Tropic Responses Controlling Leaf Orientation in the Fern Danaea wendlandii (Marattiaceae)

Abstract

Fertile leaves (sporophylls) of the eusporangiate herbaceous layer fern Danaea wendlandii emerge from the shoot apex with a horizontal orientation. Croziers of sterile leaves (trophophylls) emerge from the terminal bud oriented vertically. Sporophylls maintain a horizontal orientation as the leaflets uncoil and the rachis unrolls. This is followed by formation of a petiolar hook and vertical reorientation of the mature sporophyll. Both the vertical orientation of young trophophylls and the horizontal orientation of young sporophylls are gravitropic responses. Neither type of leaf responds to directional light stimulus. Negative gravitropism causing downward curvature of young sporophylls occurs only in the light, and in the absence of light, sporophyll croziers curve upward. Amyloplast sedimentation patterns in cells of the petiole and rachis indicate that a statolith mechanism may be involved in the orientation of young leaves of D. wendlandii.