Figure 12
The postulated fibril reorientation-slippage model undergoing large elongation rates during (a) steady growth and (b) the phototropic response. The black dashed lines in (a) represent the hypothesized direction of the microfibrils at different locations of the growth zone in a fast growing sporangiophore. It is envisioned that the microfibrils are deposited in a somewhat transverse direction (deposition zone) and begin to reorient toward the longitudinal direction because of the longitudinal deformation that occurs during elongation growth (reorientation zone). Rotation is produced in the reorientation zone. In a fast growing sporangiophore it is envisioned that the fibrils reorient to the longitudinal direction in the lower section of the growth zone and can only slide by each other without producing rotation (slippage zone). During the phototropic response (b), it is hypothesized that there is an increase in the rate of deposition of microfibrils (red dashed lines) and the deposition zone is increased in size (length). The increase in the deposition zone produces an increase in the length of the reorientation zone that extends into the fibril slippage zone. This in turn produces an increase in rotation rate during the phototropic response.
