ROOT DIMORPHISM IN LUDWIGIA PEPLOIDES (ONAGRACEAE): STRUCTURE AND GAS CONTENT OF MATURE ROOTS

Two distinct root types are borne on submerged nodes of the non-woody amphibious plant, Ludit'igia peploides (HBK) Raven. The anatomy of downward-growing roots (DGR) and the negatively geotropic upward-growing roots (UGR) demonstrates that structural elements shared by roots can be altered to produce highly modified organs differing in morphology, growth orientation, and in function. Both root types emerge contemporaneously from nodes and both have a similar pattern of cell lineages at the pole of the procambial cylinder. Differences between tissues of both roots become conspicuous with increasing distance from the root-cap junction. In the distal cap of UGR, cells are spherical with much space between them unlike the more familiar tightly packed elements of DGR caps. In DGR, an uninterrupted epidermis surrounds the compact cortex and the hexarch stele occupies 25% of the root diameter. During primary growth, the DGR axis remains thin. In UGR, an open meshwork of cells covers the entire root surface, including the cap. The small stele has three or four xylem poles and fills only 5% of the UGR diameter. The unbranched axes of UGR swell and become spongy as strutlike arms radiate from cells of the primary cortex. Gas-filled space between the arms comprises 88% of the axis volume in mature UGR. Although connections between shoot lacunae and spaces in UGR cannot be seen in the light microscope, gas can pass (25 ml/h) between nodes submerged in anaerobic substrate (02 =0. 15 ppm) and UGR which reach the water surface. The porous axis of UGR thus provides a conduit for atmospheric gases into nodes which in turn produce the roots, vegetative branches, and reproductive shoots of this species. PLANT ROOTS are diverse in appearance, anatomy, growth pattern, and function (Guttenberg, 1968) and there are many examples where roots of different forms are borne on a single plant. In extreme cases of dimorphism, root meristems can reorganize to produce shoots as in Selaginella (Webster and Steeves, 1967) and possibly in Nasturtium (Ballade, 1970) and Anthurium longifolium (Goebel, 1878). More commonly, roots on a single plant remain as roots but develop striking variations in form and behavior. Mangroves, many climbing aroids, and amphibious plants such as Typha and Ludwigia bear roots differing in external appearance and direction of growth (Troll, 1943; Guttenberg, 1968; Bristow, 1975; Jenik, 1978). This study was carried out in order to document a striking example of how root anatomy may diversify, i.e., to determine at least one structural basis for variation in root form. Additionally, results are used to evaluate previous ideas about root I Received for publication 5 April 1980; revision accepted 22 September 1980. I am grateful to Drs. Donald R. Kaplan, Lewis J. Feldman and Watson M. Laetsch for their advice and many discussions. This work was partially funded by a Sigma Xi Grant-in-Aid of Research. 2 Present address: Department of Biology, Tufts University, Medford, Massachusetts 02155. function in this dimorphic system (Hartsema, 1927). Ludwigia peploides is a prostrate amphibious plant anchored in water-logged soil. It was chosen for this study because of its vigorous growth and the rapid development of two distinct root types from submerged nodes. While one form resembles the seminal roots of Zea mays, the other grows upward and develops a swollen axis which fills with gas. In this study, the structure of both root types is documented in order to demonstrate the anatomical features characteristic of each ioot form. These findings define the mode by which a similar structural plan shared by roots can be altered, producing highly modified organs differing in appearance, growth orientation, and presumably in physiology. Subsequent contributions will consider the development of each root type from primordia embedded within nodes and the role of dimorphic roots in the biology of this semi-aquatic plant. MATERIALS AND METHODS-Histology Roots were collected from nodes of shoots growing in warm water (20-25 C) of the Sacramento River Delta 4 miles west of Stockton, California. Roots growing at similar rates (1 cm per day) were cut from nodes (Fig. 2, 3), fixed in FAA, dehydrated in ethanol, and in-