THE EPIDERMAL SURFACE OF THE MAIZE ROOT-TIP .1. DEVELOPMENT IN NORMAL ROOTS

The surface of the meristematic epidermis of maize roots is tri-partite. A helicoidal primary wall follows the contours of the tops of the columnar epidermal cells and is continuous with their buttressed anticlinal walls. Two overlying layers form a smooth covering over the root which obscures the cell outlines. This compound surface is similar architecturally to outer epidermal surfaces of shoots. The two outer layers are distinct structurally and in their staining properties from the wall and are together here referred to as the pellicle. Both pellicle layers are fibrillar but not helicoidal. Their development begins in the boundary between the cap and the root proper and they reach maximum thickness over the meristematic region. The outer layer then disintegrates and is absent from the elongation zone. The inner layer thins irregularly as the columnar cells elongate to their final tabular form and usually persists only over the groove above anticlinal walls and at the base of root hairs. The cell wall thins to about half its maximum thickness during this elongation. Emerging root hairs broach the pellicle and the original primary wall. Remnants of both these layers form a short, tight collar at the base of each hair; this collar adheres to the primary wall of the hair which is continuous with a new, thin wall which is formed interior to the original outer wall of the parent cell. Failure to recognize the complex structure and transitory nature of the epidermal pellicle has led to confusion in the literature regarding the nature of root-surface and rhizosphere mucilaginous components and their origin. These interpretations are compared with those arising from this study.