Structure of the secondary xylem and development of a cambial variant in Serjania mexicana (Sapindaceae)

The lianas in the family Sapindaceae are known for their unique secondary growth which differs from climbing species in other plant families in terms of their cambial variants. The present study deals with the stem anatomy of self-supporting and lianescent habit, development of phloem wedges, the ontogeny of cambial variants and structure of the secondary xylem in the stems of Serjania mexicana (L.) Willd. Thick stems (15–20 mm) were characterized by the presence of distinct phloem wedges and tangentially wide neo-formed cambial cylinders. As the stem diameter increases, there is a proportional increase in the number of phloem wedges and neo-formed vascular cylinders. The parenchymatous (pericyclic) cells external to phloem wedges that are located on the inner margin of the pericyclic fibres undergo dedifferentiation, become meristematic and form small segments of cambial cylinders. These cambia extend tangentially into wide and large segments of neoformations. Structurally, the secondary xylem and phloem of the neo-formed vascular cylinders remain similar to the derivatives produced by the regular vascular cambium. The secondary xylem is composed of vessels (wide and narrow), fibres, axial and ray parenchyma cells. The occurrence of perforated ray cells is a common feature in both regular and variant xylem.

Linking the Evolution of Development of Stem Vascular System in Nyctaginaceae and Its Correlation to Habit and Species Diversification

Background: The presence of alternative patterns of secondary growth in stems of Nyctaginaceae has been known for a long time. Still, the interpretation of types of cambial variants are controversial. The knowledge on stem anatomical diversity in Nyctaginaceae, which is diverse also in habits, offers the unique opportunity not only to investigate the evolution of complex developments, but also to address how these anatomies shifted within habits and how the acquisition of novel cambial variants and habit transitions impacted the diversification of the family. Methods: We integrated developmental data with a phylogenetic framework to investigate the diversity and evolution of stem anatomy in Nyctaginaceae using phylogenetic comparative methods, reconstructing ancestral states, and examining whether anatomical shifts correspond to species diversification rate shifts in the family. Results: Two types of cambial variants, interxylary phloem and successive cambia, were recorded in Nyctaginaceae, which result from four different ontogenies. These ontogenetic trajectories depart from two distinct primary vascular structures (regular or polycyclic eustele) yet, they contain shared developmental stages which generate stem morphologies with deconstructed boundaries of morphological categories (continuum morphology). Unlike our a priori hypotheses, interxylary phloem is reconstructed as the ancestral character for the family, with three ontogenies characterized as successive cambia evolving in few taxa. Cambial variants are not contingent in habits, and their transitions are independent from species diversification.Conclusions: Our findings suggests that multiple developmental mechanisms, such as heterochrony and heterotopy generate the transitions between interxylary phloem and successive cambia. Intermediate between these two extremes are present in Nyctaginaceae, suggesting a continuum morphology across the family as a generator of anatomical diversity.

The stem anatomy of the Clematis species (Ranunculaceae) in Taiwan

Background Studies on the stem anatomical characteristics of Taiwanese species from the Clematis genus (Ranunculaceae) are scarce. This study aimed to investigate and compare the patterns of secondary growth in stems of 22 Clematis species. Results The rhytidome is composed of periderm and non-conducting phloem and formed either cogwheel-like or continuous segment bark. Key features of the genus were stem with an irregular conformation, wedge-like phloem and rays, indentations in the axial parenchyma, ray dilatation, and narrow rays. Approximately eight Clematis species formed bark arc shape, which developed the cogwheel- like rhytidome. There were with approximately 27% of the Clematis species in Taiwan having 12 vascular bundles. The vessels dispersed throughout the stem were semi-ring-porous in most species but were ring-porous in others. No species had diffuse-porous vessels. The vessel restriction pattern was only found in the two shrubs, C. psilandra and C. tsugetorum. The primary xylem ring was located around the pith in C. uncinata var. uncinata, making its pith cavity hexagon in shape. Four species had the pith cavity feature. Narrow rays that occurred in the secondary xylem increased with increasing stem diameter. Conclusions The cambial variants described in this study provide a foundation for further morphological studies of the Clematis genus.

Ontogeny of divided vascular cylinders in Serjania: the rise of a novel vascular architecture in Sapindaceae

Sapindaceae lianas are remarkable for the diversity of cambial variants found in their stems. One of the family’s exclusive cambial variant is the divided vascular cylinder, which occurs in eight species of the genus Serjania. This cambial variant is marked by 5 peripheral vascular cylinders around a large pith. We performed a comparative developmental analysis, integrating traditional plant anatomy techniques with high-resolution X-ray micro-computed tomography to investigate the structure and development of the stems of three species with divided vascular cylinder. Our observations showed that the initial stages of stem development were similar to those described in the literature, however, on later developmental stages a central vascular cylinder appears in all species. The ontogeny of these stems are marked by three main processes: (i) dissection of vascular tissue from the peripheral vascular cylinders; (ii) development of new cambial arcs through the redifferentiation of pith cells; and (iii) recruitment of cambial cells from the inner portions of the vascular cambium of the peripheral vascular cylinders, forming a novel central vascular cylinder where the pith was, surrounded by five initial peripheral cylinders. As an ulterior developmental stage, some older stems also develop neoformations and connections between the different vascular cylinders. While our findings support previous descriptions of divided vascular cylinders, this is the first study illustrating the formation of the central vascular cylinder in this cambial variant. Our observations further corroborate that Serjania is the lineage with the highest and some of the most complex forms of cambial variants among all vascular plants.

Anatomy of vegetative organs in Allionia (Nyctaginaceae), with emphasis on the vascular system

Allionia is a small genus within the tribe Nyctagineae (Nyctaginaceae) which has a controversial, infrageneric delimitation. Here, we investigated the two known species of Allionia in order to characterize the anatomy of leaves, stems and roots, with further notes on vascular system development. Additionally, the present study aimed to broaden our knowledge of stem vascular diversity and to survey for anatomical features with diagnostic value in distinguishing A. choisyi from A. incarnata. Leaf anatomy of other Nyctagineae taxa was also analysed. Anatomical and ontogenetic observations from the vegetative organs in Allionia revealed no diagnostic features to distinguish the two species. We illustrated the occurrence of Kranz anatomy, which in Nyctaginaceae is only known in Allionia, Boerhavia, and Okenia. The stem primary vascular system was unusual in showing a polycyclic eustele (medullary bundles + continuous concentric procambium). Likewise, mature stems and roots show vascular cambial variants (successive cambia) that arise from the pericycle. The anatomy and histochemistry of multicellular glan-dular trichomes observed in aerial organs were presented. Raphids were seen in all organs. Although no strong xerophytic features were observed in Allionia, several characteristics can be associated with their arid habitats. Our findings on the vascular system of Allionia showed the two species to be much the same and reinforced earlier findings that the stem anatomy of Nyctaginaceae is complex and intriguing.

Wood anatomy of the neotropical liana lineage Paullinia L. (Sapindaceae)

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.

Procambial and cambial variants in Serjania and Urvillea species (Sapindaceae: Paullinieae)

For the purpose of comparing the structure of vascular cylinders, and procambial and cambial variants of Serjania Miller and Urvillea Kunth species, we studied the stems of five species, in order to add to the structural knowledge of these genera belonging to the widely distributed Sapindaceae family. Stems of sampled species were collected at “Estação Ecológica do Caiuá,” Diamante do Norte (PR), Brazil; and were analyzed using traditional anatomical techniques. We analyzed five species of two genera from the Paullinieae tribe: three species of Serjania (S. communis, S. fuscifolia, and S. meridionalis) and two of Urvillea (U. leavis and U. ulmacea); in which we describe the primary and secondary growths, the number of peripheral cylinders, the type of cylinders concerning its origin, the type of stem, and the origin of the cambial or tissue variant. Serjania stems exhibit the compound and corded types, while the cleft and mixed (cleft/corded) types have been recorded in Urvillea species. All species have peripheral vascular cylinders in which the stems of S. communis and S. fuscifolia form the procambial and cambial variants during primary growth, whereas the stems of S. meridionalis, U. leavis, and U. ulmacea develop the cambial variants during secondary growth.

Wood anatomy and conductivity in lianas, shrubs and trees of Bignoniaceae

This work compares potential xylem hydraulic efficiency among Bignoniaceae lianas, shrubs and trees. Five species from each growth habit were analysed to determine variance among habits based on quantitative and qualitative wood anatomical features. Potential hydraulic conductivity was calculated for each species in order to compare efficiency of water transport. Cambial variants are present in the Bignonieae tribe, as phloem wedges in lianas and phloem arcs in shrubs. Lianas present vessel dimorphism, quantitatively evidenced by the ratio of maximum by minimum vessel diameter of about 20, higher percentage of vessel area and lower percentage of fibres compared with the self-supporting species studied here. Potential hydraulic conductivity is higher in lianas due to the presence of wider vessels and it is hypothesised that the narrow vessels can function as back-up for water conduction when wider vessels are cavitated.