Pleonotoma

Pace, Marcelo R., Marcati, Carmen R., Lohmann, Lúcia G. & Angyalossy, Veronica, 2023, Bark anatomy of lianescent Bignoniaceae: a generic synopsis, Adansonia (3) 45 (12), pp. 167-210 : 182

IV. Pleonotoma clade

TAXONOMIC INFORMATION. — This clade contains a single genus, Pleonotoma , with four phloem wedges in transversal section.

TOTAL NUMBER OF SPECIES IN THIS CLADE. — 16 species ( Lohmann & Taylor 2014; Gomes et al. 2020).

STUDIED SPECIES. — Five species, Pleonotoma dendrotricha Sandwith , P. melioides (S.Moore) A.H.Gentry , P. orientalis Sandwith , P. tetraquetra (Cham.) Bureau , and P. stichadenia K.Schum.

Regular phloem

Fibers are absent, with sclereids found otherwise ( Fig. 9A ). Sclereids are arranged in clusters, differentiating close to the cambial zone ( Fig. 9A, C ). Because these sclereids have tapering ends, they are considered fibersclereids ( Fig. 9C ). Assemblages present ( Fig. 9B arrowhead).

Variant phloem

General configuration. Fibrous ( Fig. 9D ) to semi-fibrous ( Fig. 9E ), typically with one tangential row of sieve tubes surrounded by phloem parenchyma and fibers ( Fig. 9E ), presenting, therefore, a tangential arrangement ( Fig. 9E ).

Sieve-tube elements. As seen in transverse section, each sieve element is associated with 1-3 companion cells ( Fig. 9E, F ). The sieve elements are generally in multiples of two, ranging from solitary to multiples of 4-5 in a tangential arrangement ( Fig. 9E ), sometimes in clusters. Sieve elements of two distinct diameters can be found adjacent to each other ( Fig. 9E ). Sieve elements of narrower radial diameter can be, moreover, found associated with a parenchyma band.As seen in longitudinal section, the sieve elements are long (> 1 mm) and their end walls are inclined, bearing sieve plates with more than 30 sieve areas.

Axial parenchyma. The phloem parenchyma is sieve-tube-centric to sieve-tube-centric incomplete ( Fig. 9E, F ). A parenchyma band of 3-4 rows of cells is associated with narrower sieve elements.In addition, radial rows of phloem parenchyma cells are often present, sometimes lignified ( Fig. 9E ).

Fibers. Present as either fiber bands of 2-3 rows of cells ( Fig. 9E ) or as the fibers constituting the ground tissue where all other cells are embedded ( Fig. 9D ).

Rays. The limiting rays are lignified only to the xylem face ( Fig. 9E ). The wedge rays are non-lignified, even when crossing the fiber bands ( Fig. 9E, F ).

Crystals. Prismatic crystals are abundant, both in the phloem and ray parenchyma ( Fig. 9F ), and even more common in the ray cells. The number of crystals per cells is variable, ranging from a single crystal per cell that occupies almost the entire cell lumen, to many (c. eight), but much smaller crystals per cell ( Fig. 9F ).

Periderm

A single periderm formed. The phellem is stratified, composed of thin and thick-walled cells in alternation. The phelloderm is thick, with over three cell layers, non-stratified ( Table 2 ).

Figures

FIG. 9. — Secondary phloem of Pleonotoma: A, B, D-F, transverse sections; C, longitudinal radial section; A-C, E, F, Pleonotoma tetraquetra; D, Pleonotoma melioides; A, overall view of the regular phloem.Sclerenchyma formed exclusively by sclereids,differentiating next to the cambial zone,in the conducting phloem; B, detail of the conducting phloem, sieve tubes are very narrow. Sclereids differentiating. Acicular crystals present in axial and ray parenchyma; C, heterocellular rays. Sclereids differentiating from axial parenchyma cells, close to the cambium. Mature sclereids also present; D, overall view of the phloem wedge, fibrous phloem; E, variant phloem marked by rectangular fibers,sieve tubes solitary or in radial and tangential multiples of 2-3, parenchyma sieve-tube-centric; F, detail of variant phloem. Sieve tubes with one or two companion cells lying on the same side of the sieve tube. Axial parenchyma sieve-tube-centric.Prismatic crystals present in both axial and ray parenchyma cells. Scale bars: A, C, 200 μm; B, F, 50 μm; D, 500 μm; E, 100 μm.