Wood Anatomy of Noteworthy Species of Ludwigia (Onagraceae) with Relation to Ecology and Systematics

Abstract

Ludwigia anastomosans, a tree to 10 m, is studied because it is unusual in the genus in its arborescent habit. It proves to have unusually wide vessels; it also has interxylary phloem, hitherto reported for only one species of the genus. Wood anatomy of Ludwigia peduncularis and L. torulosa shows that they may be more closely related than so far indicated. Vestigial bars on perforation plates of L. torulosa are the first observed in Onagraceae and are believed to represent an instance of paedomorphosis, but also retention of a primitive feature. Other indicators of paedomorphosis in Ludwigia are abundance of erect ray cells and notably long vessel elements. The hypothesis that degree of vessel grouping is related to ecology in taxa having fiber-tracheids or libriform fibers is validated by Ludwigia, which has the lowest degree of vessel grouping for the family and is essentially aquatic. Other anatomical features reflective of ecology, combined in the Mesomorphy ratio, present a not dissimilar pattern that can be integrated with that given by vessel grouping if one takes into account probable transpiration rates and temperature regimes as well as water availability. Data on wood anatomy of eight species of Ludwigia have been presented earlier (Carlquist, 1975, 1 982a). That number seems small unless one takes into account the fact that Ludwigia is predominantly herbaceous; the most familiar species are nonwoody herbs of very wet habitats such as ponds, ditches, and streams. One of the species in the present study is a notable exception: L. anastomosans (DC.) Hara is a tree. The data on the collection studied here describe it as a tree 10 m tall with a trunk 15 cm dbh. Wood anatomy is of special interest because of this habit. In fact, the results obtained below demonstrate that the wood of L. anastomosans differs appreciably from that of other Ludwigia species. Dr. Elsa Zardini, who collected the material of L. anastomosans, also kindly supplied material of two other species because she wished to see if wood anatomy demonstrated the degree of relationship between them. Dr. Zardini has contemplated the idea that L. peduncularis (Griseb.) Gomez may be closely related to L. torulosa (Am.) Hara. The wood anatomy of Ludwigia is of considerable interest with respect to ecology because Ludwigia characteristically grows in very wet places. Ludwigia anastomosans was collected in bamboo clumps by a blackwater stream in the Parque Natural de Caraca, Minas Geraes, Brazil. The material of L. peduncularis came from ditches near Havana, Cuba. The L. torulosa specimen was collected in a natural pond 17 km south of Tumeremo, Distrito Roscia, Estado Bolivar, Venezuela. In taxa with libriform fibers such as Onagraceae, Carlquist (1 984a) hypothesized degree of vessel grouping to be in direct proportion to adaptation to dry conditions. In this case Ludwigia species ought to exhibit a low degree of vessel grouping. Although figures for vessels per group were developed in the earlier survey of woods of the family (Carlquist, 1975), no comparisons were made between those figures and ecological regimes occupied by the various species. Ludwigia is a group of interest with relation to paedomorphosis in wood anatomy. This is, in turn, related to habit and ecology. The species in the present study were examined in this context to see if woodiness in Ludwigia is primary or secondary. Although a study on wood anatomy can be expected to reveal new records for anatomical features, two in the present study proved of especial interest and worthy of discussion: occurrence of interxylary phloem and presence of vestigial bars on some perforation plates. I Grants from the National Science Foundation to Peter H. Raven made possible the collection and selection of the material for this research. 2 Rancho Santa Ana Botanic Garden, Claremont, California 91711, U.S.A., and Department of Biology, Pomona College, Claremont, California 9171 1, U.S.A. ANN. MISSOURI BOT. GARD. 74: 889-896. 1987. This content downloaded from 207.46.13.151 on Fri, 25 Mar 2016 08:49:45 UTC All use subject to http://about.jstor.org/terms 890 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 74 MATERIALS AND METHODS Voucher specimens are located at the Missouri Botanical Garden. Appreciation is expressed to Dr. Elsa Zardini for providing dried wood samples suitable for study. For L. peduncularis and L. torulosa, samples represented basal portions, but diameter was small (2 and 4 mm, respectively). The material of L. anastomosans was from a branch about 1.5 cm in diameter. Although this is much less than the 15 cm diameter reported for trunks of this species, -the branch material is considered here to represent an essentially mature pattern. Woods were boiled in water, stored in 50% ethyl alcohol, and sectioned on a sliding microtome. Sections prepared in this way were, in part, satisfactory, but cell collapse on account of thinness of wood cells was excessive in some instances. Therefore an alternative method involving further softening, embedding in paraffin, and sectioning on a rotary microtome (Carlquist, 1 982b) was employed. Sections prepared by both techniques were stained in a safranin-fast green combination. Macerations were prepared with Jeffrey's Fluid and stained with safranin. Means are based upon 25 measurements (fewer if feature is scarce) except for vessel wall thickness, libriform fiber diameter, and libriform fiber wall thickness, in which a few typical cells were measured. Vessel diameter includes the wall, although lumen diameter may be preferable for some purposes and may be calculated by subtracting wall thickness from the data presented here. Mean values for vessel grouping are obtained on the following basis: a solitary vessel = 1.0, a pair of vessels in contact = 2.0, etc. Bark was not observed specifically in the present study, although sections of the stem of L. torulosa included portions of the spongy stem covering that proves to be aerenchyma like that figured for Jussiaea repens L. (now a Ludwigia) by Metcalfe & Chalk (1950), and studied in detail by Ellmore (1981) for L. peploides (HBK) Raven. ANATOMICAL DESCRIPTIONS Ludwigia anastomosans (DC.) Hara, Zardini & Gentry 2175 (Figs. 1-6). Growth rings inconspicuous, and probably related to water level of the riparian habitat (Fig. 1). Mean number of vessels per mm2, 51. Mean number of vessels per group, 1.24; vessels tending to be grouped into radial multiples (Figs. 1, 3). Mean-vessel diam., 109 gim. Mean vessel wall thickness, 2.5 gim. Mean vessel element length, 458 Aim. Perforation plates simple (Figs. 2, 4). Lateral wall pitting of vessels alternate, pits crowded and circular to polygonal in outline, about 12 Am diam. on vessel-vessel interfaces (Figs. 4, 5). Vessel-axial parenchyma and vesselray pitting alternate to scalariform, pit apertures long (sometimes scalariform), pit apertures wide (gaping). Pitting with relatively conspicuous vesturing on vessel-vessel pits (Fig. 5, upper right), vesturing less pronounced on vessel-axial parenchyma and vessel-ray pits. Occasional vessel-vessel pits in aberrant patterns (Fig. 6). Tyloses abundant (Figs. 1, 3). Imperforate tracheary elements all libriform fibers because pits apparently simple, although a few exceptional pits with small borders also observed. Many libriform fibers septate. Mean libriform fiber diam., 28 gim. Mean libriform fiber wall thickness, 2.3 ,im. Mean libriform fiber length, 588 gim. Numerous libriform fibers with gelatinous walls (Fig. 6) and therefore probably reaction wood. Axial parenchyma-vasicentric scanty. Bands of phloem-containing axial parenchyma present in marginal positions (end of-growth rings) or scattered without any relation to growth rings (Figs. 1, 3). Rays multiseriate anduniseriate, the former slightly more frequent (indicated by relative heights in Fig. 2). Ray cells predominantly erect and square (Fig. 2), a few procumbent cells present in multiseriate portions of multiseriate rays. Mean multiseriate ray height, 2,707 gim. Mean uniseriate ray height, 349 gim. Mean width multiseriate rays at widest point, 3.95 cells. Ray cell walls moderately thin, lignified. Wood nonstoried. Raphides present in phloem-containing axial parenchyma strands. Ludwigia peduncularis (Griseb.) Gomez, Ekman 13416 (Figs. 7, 8). Growth rings absent (portion studied probably less than one year's accumulation). Mean number of vessels per mm2, 62. Mean number of vessels per group, 1.30. Mean vessel diam., 50 gim. Mean vessel wall thickness, 1.8 gim. Mean vessel element length, 484 gim. Perforation plates simple. Lateral walls of vessels with crowded alternate elliptical pits (about 4 x 8 gin) with pointed ends and narrow apertures (vessel-vessel contacts). Vessel-axial parenchyma and vesselray pitting similar but with pits longer (appearing pseudoscalariform) and wider, and with wider This content downloaded from 207.46.13.151 on Fri, 25 Mar 2016 08:49:45 UTC All use subject to http://about.jstor.org/terms 19871 CARLQUIST-WOOD ANATOMY OF LUDW IGI. 891