Ludwigia ravenii (Onagraceae), a New Species from the Coastal Plain of the Southeastern United States

Abstract

Ludwigia ravenii, a uniform new species with scattered distribution in the Coastal Plains from Virginia south to northeastern Florida, belongs to the diverse sect. Microcarpium and bears a striking superficial resemblance to L. pilosa and, to some degree, L. sphaerocarpa. Careful morphological study and field observations reveal that they differ significantly in floral details and in breeding system. Ludwigia ravenii is one of eight tetraploid taxa in sect. Microcarpium, which is a polyploid complex. The other seven tetraploid taxa are interfertile; there are presumably no genetic barriers between L. ravenii and them. In the course of a biosystematic study of Lud- wigia L. sect. Microcarpium Munz, a number of atypical specimens initially identified as L. pi- losa Walter were discovered. Although they su- perficially resemble L. pilosa in general aspect and in the presence of hirtellous pubescence, detailed analysis of these plants reveals signif- icant differences in sepal shape and color, sta- men and style size, and microscopic seed char- acters, and supports their recognition as a new species. 0.3-0.35 mm longae; filamenta 0.7-1.1 mm lon- ga. Pollen in tetradis effundata. Discus nectarii viridis. Stylus 0.25-0.5 mm longus; stigma 0.25- 0.45 mm crassum. Capsula oblongo-ovoidea, 4- 5(3-5.3) mm longa, 2-3.5(-4) mm crassa. Brac- teolae (1.5-)2-4.3 mm longae. Semina 0.5-0.7 mm longa, 0.25-0.35 mm crassa, cellulis super- ficialibus oblongis transverse elongatis. Auto- compatabilis, plerumqute autogogama. Nume- rus chromosomaticus gameticus, n = 16. Erect herb perennating by stolons; stems