X. Tanaecium clade

TAXONOMIC INFORMATION. — This clade contains a single genus, Tanaecium, with four phloem wedges in transversal section and a single species, Tanaecium tetramerum (A.H.Gentry) Zuntini & L.G.Lohmann, that lacks phloem wedges and only shows a regular phloem. Tanaecium currently includes species from seven previously recognized genera (Table 1), Arrabidaea, Ceratophytum Pittier, Pseudocatalpa A.H.Gentry, Spathicalyx J.C.Gomes, Sphingiphila A.H.Gentry, Paragonia Bureau, and Periarrabidaea A.Samp.

TOTAL NUMBER OF SPECIES IN THIS CLADE. — 21 species (Frazão & Lohmann 2019).

STUDIED SPECIES. — Three species, Tanaecium bilabiatum (Sprague) L.G.Lohmann, T. pyramidatum (Rich.) L.G.Lohmann, and T. tetramerum (A.H. Gentry) Zuntini & L.G.Lohmann.

Regular phloem

Thin to thick fiber bands, assemblages generally present (Fig. 13A), but absent in T. tetramerum . Exclusively thin fiber bands occur in Tanaecium bilabiatum and T. tetramerum (Fig. 13D).

Variant phloem

General configuration. Semi-fibrous (Fig. 13B, C) to fibrous, with semi-fibrous species ( Tanaecium pyramidatum) exhibiting fiber bands alternating with 4-8 rows of sieve elements and phloem parenchyma (Fig. 13B, C), and fibrous species ( Tanaecium bilabiatum) showing a matrix of fibers that intermingle all other cells. The axial elements exhibit a diffuse to tangential arrangement (Fig. 13B, C).

Sieve-tube elements. As seen in transversal section, each sieve element is associated with 1-2 companion cells (Fig. 13C). The sieve elements are generally solitary or in multiples of 2-3 cells (Fig. 13C). As seen in tangential section, the sieve elements are short (<1 mm) and their end walls are inclined, bearing compound sieve plates with 16-18 sieve areas.

Axial parenchyma. The phloem parenchyma is sieve-tube-centric incomplete. In Tanaecium pyramidatum, the phloem parenchyma is much more abundant, also forming radial rows (Fig. 13B, C) that may be lignified in some portions (Fig. 13B, C), completely lignified in the nonconducting phloem. In T. bilabiatum, incomplete tangential rows may be found near the sieve elements.

Fibers. In T. pyramidatum the fiber band has 1-2 rows of cells (Fig. 13B, C), while in T. bilabiatum the fibers form a matrix that intermingle all other cells.

Rays. The limiting rays are lignified to both xylem and phloem faces, with a radial row non-lignified between them. The wedge rays have randomly alternating portions lignified and non-lignified (Fig. 13B).

Crystals. Absent.

Periderm

A single periderm is formed. The phellem is stratified in Tanaecium pyramidatum and T. tetramerum, non-stratified and thin-walled in T. bilabiatum (Table 2). The phelloderm is thick, with over three layers of cells, non-stratified. Lenticels were only observed in T. pyramidatum, non-stratified, with lignified filling cells (Table 2).