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

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Abstract
Two distinct root types are borne on submerged nodes of the non‐woody amphibious plant, Ludwigia 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 (O2 = 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.
Funding Information
  • Sigma Xi