Root Hair Zone of Soil-grown Roots

Abstract

PATHOGENS which invade the roots of higher plants from the soil, in general, enter through the root hair zone of the seedling or the feeder roots of the older plant. Recent examination of certain common soil-grown species of dicotyledons and monocotyledons, namely, broad bean (Vicia faba), castor bean (Ricinus communis), mustard (Brassica nigra), purslane (Portulaca oleracea), radish (Raphanus sativus), squash (Cucurbita pepo), willow (Salix sp.), onion (Allium cepa), and wheat (Triticum aestivum), indicates that the description of the root hair zone in current text-books is inadequate and requires considerable modification in respect to the extent of the region of absorption and the details of tissue structure¹. In present-day diagrams of the primary root, the root hair zone is invariably short. The xylem elements are devoid of contents. It is generally stated that the epidermis of the root differs from that of the shoot in the absence of a cuticle. It is somewhat difficult to reconcile the limitation of the absorptive region with the extent of the root systems which have been examined in detail by numerous investigators². In rye grass, for example, the estimated total length of the 4.5 × 10⁹ root hairs amounts to 6,603 miles, with a surface area of 432 sq. ft. (ref. 3).