Priorities for Bark Anatomical Research: Study Venues and Open Questions.

The bark fulfils several essential functions in vascular plants and yields a wealth of raw materials, but the understanding of bark structure and function strongly lags behind our knowledge with respect to other plant tissues. The recent technological advances in sampling and preparation of barks for anatomical studies, along with the establishment of an agreed bark terminology, paved the way for more bark anatomical research. Whilst datasets reveal bark's taxonomic and functional diversity in various ecosystems, a better understanding of the bark can advance the understanding of plants' physiological and environmental challenges and solutions. We propose a set of priorities for understanding and further developing bark anatomical studies, including periderm structure in woody plants, phloem phenology, methods in bark anatomy research, bark functional ecology, relationships between bark macroscopic appearance, and its microscopic structure and discuss how to achieve these ambitious goals.

Cryptocarya chinensis from the Upper Pleistocene of South China and its biogeographic and paleoecological implications.

Anatomical structure of mummified wood of Cryptocarya (Lauraceae) from the Upper Pleistocene of Maoming, South China and the woods of 15 extant species of Cryptocarya from China and Malaysia were examined. The fossil wood has been convincingly attributed to extant species Cryptocarya chinensis (Hance) Hemsl. This is the first reliable fossil record of Cryptocarya in Asia. The finding combined with the results of Biomod2 species distribution modeling suggest that the range of C. chinensis in the Late Pleistocene in South China and North Vietnam was very restricted due to increased continental aridity and enhanced temperature seasonality in this region. Thus, modern populations of C. chinensis in Maoming can be considered as glacial relicts. The mines (larval tunnels) produced by the larvae of flies from the genus Phytobia Lioy (Agromyzidae, Diptera) were observed in fossil wood under study. These cambial miners have never been reported in Cryptocarya.

Glacial expansion of cold-tolerant species in low latitudes: megafossil evidence and species distribution modelling.

Fossil wood of Chinese white pine (Pinus armandii Franch.) from the Late Pleistocene deposits of Maoming Basin of South China provides the first megafossil evidence for glacial expansion of the range of a cold-tolerant species in low latitudes.

Mummified Wood of Juniperus (Cupressaceae) from the Late Miocene of Taman Peninsula, South Russia.

Juniperus L. is the second-largest genus of conifers, having the widest distribution of all conifer genera. Its phytogeographic history is, however, obscure due to its very poor fossil record. We described a wood of Juniperus sp. from the lower Maeotian sediments of the Popov Kamen section, Taman Peninsula, South Russia, in order to clarify its taxonomic position shedding light on the phytogeographic history of the genus. This fossil wood was well-preserved by mummification, which allowed for it to be studied by the same methods as used for the anatomical examination of modern woods. The wood from the Popov Kamen section shows the greatest similarity to the extant Mediterranean species J. excelsa, belonging to the section Sabina. This is the first reliable macrofossil evidence of the sect. Sabina from Eurasia convincingly dated to the Miocene. The age of the mummified wood from the Popov Kamen section is consistent with molecular dating of diversification of the lineage comprising juniper species of the sect. Sabina from Europe, Asia and eastern Africa. The wood of Juniperus sp. has not been buried in situ, as it was found in the relatively deep-water marine sediments. The available coeval pollen series and macrofossils of Cupressaceae from the surrounding regions suggest that this wood was likely transferred by sea current from the northwestern side of the Black Sea, which was a part of the Eastern Paratethys.

Stem and Leaf Anatomy of Aragoa (Plantaginaceae): In Search of Lost Rays.

Aragoa is a shrubby genus endemic to páramo in the northern Andes representing the sister group to Plantago and Limosella. Stem and leaf structure of Aragoa corrugatifolia were studied to clarify the evolutionary pathways and ecological significance of their anatomical traits. Aragoa and Plantago share a non-fascicular primary vascular system, rayless wood and secondary phloem, and anomocytic stomata. Aragoa is distinctive from most Plantaginaceae in the presence of cortical aerenchyma and of helical thickenings in vessels. Its procambium emerges in the primary meristem ring as a continuous cylinder. The view on the ring meristem and procambial strands as developmental stages in the formation of a primary vascular system is not relevant for Aragoa, and probably for other Plantaginaceae. The raylessness is synapomorphic for the crown clade of Plantaginaceae comprising Aragoa, Littorella, Plantago, Veronica, Picrorhiza, Wulfenia, and Veronicastrum. The loss of rays is thought to be predetermined by procambium rather than by the vascular cambium. The extremely narrow vessels with helical thickenings are presumably adaptive to hydric and thermic conditions of páramo. Cortical aerenchyma is thought to be a response to the local hypoxia caused by the water retained by ericoid leaves. Trichomes on juvenile leaves are expected to be the traits of considerable taxonomic importance.

Derived woodiness and annual habit evolved in African umbellifers as alternative solutions for coping with drought.

BACKGROUND: One of the major trends in angiosperm evolution was the shift from woody to herbaceous habit. However, reversals known as derived woodiness have also been reported in numerous, distantly related clades. Among theories evoked to explain the factors promoting the evolution of derived woodiness are moderate climate theory and cavitation theory. The first assumes that woody habit evolves in response to mild climate allowing for prolonged life span, which in turn leads to bigger and woodier bodies. The second sees woodiness as a result of natural selection for higher cavitation resistance in seasonally dry environments. Here, we compare climatic niches of woody and herbaceous, mostly southern African, umbellifers from the Lefebvrea clade to assess whether woody taxa in fact occur in markedly drier habitats. We also calibrate their phylogeny to estimate when derived woodiness evolved. Finally, we describe the wood anatomy of selected woody and herbaceous taxa to see if life forms are linked to any particular wood traits. RESULTS: The evolution of derived woodiness in chamaephytes and phanerophytes as well as the shifts to short-lived annual therophytes in the Lefebvrea clade took place at roughly the same time: in the Late Miocene during a trend of global climate aridification. Climatic niches of woody and herbaceous genera from the Cape Floristic Region overlap. There are only two genera with distinctly different climatic preferences: they are herbaceous and occur outside of the Cape Floristic Region. Therefore, studied herbs have an overall climatic niche wider than their woody cousins. Woody and herbaceous species do not differ in qualitative wood anatomy, which is more affected by stem architecture and, probably, reproductive strategy than by habit. CONCLUSIONS: Palaeodrought was likely a stimulus for the evolution of derived woodiness in the Lefebvrea clade, supporting the cavitation theory. The concurrent evolution of short-lived annuals withering before summer exemplifies an alternative solution to the same problem of drought-induced cavitation. Changes of the life form were most likely neither spurred nor precluded by any qualitative wood traits, which in turn are more affected by internode length and probably also reproductive strategy.

Revisiting the anatomy of the monocot cambium, a novel meristem.

MAIN CONCLUSION: The monocot cambium is semi-storied, and its cells do not undergo rearrangement. The monocot cambium is a lateral meristem responsible for secondary growth in some monocotyledons of Asparagales. It is an unusual meristem, not homologous with the vascular cambia of gymnosperms and non-monocotyledonous angiosperms. Owing to the limited information available on the characteristics of this meristem, the aim of this study was to survey the structure of the monocot cambium in order to clarify the similarities and dissimilarities of this lateral meristem to the vascular cambium of trees. Using the serial sectioning analysis, we have studied the monocot cambium of three species of arborescent monocotyledons, i.e., Quiver Tree Aloe dichotoma, Dragon Tree Dracaena draco, and Joshua Tree Yucca brevifolia, native to different parts of the world. Data showed that in contrast to the vascular cambium, the monocot cambium is composed of a single type of short initials that vary in shape, and in tangential view display a semi-storied pattern. Furthermore, the cells of the monocot cambium do not undergo rearrangement. The criteria used in identifying monocot cambium initial cell are also discussed.

Parallel evolution of arborescent carrots (Daucus) in Macaronesia.

PREMISE: Despite intensive research, the pathways and driving forces behind the evolution of derived woodiness on oceanic islands remain obscure. The genus Daucus comprises mostly herbs (therophytes, hemicryptophytes) with few rosette treelets (chamaephytes) endemic to various Macaronesian archipelagos, suggesting their independent evolution. To elucidate the evolutionary pathways to derived woodiness, we examined phylogenetic relationships and the habit and secondary xylem evolution in Daucus and related taxa. METHODS: Sixty taxa were surveyed for molecular markers, life history, and habit traits. Twenty-one species were considered for wood anatomical characters. A dated phylogeny was estimated using Bayesian methods. The evolution of selected traits was reconstructed using parsimony and maximum likelihood. RESULTS: Daucus dispersed independently to the Canary Islands (and subsequently to Madeira), Cape Verde, and the Azores in the late Miocene and Pleistocene. Life span, reproductive strategy, and life form were highly homoplastic; the ancestor of Daucus was probably a monocarpic, biennial hemicryptophyte. Rosette treelets evolved independently in the Canarian-Madeiran lineage and in Cape Verde, the latter within the last 0.13 Myr. Treelets and hemicryptophytes did not differ in wood anatomy. Pervasive axial parenchyma in wood occurred more often in polycarpic rather than monocarpic species. CONCLUSIONS: Life span and life form in Daucus are evolutionarily labile and may change independently of wood anatomy, which is related to plant reproductive strategy rather than to life form. Insular woodiness may evolve rapidly (as demonstrated in D. bischoffii), and in Daucus, it does not seem to be an adaptation to lower the risk of xylem embolism.

Araucarioid wood from the late Oligocene-early Miocene of Hainan Island: first fossil evidence for the genus Agathis in the Northern Hemisphere.

Although many fossil and molecular data suggest migrations from Malesia and Asia to Australia appear to dominate floristic exchange between Australian and Asian rainforests, evidence is emerging that demonstrate dispersal of plant groups from Australia to Asia. In this paper, a new species Agathis ledongensis sp. nov. is described on the basis of silicified wood from the late Oligocene-early Miocene of the Qiutangling Formation in Ledong, Hainan Island, South China. It is the first fossil record of Agathis in the Northern Hemisphere, and the only known fossil evidence of its dispersal outside of Gondwana. The close affinity of the fossil wood from Ledong with the genus Agathis was confirmed by comparing quantitative traits in 31 wood samples of 20 species representing all three extant genera of the Araucariaceae. The percentage of tracheids with uniseriate pitting on radial walls is shown as an additional diagnostic trait for separating Agathis and Wollemia from Araucaria. The wood of Agathis ledongensis provides evidence for the dispersal of this important plant group from Australia, or another Gondwanan terrane, to eastern Asia based on reliable fossil data. It records the occurrence of this genus in Hainan Island by the early Miocene, i.e. at the beginning of the formation of the island chains between Australia and the South-East Asia and thus the provision of a land migration route. As the land routes between these continents were restricted at that time, the migration of Agathis to Malesia and Asia was presumably facilitated by long-distance dispersal of its winged seeds by wind.

Gradual vs. abrupt reduction of carpels in syncarpous gynoecia: A case study from Polyscias subg. Arthrophyllum (Araliaceae: Apiales).

PREMISE OF THE STUDY: Revealing the relative roles of gradual and abrupt transformations of morphological characters is an important topic of evolutionary biology. Gynoecia apparently consisting of one carpel have evolved from pluricarpellate syncarpous gynoecia in several angiosperm clades. The process of reduction can involve intermediate stages, with one fertile and one or more sterile carpels (pseudomonomery). The possible origin of monomery directly via an abrupt change of gynoecium merism has been a matter of dispute. We explore the nature of gynoecium reduction in a clade of Araliaceae. METHODS: The anatomy and development of unilocular gynoecia are investigated using light and scanning electron microscopy in two members of Polyscias subg. Arthrophyllum. Gynoecium diversity in the genus is discussed in a phylogenetic framework. KEY RESULTS: Unilocular gynoecia with one fertile ovule have evolved at least four times in Polyscias, including one newly discovered case. The two unilocular taxa investigated are unicarpellate, without any traces of reduced sterile carpels. Carpel orientation is unstable, and the ovary roof and style contain numerous vascular bundles without clearly recognizable dorsals or ventrals. In contrast to pluricarpellate Araliaceae and Apiaceae, the cross zone is apparently oblique in the unicarpellate species. CONCLUSIONS: No support was found for gradual gynoecium reduction via pseudomonomery. The abrupt origin of monomery via direct change of gynoecium merism and the unstable carpel orientation observed are related to the general lability of the flower groundplan in Polyscias. The apparent occurrence of the unusual oblique cross zone in unicarpellate Araliaceae can be explained by developmental constraints.

First record of Podocarpoid fossil wood in South China.

A new species of fossil conifer wood, Podocarpoxylon donghuaiense sp. nov., is described from the late Eocene of Nadu Formation in Baise Basin of the Guangxi Province, South China. This fossil wood is characterized by distinct growth rings, circular to oval tracheids in cross section, 1-2-seriate opposite pits on radial tracheid walls, uniseriate (rarely biseriate) rays, smooth end walls of ray parenchyma cells, and the absence of resin ducts, suggesting its affinity to Podocarpaceae. The new species is distinctive from other Cenozoic woods ascribed to this family by the combination of distinctive growth rings, the absence of axial parenchyma, the occurrence of bordered pits on tangential tracheid walls, and the occurrence of 3-4 cuppressoid or taxodioid pits on cross-fields. This represents the first record of podocarpoid fossil wood in South China and provides fossil evidence for the early dispersal and diversification of Podocarpaceae in eastern Asia as well as for mild temperate seasonal climate in this region during the late Eocene.

Camellia nanningensis sp. nov.: the earliest fossil wood record of the genus Camellia (Theaceae) from East Asia.

A new species Camellia nanningensis was described on the basis of well-preserved mummified wood from the upper Oligocene Yongning Formation of Nanning Basin in Guangxi Province, South China. This represents the most ancient fossil wood assigned to Camellia, and the earliest fossil record of the family Theaceae in China. This fossil material shows that Camellia occurred in China as early as the late Oligocene, suggesting more ancient radiation of this genus than estimated by molecular dating.

False paracarpy in Seemannaralia (Araliaceae): from bilocular ovary to unilocular fruit.

BACKGROUND AND AIMS: Seemannaralia appears to be fundamentally different from all other Araliaceae in the presence of a well-developed symplicate zone in its gynoecium, as well as in the ovule insertion in the symplicate zone (rather than in the cross-zone). The present investigation re-examined the floral structure of Seemannaralia with emphasis on the morphology and evolution of its gynoecium. METHODS: Flowers and fruits of Seemannaralia gerrardii at various developmental stages were examined using light microscopy and scanning electron microscopy. KEY RESULTS: Ovaries in the flowers of Seemannaralia are bilocular. Each ovary locule corresponds to a carpel whose ascidiate part is distinctly longer than the plicate part. Each carpel contains one fertile ovule attached to the cross-zone, and one sterile ovule as well. The fruit is unilocular: its central cavity is occupied by a single large seed. In the course of fruit development, the growth of one ovule stops while another ovule develops into the mature seed. When this ovule outgrows the available space in the locule, the septum is ruptured, forming a united cavity of two carpels. CONCLUSIONS: Despite literature data, the synascidiate zone is well developed in the gynoecium of Seemannaralia, and the ovules are attached to the cross-zone. Its preanthetic and anthetic gynoecium has nearly the same structure as gynoecia of most other Araliaceae. The Seemannaralia fruit resembles the paracarpous gynoecium but its ground plan is very different because the central cavity is formed by mechanical rupture of the septum. The term 'pseudoparacarpy' ('false paracarpy') is proposed to describe this condition, which has not been reported to date for indehiscent fruits in any taxa other than Seemannaralia. In this genus, the pseudoparacarpy has probably resulted from a decrease in seed number in the course of the transition from zoochory to anemochory.