CORTICAL AND CAP SEDIMENTATION IN GRAVITROPIC EQUISETUM ROOTS

Although the rootcap is required for gravitropic sensing, various clas sical and contemporary data raise the question of whether additional s ensing occurs away from the cap in roots. Roots of Equisetum hyemale L . (horsetail) were examined by light and electron microscopy to determ ine which cell components were distributed with respect to gravity bot h in and away from the rootcap. Adventitious roots from stem cuttings were gravitropic in a vertical orientation or if reoriented to the hor izontal. Obvious amyloplast sedimentation was found in vertical and in reoriented roots 1) in cells in the center of the rootcap and 2) in y oung, elongating cortical cells located in two to three cell layers ou tside the endodermis. These cortical amyloplasts were smaller than cap amyloplasts and, unlike central cap amyloplasts, were occasionally fo und in the top of the cell. The nucleus was also sedimented on top of the amyloplasts in both cell types, both in vertical and in reoriented roots. Sedimentation of both organelles ceased as cortical cells elon gated further or as cap cells became peripheral in location. In both c ell types with sedimentation, endoplasmic reticulum was located in the cell periphery, but showed no obvious enrichment near the lower part of the cell in vertical roots. This is the first modem report of sedim entation away from the cap in roots, and it provides structural eviden ce that gravitropic sensing may not be confined to the cap in all root s.