Phloem wedges in Malpighiaceae: origin, structure, diversification, and systematic relevance

Abstract

Background: Phloem wedges furrowing the wood are one of the most notorious, widespread types of cambial variants in Angiosperms. Many lianas in Malpighiaceae show these variations in the arrangement of the secondary tissues. Here we explore their ontogeny, structure, and evolution in Malpighiaceae, where phloem wedges appeared multiple times, showing how they have contributed to the anatomical diversification of the family. Using a broad sampling with 143 species from 50 genera, covering all major lineages in Malpighiaceae, we crossed data from ontogeny, stem anatomy, and phylogenetic comparative methods to determine ontogenetic trajectories, final anatomical architectures, and evolution within the most recent phylogeny for the family.

Results: Phloem wedges appeared exclusively in lianas and disappeared in shrub lineages nested within liana lineages. At the onset of development, the vascular cambium is regular, producing secondary tissues homogeneously across its girth, but soon, portions of the cambium in between the leaf insertions switch their activity producing less wood and more phloem, initially generating phloem arcs, which progress into phloem wedges. In the formation of these wedges, two ontogenetic trajectories were found, one that maintains the continuity of the cambium, and another where the cambium gets dissected. Phloem wedges frequently remain as the main cambial variant in several lineages, while in others there are additional steps toward more complex cambial variants, such as fissured stems, or included phloem wedges, the latter a novel type of interxylary phloem first described for the family.

Conclusions: Phloem wedges evolved exclusively in lianas, with two different ontogenies explaining the 10 independent origins of phloem wedges in Malpighiaceae. The presence of phloem wedges has favored the evolution of even more complex cambial variants such as fissured stems and interxylary phloem.

Keywords: Cambial variant; Discontinuous cambium; Lianas; Limiting rays; Malpighiaceae; Phloem; Phloem wedges; Vascular cambium; Xylem.

Fig. 1

Terminology adopted for phloem wedges. A Stem with phloem wedges well-developed, two regions are identified: interwedges (Iw) and phloem wedges (W). Interwedges are the areas between phloem wedges and where the cambium maintains its regular activity. Phloem wedges are regions derived from a variant cambial portion that produces more secondary phloem and less secondary xylem. B Phloem wedge (W). Phloem wedges are commonly flanked by limiting rays (Lr), which are tall, wide multiseriate rays. C Phloem wedges well-developed with cambium disrupted. Mechanical pressure causes the inclusion of inner portions of wedges, known as interxylary phloem (Ip). D The transformation of flanking regions of the wedge from a regular to a variant activity causes wedges to have a stepwise pattern. Magenta dotted line outlines this pattern. Portions of the variant cambium (Cp) remain on the bottom of the wedges. Color key: black = pith limits, blue = secondary phloem, grey = dermal tissue, red = vascular cambium, orange = limiting rays

Fig. 2

Ancestral character state reconstruction of lianescent habit and phloem wedges in Malpighiaceae. Trees show the occurrence of lianas (left) and stem with xylem furrowed by phloem wedges (right) in Malpighiaceae. Pie charts on nodes show the posterior probabilities of every state. For habit reconstruction (left), the purple color represents self-supporting plants (trees, shrubs, and sub-shrubs) and yellow represent climbing plants. For phloem wedges reconstruction (right) purple color represents the absence of phloem wedges and yellow its presence. Climbing plants appear twice in Malpighiaceae while phloem wedges appear ten times. Most of the phloem wedges are concentrated in tetrapteroids and stigmaphylloids, except for Tristellateia (Bunchosia clade) and Hiraea (Hiraea clade)

Fig. 3

The external appearance of stems with xylem furrowed by phloem wedges. A, B Heteropterys cordifolia. A Thick stem of H. cordifolia holding onto adjacent trees and its own younger branches. B Cross-section of H. cordifolia stem showing the match of stem depressions with a phloem wedge (arrows). C–E Mascagnia sepium. C Phloem wedges can be detected morphologically since they generate concavities visible in the stem surface (arrow). D Phloem wedges match with the leaves in the stem (arrow showing leaf scar), and progress in a spiral fashion. E Coincidence of a stem depression with the development of a phloem wedge (arrow)

Fig. 4

Two different ontogenetic trajectories are described in stems with xylem furrowed by phloem wedges in Malpighiaceae. A Ontogeny 1. Stems with phloem wedges that keep a single and continuous cambium (red line). After phloem wedges keep growing, these can be further developed into fissured stems (ontogeny 1a), or the tissue produced by regular cambia adjacent to the wedges may exert mechanical pressure including portions of the wedge (ontogeny 1b), this interxylary phloem keeps reminiscences of the active vascular cambium (black arrows). B Stems with phloem wedges that disrupt the cambium continuity and include portions of phloem wedges, resulting in interxylary phloem (black arrows). Color key: black = pith, blue = secondary phloem, grey = cortex, red = vascular cambium

Fig. 5

Light micrographs of stem cross-sections in Malpighiaceae showing phloem wedges diversity. All sections were double-stained in safranin and Astra-blue. A Peixotoa sericea with 6 phloem wedges. Note xylem under the phloem wedges is different from that of interwedges, with fewer vessels and more fibers. B Mascagnia sepium showing approximately 15 phloem wedges some deeper than the others, because of their different formation onset times. C Niedenzuella multiglandulosa showing 8–12 phloem wedges of different ages, some being deeper than the others D Stigmaphyllon blanchetii showing 6 phloem wedges generating marked concavities in the stem. E Mascagnia divaricata showing at least 4 well-formed phloem wedges. F Alicia anisopetala showing 9 well-formed phloem wedges. G Tristellateia greveana showing 8 phloem wedges, without apparently forming depressions in the stem. H Tristellateia australasiae showing 8 well-formed phloem wedges slightly marking concavities on the stem. Images not to scale

Fig. 6

Light micrographs show the formation of phloem wedges that preserve a continuous cambium (Ontogeny I). All sections were double-stained in safranin and Astra-blue. A–D Development of the stem of Mascagnia sepium. A Onset of development with regular secondary growth. B Shallow arcs of phloem are formed approximately 6 to 8 shallow arcs are visible. C Phloem arcs get deeper and turn into phloem wedges, some deeper than the others, given their different time of onset formation. D Adult stem with deep phloem wedges that furrow the xylem, some deeper than the others. E Mezia mariposa phloem wedges (arrowheads). F Peixotoa sericea with phloem wedges with different stages of development, some deeper than others (yellow dotted line and arrowheads), phloem wedges match with outer depressions of the stem (black arrowhead). G Niedenzuella multiglandulosa cambium is continuous throughout the phloem wedge (Pw), evidenced by the formation of inclined cells of phloem and xylem (yellow dotted lines and arrowhead). H M. sepium showing the formation of xylem and phloem perpendicularly to the previous tissues formed in the stem (yellow dotted lines and arrowhead), evidencing that the cambium is continuous, i.e., delineates the entire phloem wedge (Pw). I Mascagnia divaricata variant cambium produces under the phloem wedges (Pw) larger bands of non-lignified parenchyma (nlp) (arrowhead) than regular cambium. J Peixotoa sericea xylem under the phloem wedge (Pw) contains fewer vessels and more fibers, phloem wedges match with outer depressions of the stem (black arrowhead). K Mascagnia sepium parenchyma proliferation (pa) under the phloem wedges (Pw) proliferates (arrowhead) breaking up the inner xylem, stimulating proliferation also of the pith (p), in a continuum. Scale bars A–C, I–J = 1 mm, D = 5 mm, E, F = 200 µm, G, H = 400 µ, K = 500 µm

Fig. 7

Light micrographs showing phloem wedges inclusion in cross-sections. All sections were double-stained in safranin and Astra-blue. A, B Niedenzuella multiglandulosa. A The inclusion of wedges portions is given by the mechanical pressure exerted by the adjacent wood at both sides of the phloem wedges (W) (yellow dotted lines and arrowheads). B Once the portion of the phloem wedge is included (Ip), the adjacent regions of the wedges are in contact (yellow dotted lines and arrowheads). Scale bars A = 1 mm, B = 500 µm

Fig. 8

Light micrographs of stem cross-section showing phloem wedges formation with cambium disruption (Ontogeny II). All sections were double-stained in safranin and Astra-blue. A–D Development of phloem wedges in Tristellateia greveana. A Onset of development with a single, continuous cambium with regular activity. B Formation of shallow phloem invaginations, i.e., the onset of differential production between secondary xylem and phloem. C Shallow phloem arcs turn into phloem wedges. D Final architecture with well-formed phloem wedges and phloem inclusion. E–I Tristellateia greveana. E Onset of cambium disruption in the wedge region (W), evidenced by the formation of inclined cells of xylem (yellow dashed lines) and cells with a parallel course to those produced by regular cambium, limiting rays throughout wedges (arrowhead). F Since early stages it is possible to see the disruption between regular cambium (yellow dashed lines) and variant cambium (black dashed lines). G Phloem wedges with a stepwise pattern (yellow dashed lines). Note that the wood produced by the variant cambium in the wedge region (W) is like that produced by the regular cambium in the interwedge region (I). H The wood that flanks the phloem wedge (Pw) begins to exert pressure on both sides of the wedge (yellow dashed lines and black arrowheads). I Mechanical pressure eventually embeds a portion of the phloem wedge (Ip) within the wood (arrowhead), the adjacent regions of the wedges are in contact (yellow dashed lines), the interxylary phloem (Ip) has minute reminiscences of variant cambium. Scale bars A–D = 1 mm E–I 200 µm