Root Hairs, Cuticle, and Pits.

The filamentous roots of mustard (Raphanus sativus), radish (Brassica nigra), squash (Cucurbita pepo), and wheat (Triticum aestivum) are covered throughout their length with living nucleated root hairs which may measure 1600 micro or more. The outer walls of piliferous and nonpiliferous cells consist of successive layers of mucilage, cutin, and the cellulose-pectic framework of the cell. Plasmodesmata and pits occur on all cell walls. Under the electron microscope individual pores and pits in the microfibrillar wall are evident throughout the length of the root hair. The "semipermeable membrane" of the root hair zone is thus structurally complex.