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(1) Background: the cambium has seasonal activity, forming earlywood and early phloem with relatively wide conducting cells, which will function during the most favorable season, and latewood and late phloem with narrower conducting cells, which typically function during the less favorable season. However, few studies have focused on when these two contrasting tissue types are formed in relation to climatic conditions. (2) Methods: the senior author of this paper made weekly collections for an entire year of four specimens per collection back in the 1960s, using traditional anatomical methods to study in detail what the cambium was producing progressively. (3) Results: annual growth rings are evident in both secondary xylem and secondary phloem. The cambium resumes activity in early April, with simultaneous formation of wood and secondary phloem. Both latewood and late phloem production are initiated in early June, the peak of the favorable season. The cambium ends its activity in early August. Phloem growth rings are marked by radially narrow sieve elements interspersed among a band of axial parenchyma with dark contents. Most specimens produce only one fiber band per season. This feature may be used as an indirect phloem growth ring marker. Wood growth rings are marked by very wide vessels and thick-walled, radially narrow fibers. (4) Conclusions: growth rings are evident in both secondary xylem and secondary phloem. The trees produce their latewood and late phloem long before the beginning of autumn, indicating that they prepare ahead of the selective regime, a phenomenon most likely dependent on the photoperiod. Living sieve elements are present yearlong.

Evolutionary developmental biology (evo-devo) explores the link between developmental patterning and phenotypic change through evolutionary time. In this review, we highlight the scientific advancements in understanding xylem evolution afforded by the evo-devo approach, opportunities for further engagement, and future research directions for the field. We review evidence that 1) heterochrony-the change in rate and timing of developmental events, 2) homeosis- the ontogenetic replacement of features, 3) heterometry-the change in quantity of a feature, 4) exaptation- the co-opting and repurposing of an ancestral feature, 5) the interplay between developmental and capacity constraints, and 6) novelty- the emergence of a novel feature, have all contributed to generating the diversity of woods. We present opportunities for future research engagement, which combine wood ontogeny within the context of robust phylogenetic hypotheses, and molecular biology. This article is protected by copyright. All rights reserved.

Premise of research. The alternative patterns of secondary growth (vascular cambial variants) in stems of Nyctaginaceae are outstanding and have been widely investigated since the late nineteenth c...

The liana genus Paullinia L. is one of the most speciose in the neotropics and is unusual in its diversity of stem macromorphologies and cambial conformations. These so-called “vascular cambial variants” are morphologically disparate, evolutionarily labile, and are implicated in injury repair and flexibility. In this study, we explore at the finer scale how wood anatomy translates into functions related to the climbing habit. We present the wood anatomy of Paullinia and discuss the functional implications of key anatomical features. Wood anatomy characters were surveyed for 21 Paullinia species through detailed anatomical study. Paullinia woods have dimorphic vessels, rays of two size classes, and both septate and non-septate fibers. Fibriform vessels, fusiform axial parenchyma, and elements morphologically intermediate between fibers and axial parenchyma were observed. Prismatic crystals are common in the axial and/or ray parenchyma, and laticifers are present in the cortex and/or the early-formed secondary phloem. Some features appear as unique to Paullinia or the Sapindaceae, such as the paucity of axial parenchyma and the abundance of starch storing fibers. Although many features are conserved across the genus, the Paullinia wood anatomy converges on several features of the liana-specific functional anatomy expressed across distantly related lianas, demonstrating an example of convergent evolution. Hence, the conservation of wood anatomy in Paullinia suggests a combination of phylogenetic constraint as a member of Sapindaceae and functional constraint from the liana habit.

—Aspidosperma flaviflorum, is described as a new species from the submountain semideciduous forest in the municipality of Porto Murtinho in the state of Mato Grosso do Sul, Brazil. This new taxon is described and compared with the most morphologically similar species, A. quirandy and A. tomentosum. In its wood anatomy, A. flaviflorum is unique within the genus by the very scanty axial parenchyma and the lack of a line of axial parenchyma delimiting the growth rings.

PREMISE Medullary bundles, i.e., vascular units in the pith, have evolved multiple times in vascular plants. However, no study has ever explored their anatomical diversity and evolution within a phylogenetic framework. Here, we investigated the development of the primary vascular system within Nyctaginaceae showing how medullary bundles diversified within the family. METHODS Development of 62 species from 25 of the 31 genera of Nyctaginaceae in stem samples was thoroughly studied with light microscopy and micro-computed tomography. Ancestral states were reconstructed using a maximum likelihood approach. RESULTS Two subtypes of eusteles were found, the regular eustele, lacking medullary bundles, observed exclusively in representatives of Leucastereae, and the polycyclic eustele, containing medullary bundles, found in all the remaining taxa. Medullary bundles had the same origin and development, but the organization was variable and independent of phyllotaxy. Within the polycyclic eustele, medullary bundles developed first, followed by the formation of a continuous concentric procambium, which forms a ring of vascular bundles enclosing the initially formed medullary bundles. The regular eustele emerged as a synapomorphy of Leucastereae, while the medullary bundles were shown to be a symplesiomorphy for Nyctaginaceae. CONCLUSIONS Medullary bundles in Nyctaginaceae developed by a single shared pathway, that involved the departure of vascular traces from lateral organs toward the pith. These medullary bundles were encircled by a continuous concentric procambium that also constituted the polycyclic eustele, which was likely a symplesiomorphy for Nyctaginaceae with one single reversion to the regular eustele.

Rhabdadenia (Apocynaceae) includes three species, Rhabdadenia biflora, Rhabdadenia madida, and Rhabdadenia ragonesei. These species are morphologically similar and phenotypically plastic in their vegetative organs and flower color, and despite recent taxonomic revisions of the genus, species identification and delimitation remains a challenge. In this study, we investigate leaf and stem anatomy of all Rhabdadenia species to establish unequivocal taxonomically diagnostic characters. We encountered distinct anatomical differences in (1) leaf characters, notably the stomata distribution, which is restricted to the main veins in R. ragonesei and spread across the entire lamina for the other two species; (2) the presence and form of papillae, which are dome type in R. biflora, cone type in R. madida, yet absent in R. ragonesei; (3) the number of layers of palisade parenchyma, which are two in R. biflora and one in both R. ragonesei and R. madida; (4) the shape of the vascular bundle in the midrib, which is concave-convex in R. ragonesei and convex-concave in R. madida and R. biflora; (5) the leaf blade margins, which are sharply arched downward in R. madida, slightly flexed in R. ragonesei, and straight in R. biflora. A dichotomous identification key is presented. This work enables unequivocal, rapid identification of Rhabdadenia species using leaf anatomy, even in the absence of reproductive parts. Our work also determined that Rhabdadenia is distinct within Apocynaceae by the presence of articulated anastomosed ramified laticifers, as opposed to the unarticulated laticifers that are typical of the family.

A taxonomic revision ofMcvaughiais presented, including the description of a new species from the state of Piauí, Brazil, and notes on wood, secondary phloem, leaf, and floral morpho-anatomy. We present a key to the species, full morphological descriptions, a distribution map, and notes on distribution, ecology, etymology, and conservation status for each species.

Stem ontogeny and structure of two neotropical twining vines of the genus Callaeum are described. Secondary growth in Callaeum begins with a typical regular cambium that gradually becomes lobed as a result of variation in xylem and phloem production rates in certain portions of the stem aligned with stem orthostichies. As development progresses, lignified ray cells of the initially formed secondary xylem detach on one side from the adjacent tissues, forming a natural fracture that induces the proliferation of both ray and axial nonlignified parenchyma. At the same time, parenchyma proliferation takes place around the pith margin and generates a ring of radially arranged parenchyma cells. The parenchyma generated in this process (here termed disruptive parenchyma) keeps dividing throughout stem development. As growth continues, the parenchyma finally cleaves the lignified axial parts of the vascular system into several isolated fragments of different sizes. Each fragment consists of xylem, phloem and vascular cambium and is immersed in a ground matrix of disruptive parenchyma. The cambium present in each fragment divides anticlinally to almost encircle each entire fragment and maintains its regular activity by producing xylem to the centre of the fragment and phloem to the periphery. Additionally, new cambia arise within the disruptive parenchyma and produce xylem and phloem in various polarities, such as xylem to the inside and phloem to the outside of the stem, or perpendicularly to the original cambium. Unlike the very distinctive stem anatomical architecture resulting from this cambial variant in Callaeum, its secondary xylem and phloem exhibit features typical of lianas. These features include very wide conducting cells, abundant axial parenchyma, high and heterocellular rays and gelatinous fibres.

Background: Lianas present many interesting structural features that are linked to their climbing habit. Having lost substantial amounts of supporting tissue, these plants depend on external structures for support. Meanwhile, during their evolutionary history, they have gained additional conductive and storage tissues. The wood of lianas generally includes wider vessels, larger amounts of axial parenchyma, larger rays, and longer fibres than those of trees. Cambial variants represent another key anatomical feature of lianas. Aims: In this paper, we review various aspects of liana biology, including those associated with their vascular system and water conduction, secondary growth and seasonal responses to environmental variability, as well as aspects related to the evolution of their cambial variants. Methods: Examples from the Bignoniaceae and Leguminosae, the two most abundant liana taxa in the Neotropics, are presented in a series of case studies, bringing new data, such as the activity of the cambium ...

PREMISE OF THE STUDY The phloem is a plant tissue with a critical role in plant nutrition and signaling. However, little is still known about the evolution of this tissue. In lianas of the Bignoniaceae, two distinct types of phloem coexist: a regular and a variant phloem. The cells associated with these two phloem types are known to be anatomically different; however, it is still unclear what steps were involved in the evolution of such differences. METHODS Here we studied the anatomical development of the regular and variant phloem in representatives of all 21 genera of Bignonieae and used a phylogenetic framework to investigate the timing of changes associated with the evolution of each phloem type. KEY RESULTS We found that the variant phloem always appears in a determinate location, between the leaf orthostichies. Furthermore, the variant phloem was mostly occupied by very wide sieve tubes and generally included a higher concentration of fibers, indicating an increase in conduction and mechanical support. On the other hand, the regular phloem included much more parenchyma, more and wider rays, and tiny sieve tubes that resembled terminal sieve tubes from plants with seasonal formation of vascular tissues; these findings suggest reduced conduction and higher storage capacity in the regular phloem. CONCLUSIONS Overall, differences between the regular and variant phloem increased over time, leading to further specialization in conduction in the variant phloem and an increase in storage specialization in the regular phloem.