in growth rate during gravitropism in maize
roots (Zea mays L.) are unaffected by the orientation of cuts which
remove epidermal and cortical tissue in the growing zone (Björkman
and Cleland, 1988, Planta 176, 513-518). We concluded that the
epidermis and cortex are not essential for transporting a
growth-regulating signal in gravitropism or straight growth, nor for
regulating the rate of tissue expansion. This conclusion has been
challenged by Yang et al. (1990, Planta 180, 530-536), who contend that
a shallow girdle around the entire perimeter of the root blocks
gravitropic curvature and that this inhibition is the result of a
requirement for epidermal cells to transport the growth-regulating
signal. In this paper we demonstrate that the entire epidermis can be
removed without blocking gravitropic curvature and show that the
position of narrow girdles does not affect the location of curvature.
We therefore conclude that the epidermis is not required for transport
of a growth-regulating substance from the root cap to the growing zone,
nor does it regulate the growth rate of the elongating zone of
roots.