The Vanguerieae is a tribe consisting of about 500 species ordered in 27 genera. Although this tribe is mainly represented in Africa and Madagascar, Vanguerieae also occur in tropical Asia, Australia, and the isles of the Pacific Ocean. This study gives a detailed wood anatomical description of 34 species of 15 genera based on LM and SEM observations. The secondary xylem is homogeneous throughout the tribe and fits well into the Ixoroideae s.l. on the basis of fibre-tracheids and diffuse to diffuse-in-aggregates axial parenchyma. The Vanguerieae include numerous geofrutices that are characterised by massive woody branched or unbranched underground parts and slightly ramified unbranched aboveground twigs. The underground structures of geofrutices are not homologous; a central pith is found in three species (Fadogia schmitzii, Pygmaeothamnus zeyheri and Tapiphyllum cinerascens var. laetum), while Fadogiella stigmatoloba shows central primary xylem which is characteristic of roots. Comparison of underground versus aboveground wood shows anatomical differences in vessel diameter and in the quantity of parenchyma and fibres.
Recent studies on the functional significance of pit membranes in water conducting cells have renewed general interest in their micromorphology. At least two types of pit membrane thickenings have been described in angiosperm families, i.e. genuine tori and pseudo-tori. This study explores the distribution and morphology of pit membrane thickenings in 69 species and 23 genera within Oleaceae using light and electron microscopy. Torus-bearing pit membranes are confirmed for Osmanthus, and new records are reported for Chionanthus retusa, Picconia azorica, and P. excelsa, but not for the other species studied of Chionanthus. This infrageneric variation suggests that tori represent a plastic feature that has evolved more than once within the family as the result of functional adaptation to efficient and safe water transport. Pseudo-tori are observed in species of Abeliophyllum, Fontanesia, Forsythia, Jasminum, Ligustrum, Menodora, and Syringa. Based on structural differences, we state that tori and pseudo-tori can be distinguished as non-homologous features.
Subfamily Spathelioideae of Rutaceae constitutes a well-supported early branching clade of eight small woody genera that were formerly assigned to five different Sapindalean/Rutalean families. This study brings together detailed wood anatomical information on all eight genera (for four the wood anatomy is described for the first time in detail). Wood anatomy strongly supports the inclusion of all Spathelioid genera in Rutaceae and underpins the molecular phylogeny with a set of interesting apomorphies at different nodes of the cladogram. The wood anatomy of Cneorum tricoccon with its semi-ring porosity, dendritic vessel pattern, vascular tracheids and helical vessel wall thickenings stands out in Spathelioideae. This wood anatomical syndrome is hypothesized to be due to adaptive evolution for hydraulic safety and efficiency of this species in a typical Mediterranean climate, where similar syndromes have evolved in many unrelated clades of woody dicots. In at least six unrelated genera of Rutaceae outside Spathelioideae from Mediterranean or cool temperate and montane climates, the syndrome has also evolved in presumably parallel, adaptive evolution.
Xylem vessels interconnect to form the vessel network that is responsible for long-distance water transport through the plant. As plants dehydrate, the water column within vessels cavitates and gas emboli form, which block transport through embolized vessels. The impact of vessel blockages on transport through the xylem tissue depends upon vessel size and the arrangement and connections between vessels in the network. We examined if there was a correlation between vessel length and diameter within poplar stem xylem tissue using both silicone-injection and analysis of tissue volumes scanned using high-resolution computed tomography (microCT). We then used microCT to scan intact stems sampled over varying water potentials to examine if larger vessels, which would have the greatest impact on hydraulic transport, were more vulnerable to cavitation and embolism than smaller vessels. Within the xylem tissue, larger diameter vessels tended to be longer than narrow diameter vessels. Vessel size distributions indicated that most vessels were narrow and short, with fewer large vessels. Larger volume vessels tended to embolize at higher water potentials and the mean vessel volume of embolized vessels declined as water potentials declined. Hydraulic transport through the xylem tissue was near zero when about 40% of the vessels within the xylem tissue volume were embolized, suggesting important vessel network effects occur as water moves through a three-dimensional (3D) tissue. The structure of the vessel network is important in understanding the impact of emboli within vessels on the overall hydraulic function of xylem tissue.
Covariation amongst wood traits along the stem axis is important to maintain hydraulic integrity ensuring sufficient sap flow to the canopy. Here, we test how wood traits (co)vary along the trunk and whether two seasonally dry Brazilian habitats (cerrado and caatinga) influence this variation in two co-occurring species, Tocoyena formosa (Rubiaceae) and Tabebuia aurea (Bignoniaceae). The samples were collected at five heights along the main trunk of three individuals per species in both sites. We used light, scanning and transmission electron microscopy to observe the wood traits. Out of 13 wood traits, nine show relationships with sampling height: eight traits predict height in T. formosa and five in T. aurea. Contrastingly, only three traits show differences between sites and only for T. formosa. The intratrunk wood variation is reflected by the hydraulically weighted vessel diameter showing a curvilinear relationship, disagreeing with the prediction of a continuous vessel widening from tip to base. In both species, the largest vessels are linked to the thinnest intervessel pit membranes. Wood density increases basipetally for both species, being site-dependent and correlated with vessel traits in T. formosa, and site-independent and determined by fiber wall thickness in T. aurea. Furthermore, the functional role of rays was found to be different for each species, and may be related to the marked difference in ray composition. In conclusion, both species show a unique adaptation to deal with height-related constraints using species-specific co-variation amongst wood traits, while site does not contribute much to the wood variation.