Morphology and ultrastructure of the gravity-sensitive leaf sheath base of the grass Echinochloa colonum L.

When a flowering stalk of Echinochloa colonum is held horizontally, growth is initiated in the lower side of each leaf sheath base, restoring the inflorescence to an upright position. Changes in the gravity vector are perceived by specialised statolithcontaining tissue which is associated with each of the symmetrically-arranged vascular bundles within the leaf sheath bases. The morphological and ultrastructural features of these gravity-sensitive regions have been examined by light and electron microscopy. Each statocyte cell contains a large central vacuole with a thin lining of cytoplasm. Up to 50 spherical starch statoliths lie along the lowermost side of the cells and these sediment readily following geotropic stimulation. Statoliths are found in contact with the plasmalemma, or may be prevented from touching it by bands of microtubules. Dictyosomes and mitochondria are numerous, but endoplasmic reticulum is sparse. The nuclei tend to remain at the original apex of each cell. Statocytes of the leaf sheath base are compared and contrasted with those of the root tip.