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The Opuntioideae include iconic cacti whose lateral branch–branch junctions are intriguing objects from a mechanical viewpoint. We have compared Opuntia ficus-indica, which has stable branch connections, with Cylindropuntia bigelovii, whose side branches abscise under slight mechanical stress. To determine the underlying structures and mechanical characteristics of these stable versus shedding cacti junctions, we conducted magnetic resonance imaging, morphometric and anatomical analyses of the branches and tensile tests of individual tissues. The comparison revealed differences in geometry, shape and material properties as follows: (i) a more pronounced tapering of the cross-sectional area towards the junctions supports the abscission of young branches of C. bigelovii. (ii) Older branches of O. ficus-indica form, initially around the branch–branch junctions, collar-shaped periderm tissue. This secondary coverage mechanically stiffens the dermal tissue, giving a threefold increase in strength and a tenfold increase in the elastic modulus compared with the epidermis. (iii) An approximately 200-fold higher elastic modulus of the vascular bundles of O. ficus-indica is a prerequisite for the stable junction of its young branches. Our results provide, for both biological and engineered materials systems, important insights into the geometric characteristics and mechanical properties of branching joints that are either stable or easily detachable.

The periderm plays a crucial role in trees, acting as a barrier protecting internal tissues against biotic and abiotic stresses, thus having an impact on tree physiology, ecology, and general performance. It consists of the meristematic phellogen, whose activity gives rise to suberized phellem (cork) cells outwardly and the parenchymatous phelloderm inwardly. Despite the periderm importance, intra-annual and seasonal changes in phellogen activity and phellem and phelloderm differentiation are poorly recognized. Therefore, we aimed to compare periderm development and functioning in successive years in horse chestnut, utilizing standard histological methods. We distinguished six stages of periderm development, including phellogen initiation and the differentiation of its derivatives. In the following years, the phellogen was active for a similar period, but produced fewer derivative cells. Importantly, some phellogen cells lost their meristematic characteristics before the end of the season and differentiated into phellem. To maintain periderm integrity, the remaining phelloderm cells underwent divisions, leading to phellogen re-initiation. Alternatively, when all periderm cells differentiated into the phellem, the new (subsidiary) phellogen originated from the underneath collenchyma. We postulate that phellogen re-initiation could be a mechanism ensuring the functional integrity of the periderm and discuss the role of phelloderm or collenchyma cells in this process.

Key MessageAnatomical evidences suggest that differences in rooting ability among Acca sellowiana materials are explained by earlier phase change in difficult to root genotypes.Successful development of adventitious roots (AR) in cuttings imposes an important limitation to the propagation of woody plants and in some species, the ability to form AR is strongly affected by genotype. However, we lack an understanding of the differences among genotypes underlying such different responses in various species. We examined the anatomical and biochemical effects of exogenous indol-3-butyric acid and type of cutting in rooting experiments of two Acca sellowiana genotypes with contrasting rooting ability. New meristems developed outside the cambial ring, without callus formation by day fourteen and new adventitious roots grew through the cortex emerging by day 28. Both anatomically in vivo and biochemically in vitro, cuttings from the different genotypes behaved differently. We found anatomical differences between the genotypes that might explain the differences in rooting ability. An earlier development of a periderm was present in the difficult-to-root genotype. This secondary dermal tissue could be used as a reliable phase-change marker to distinguish juvenile from mature plant parts which have lost rooting capacity.

The outer bark that includes sequent periderms is referred to as rhytidome. The defense and physiological functions of rhytidome are maintained by the continuous formation of sequent periderms. To understand the mechanisms of rhytidome growth, we examined the development of sequent periderms and the corresponding changes in the cellular contents of phloem parenchyma cells in Cryptomeria japonica . New layers of rhytidome were formed in the studied trees during the two-year course of the study. Our records showed that a new layer of periderm forms annually, and therefore, rhytidome development in C . japonica can be studied by sequential sample collection in any given year. Formation of new periderm and initiation of nuclei disappearance in phloem parenchyma in the outer layers of the developing outer bark occurred simultaneously. The early disappearance of nuclei indicates that some parenchyma cells might have been in a stage of preparation for cell death before the formation of new periderm. Four developmental stages of annual rhytidome growth were identified by structural and physiological changes of the outer layers of phloem parenchyma and the growth of the new periderm.

Wounding of trees by debarking during the vegetative period sometimes results in the formation of callus tissue which develops over the entire wound surface or on parts of it. This light and transmission electron microscopy study of living lime trees found that the formation of such a surface callus is subdivided into three stages. During the first stage, numerous cell divisions take place in regions where differentiating xylem remains at the wound surface after debarking. This young callus tissue consists of isodiametric parenchymatous cells. Cambium cells, sometimes also remaining at the wound surface, collapse and do not contribute to callus formation. During the second stage, cells in the callus undergo differentiation by forming a wound periderm with phellem, phellogen and phelloderm. In the third stage, a cambial zone develops between the wound periderm and the xylem tissue laid down prior to wounding. This process is initiated by anticlinal and periclinal divisions of a few callus cells only. Later this process extends tangentially to form a continuous belt of wound cambium. Subsequently, this cambium produces both wound xylem and wound phloem and thus contributes to further thickening.

The Welton Member of the Scotch Grove Formation at Shaffton Quarry, near Camanche, Iowa, is about 427 Ma old and contains numerous metaconulariid specimens, many of which are preserved in apex-downward orientation. Some of these show an unusual, splayed, “Maltese cross” configuration. Apex-downward configurations suggest rapid burial, consistent with the soft part preservation known in other taxa from this locality. The abundance of Metaconularia at Shaffton Quarry, and of topotype specimens of M. aspersa, the generic type species, permits evaluation of the degree of individual and intracollectional variation in peridermal ornament. Variation within and among individuals precludes reliance on ornamental differences in species differentiation in most cases. In view of these results we assign all Shaffton specimens to Metaconularia manni (Roy, 1935), and revise Metaconularia based on its type material from Europe and other material from Europe and North America. An exploratory phylogenetic analysis highlights aspects of character distribution within the genus, but the small number of characters states and possible taphonomic influences upon them limit confidence in the clade topology. Subgroups within the genus are characterized by larger, discoidal papillae, and by strong transverse corrugation and sinuous rows of smaller papillae. The genus itself comprises those conulariids with an external ornamentation of simple, round, small papillae, paired internal septae along the midlines, and a thin periderm that was to some degree pliable during life.

Ophiostomatoid fungi colonize the conducting tissues of conifer stems, the phloem and the xylem. These pathogenic fungi penetrate into the stem through injuries made by xylophagous insects vectoring these pathogens. In this study the response of the phloem of Scotch pine ( Pinus sylvestris L.) to wounding (treatment 1) was compared with the response to wounding combined with application of high-molecular-weight compounds isolated from the mycelium of the ophiostomatoid fungus Ceratocystis laricicola Redfern & Minter (treatment 2). Both treatments induced the appearance of necrosis in the inner bark, the formation of periderm separating living and dead tissues, and formation of the callus alongside the wound perimeter. In addition, the bark accumulated lignin, bound proanthocyanidins, and resins, with a parallel decrease in the content of free proanthocyanidins, low-molecular-weight carbohydrates, and non-lignin components of the cell wall ( P > 0.95). The size of necrotic spots, as well as changes in the content of most substances, were significantly higher in the treatment 2 than in the treatment 1 ( P > 0.95). The accumulation of lignin in cell walls of phloem sieve cells was delayed in the treatment 2 as compared with that in the treatment 1. This suggested that the mycelial extract temporarily inhibited lignification at the early stage of the wound response. This disturbance of the cell wall protective transformation led to the hypothesis that the fungal suppressors retard the repair of inner bark injured by insects, thereby favoring the invasion of conifer tissues by ophiostomatoid fungi.

Conulariids of West Africa have received relatively little detailed attention in the literature on this widespread extinct group of marine cnidarians. Recently, however, Babcock et al. (1995) described two new species, Paraconularia feldmanni and P. sahara, from the upper part of the Mississippian (Visean) Talak Shale Formation in the Tim Mersoi Basin of northern Niger. This was the first report of the genus Paraconularia Sinclair, 1940 from Africa as well as the first assignment of Carboniferous conulariids from that continent to the species level. Since then two of the present authors have collected 24 additional conulariid specimens from the Talak Formation in the same area that yielded the two specimens described by Babcock et al. (1995). Like the previously described material, most of the new specimens, described below, have been worn or weathered to such an extent that it is difficult to identify them to the species level. In some cases, moreover, identification to the genus level is problematical, but this may ultimately be due to the fact that currently recognized conulariid genera (e.g., Moore and Harrington, 1956b) have been defined phenetically, rather than on the basis of prior analysis of the phylogenetic relationships among conulariid species. This means that at least some currently recognized conulariid genera may have been defined by similarities that are primitive or analogous.

The Middle Triassic flora of the Fremouw Formation in the central Transantarctic Mountains consists of conifers, cycads, ferns, pteridosperms, and sphenophytes. Stems with an unusual anatomy have been discovered within silicified peat from the same locality. The diameters of the stems range from 1.4 to 1.7 cm; the longest specimen is approximately 12 cm. In transverse section the vascular system consists of segments that occur as single traces or are connected in the center and anastomose at varying levels within the stem. Each segment contains a bifacial vascular cambium. Secondary tissues of each segment surround a central area of parenchyma and small tracheids presumed to represent primary xylem. Surrounding the stem is a periderm. Traces are produced near the periphery of the axis and consist of radially arranged secondary xylem and a thick periderm. The absence of leaves and reproductive organs leads to uncertain phylogenetic relationships. We are unaware of any Triassic plants with this type of vascular tissue organization, and those plants with a similar type of arrangement occur only in the Devonian and Carboniferous. Possible phylogenetic affinities with the Cladoxylales and Lycophyta are examined, but the anatomical differences, along with stratigraphic age, preclude formal assignment to any known taxon at this time. Therefore, we have assigned it to a new taxon: Hapsidoxylon terpsichorum gen. et sp. nov.

Chicory root, stem, and leaf tissues have the propensity to regenerate organogenetic nodules following a wounding treatment. Using histological methods, we described this phenomenon on leaves of Cichorium intybus L. cv. 'Witloof' cultivated in vitro. Nodule morphogenesis and bud regeneration from nodules were characterized with the assistance of light and transmission microscopy techniques. Semithin and ultrathin sections of leaves and nodules were prepared from nodule induction to bud regeneration phases. Starch and inulin in differentiating tissues were histochemically detected in semithin and handmade sections. Our study demonstrates that nodules are formed in direct contact with vascular bundles of the leaves. The nodules consist of organized structures: a vascular center is surrounded by a large parenchyma, which is delimited by a periderm of multiple cork cell layers. Inside the vascular center near phloem tissues, a secretory network of articulated pseudolaticifers develops. From parenchymatous cells, in the vicinity of vascular centers, endogenous buds can be induced. In this region, several cords of vessel initials formed the vascular connection between neoformed buds and vascular centers. Simultaneously, transfer cell specialization occurs near the neoformed vascular strands. Vascular centers play a major role in this kind of organogenesis. Sugar metabolism appears to be closely related to nodule morphogenetic events.