The epidermal surface of the maize root tip: II. Abnormalities in a mutant which grows crookedly through soil.

The mutation Ageotropic (Agt) results in defective development of the surface pellicle which overlies the young epidermal cells of mesocotyl and nodal roots of maize. In roots of plants of the parent, cv. Kys, pellicle development is normal. This structure forms a coherent, smooth covering, up to ∼ 13/μm thick, over the epidermal cells distal to the elongation zone. This pellicle is external to the helicoidal outer wall of the epidermal cells and is composed of a thick inner layer with close-packed, longitudinally oriented fibrils, and a thin outer layer with less regularly oriented fibrils. Both layers of the normal pellicle and the underlying epidermal wall are strongly stained by the periodic acid-Schiff's (PAS) reaction. In Agt plants, the pellicle is irregular and diffuse, lacks definition of the two layers and is largely amorphous with occasional wisps of fibrils. Neither this mutant pellicle nor the underlying epidermal wall are PAS-positive. Mesocotyl and nodal roots of Agt are sensitive to gravity but grow crookedly through soil. It is proposed that a normal pellicle maintains the smooth outer contour of young roots and provides stiffness in the region distal to the zone of elongation. When the pellicle is defective, the root tip is more compliant and bends result when the tip encounters small air spaces and barriers in the soil.