IX. Fridericia - Xylophragma clade

TAXONOMIC INFORMATION. — This clade includes species from two genera, Fridericia and Xylophragma, both with four phloem wedges in transversal section. Fridericia currently includes species from four previously recognized genera (Table 1), Arrabidaea, Fridericia, Piriadacus Pichon, and Sampaiella J.C.Gomes. In turn, Xylophragma includes species previously included in Arrabidaea and Xylophragma .

TOTAL NUMBER OF SPECIES IN THIS CLADE. — 67 species belonging to Fridericia (60) and Xylophragma (7) (Lohmann & Taylor 2014; Kaehler et al. 2019; Kaehler & Lohmann 2021b, 2022).

STUDIED SPECIES. — 12 species, Fridericia chica (Bonpl.) L.G.Lohmann, F. cinnamomea (DC.) L.G.Lohmann, F. conjugata (Vell.) L.G.Lohmann, F. nigrescens (Sandwith) L.G.Lohmann, F. ornithophila (A.H.Gentry) L.G.Lohmann, F. patellifera (Schltdl.) L.G.Lohmann, F. platyphylla (Cham.) L.G.Lohmann, F. samydoides (Cham.) L.G.Lohmann, F. speciosa Mart., F. triplinervia (Mart. ex DC.) L.G.Lohmann, Xylophragma myrianthum (Cham.) Sprague, and X. platyphyllum (DC.) L.G.Lohmann.

Regular phloem

Thick fiber bands, assemblages present.

Variant phloem

General configuration. Semi-fibrous (Fig. 12B) to fibrous (Fig. 12A, C), with semi-fibrous species ( Fridericia chica, F. conjugata, and F. speciosa) (Fig. 12B), and fibrous species ( Fridericia chica, F. ornithophyla, F. patellifera, F. platyphylla, F. samydoides, Xylophragma myrianthum, and X. platyphylla) showing a matrix of fibers that intermingle all other cells (Fig. 12C, D). Interestingly, in Fridericia chica we found specimens with both a semi-fibrous and a fibrous configuration. The sieve tubes and associated phloem parenchyma exhibit a tangential arrangement (Fig. 12 A-D).

Sieve-tube elements. As seen in transverse section, each sieve element is associated with one or two companion cells (Fig. 12B), rarely three, at the same side of the sieve element (Fig. 12B). The sieve elements may be solitary to multiples of 2-3 (Fig. 12 B-D), in a diffuse to tangential arrangement (Fig. 12 A-D). Such arrangement results from the way that the multiple sieve elements are organized, forming short radial groups in some places, and short tangential groups in others; while some groups form clusters of 3-4 cells, most groups exhibit a tangential tendency (Fig. 12 B-D). Narrower sieve elements with just one companion cell may be found in a parenchyma band in most species (except Fridericia platyphylla and F. speciosa). As seen in longitudinal section, the sieve elements are long (over 500 µm) and their end walls are inclined, bearing sieve plates with more than 20 sieve areas (Fig. 12E, F).

Axial parenchyma. The phloem parenchyma is typically sieve-tube-centric, surrounding the groups of sieve elements (Fig. 12 B-D). Furthermore, in most species (except Fridericia platyphylla and F. samydoides) a parenchyma band of 2-3 cells of phloem parenchyma is associated with sieve elements of radial narrower diameter (Fig. 12C arrows).

Fibers. In semi-fibrous species the fiber bands are waved (Fig. 12B), with borders displaying 5-6 cells and the middle portion displaying 1-2 cells. In fibrous species, on the other hand, the fibers intermingle all other cells with no distinct arrangement (Fig. 12C, D).

Rays. The limiting rays are lignified to both xylem and phloem faces, with a radial row non-lignified between them (Fig. 12A). The wedge rays have randomly alternating portions lignified and non-lignified (Fig. 12B). In Fridericia conjugata the rays are only lignified when crossing the fibers.

Crystals. Acicular and navicular crystals are present in the phloem and ray parenchyma, both in lignified and non-lignified cells, across the entire phloem wedge.

Periderm

A single periderm is formed in Fridericia nigrescens, F. platyphylla, F. speciosa, F. triplinervia, and Xylophragma myrianthum (Table 2). Sequent periderms (rhytidome) are formed in Fridericia chica, F. conjugata, and F. samydoides (Table 2). The phellem is non-stratified, evenly thin-walled in Fridericia chica, F. conjugata, F. samydoides, and Xylophragma myrianthum (Table 2). The phellem is non-stratified, evenly thick-walled in Fridericia platyphylla, F. speciosa, and F. triplinervia . The phellem is stratified in Fridericia nigrescens . The phelloderm is thick, non-stratified in Fridericia chica, F. conjugata, F. nigrescens, F. speciosa, F. triplinervia, and Xylophragma myrianthum (Table 2). The phelloderm is stratified in Fridericia platyphylla and F. samydoides (Table 2). Lenticels are non-stratified in Fridericia speciosa and Xylophragma myrianthum; stratified in F. samydoides (Table 2). Lenticels were not observed in the remaining species.