ANATOMY OF NODES VS. INTERNODES IN COLEUS: THE NODAL CAMBIUM

Secondary growth begins in the nodal regions before the internodal regions in Coleus, so that longitudinally discontinuous vascular cambia are formed in the 6th through the 9th or 10th nodes, where the internodal cambium becomes continuous between nodal cambia. The nodal cambia are identifiable by radial seriation in interfascicular regions, typical cytology of fusiform initials, and the presence of a ray system. Anatomical features distinct from the primary plant body are shared by the nodal and internodal cambia. Branching of primary vascular strands, restricted to procambium and phloem, is virtually confined to nodal regions. In secondary growth, vascular branching of xylem and phloem occurs in both nodes and internodes. Xylem strand branches are formed only from derivatives of vascular cambia. It is proposed that the cambium provides the secondary plant body an efficient channel for lateral auxin transport, by which branching across interfascicular regions is facilitated.     

ORIGIN AND ACTIVITY OF SUCCESSIVE CAMBIA IN CYCAS (CYCADALES)

Observations of the vascular tissue of Cycas shoots have provided supporting evidence that the first vascular cambium as well as subsequent successive cambia are simultaneously active. The establishment of the second cambium occurs during the seedling stage, and differentiates mainly within the cortical cells. However, cambial activity also occurs within phloem parenchyma cells of the first vascular cylinder. Tracheids in the first and the successive vascular cylinders are generally of the same length; however, there is a trend toward increasing length within the successive cylinders, possibly because the successive cambia are long-lived.     

濒危植物海南风吹楠营养器官解剖结构特征

该研究采用石蜡切片和光学显微技术,对海南风吹楠营养器官的解剖结构及其对环境的适应性进行了探讨。结果表明:海南风吹楠为典型异面叶,叶片中脉发达,中部分化出髓,上表皮外侧具角质层,内侧具1层内皮层,下表皮外侧无角质层,有气孔器分布,气孔器为双环型,略下陷;栅栏组织3~4层细胞,海绵组织4~6层细胞。茎的初生结构中表皮轻微角质化,维管束为外韧型,8~10个初生维管束围绕髓排列为1轮;茎的次生结构中,表皮外部角质层加厚,维管柱紧密排列连成环状,次生韧皮部和次生木质部发达,形成层细胞3~5层。根的初生结构中表皮细胞外壁加厚,外皮层细胞体积大,形状不规则,内侧具1层形成层,内皮层具凯氏带,初生木质部为多原型,呈辐射状排列。根的次生结构中木栓层细胞5~6层,木栓层内侧具1层木栓形成层,栓内层细胞3层。海南风吹楠营养器官具有一定耐阴和耐旱结构特征,同时与其生活的热带雨林沟谷中高温荫湿的环境相适应。     

Osservazioni Sull'attività Cambiale Di Nerium Oleander L.

Abstract

Observation on the cambial activity in «Nerium oleander» L. — The cambial activity of «Nerium oleander» has been observed in three localities in Southern Italy: Bari and Lucera, where the oleander is cultivated, and Policoro, in the bed of the river Sinni, where the oleander is spontaneuos. The wood is of the diffuse-porous type. Growth rings corresponding to distinct periods of growth are not distinguishable, as there are no easily distinguishable types of wood (early, intermediate, and late wood). Only occasionally the uniform wood is interrupted by narrow circular bands of a few layers of radially compressed cells. Differences in the behaviour of the cambium have been observed in the three localities. At Policoro there was a long period of activity from the middle of March until November without interruptions. At Bari the cambium was active from the middle of March until the first days of April; after a pause it was active again from the second half of May until die middle of July and again during the second half of September-October and the first ten days of October. In 1957 a slight activity was noticed also during about twenty days in December. At Lucera the active period as compared with Policoro was still shorter than at Bari; in the spring there was a delay in the resumption of activity, while the summer rest period lasted until the middle of October. The cambium was then active again until the first days of November. Some of the differences in the behaviour of the cambium in the three localities could perhaps be explained by climatic or pedologic factors, such as the rich water table in the case of the plant growing in the Sinni bed. In conclusion «N. oleander» seems to be a species adapted to a sub-tropical climate, without distinct growth rings.     

ANOMALOUS SECONDARY GROWTH IN LIANAS OF THE BIGNONIACEAE IS CORRELATED WITH THE VASCULAR PATTERN

Vascular pattern and anomalous secondary growth were studied in shoots of Clytostoma callistegoides, a liana having two types of phyllotaxy, one decussate and the other whorled. In shoots with decussate phyllotaxy, typical of bignoniaceous lianas, the vascular pattern has four major vascular strands that extend continuously from internode to internode, whereas in shoots having a whorled phyllotaxy the pattern has six major vascular strands. The first unidirectional cambium segments which result in the anomalous secondary growth were initiated precisely opposite each of the major vascular strands in both types of shoots. It is concluded that positioning of unidirectional cambium segments responsible for anomalous growth is correlated morphogenetically with the vascular pattern.     

Formation of successive cambia and stem anatomy of Sesuvium sesuvioides (Aizoaceae)

Mature stems of Sesuvium sesuvioides (Fenzl) Verdc. were found to be composed of successive rings of xylem alternating with phloem. Repeated periclinal divisions in the parenchyma outside the primary phloem gave rise to conjunctive tissue and the lateral meristem that differentiate into the vascular cambium on its inner side. After the formation of the vascular cambium, the lateral meristem external to it became indistinct as long as the cambium was functional. As the cambium ceased to divide, the lateral meristem again became apparent prior to the initiation of the next cambial ring. The cambium was exclusively composed of fusiform cambial cells with no rays. In the young saplings, the number of cambial cylinders in the axis varied from the apex to the base, indicating formation of several rings within the year. In each successive ring of the lateral meristem, small segments differentiated into the vascular cambium and gave rise to vessels, axial parenchyma, fibres and fibriform vessels towards the inside, and secondary phloem on the outer side. In the old stems, non‐functional phloem of the innermost rings was replaced by a new set of sieve tube elements formed by periclinal divisions in the cambial segments associated with the non‐functional phloem. In some places the cambial segments completely differentiate into derivatives leaving no cambial cells between the xylem and phloem. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 548–555.     

桔梗根的发育解剖学研究

以桔梗(Platycodon grandiflorum A.DC)根为材料,运用石蜡切片和半薄切片法对其根的发育过程及结构进行解剖学观察,并对不同年限根的结构进行了比较。结果表明:桔梗根的结构发育过程包括原生分生组织、初生分生组织、初生生长和次生生长4个阶段。其原生分生组织由3群原始细胞组成,表现出典型分生组织的细胞学特征;初生分生组织包括根冠原、表皮原、皮层原和中柱原;初生结构由表皮、皮层和中柱组成,其中皮层薄壁细胞占主要地位,初生木质部为二原型;次生生长主要依靠维管形成层和木栓形成层的活动来完成,其次生结构从外到内由周皮和次生维管组织组成,次生维管组织占主导地位,其中以薄壁细胞为主,维管分子少量,分散在薄壁组织中。不同年限的根的结构基本相同,但它们在主根长度和直径、周皮厚度、木质部与韧皮部面积之比等方面存在差异。     

Behind the curtain of the compartmentalization process: Exploring how xylem vessel diameter impacts vascular pathogen resistance

A key determinant of plant resistance to vascular infections lies in the ability of the host to successfully compartmentalize invaders at the xylem level. Growing evidence supports that the structural properties of the vascular system impact host vulnerability towards vascular pathogens. The aim of this study was to provide further insight into the impact of xylem vessel diameter on compartmentalization efficiency and thus vascular pathogen movement, using the interaction between Vitis and Phaeomoniella chlamydospora as a model system. We showed experimentally that an increased number of xylem vessels above 100 μm of diameter resulted in a higher mean infection level of host tissue. This benchmark was validated within and across Vitis genotypes. Although the ability of genotypes to restore vascular cambium integrity upon infection was highly variable, this trait did not correlate with their ability to impede pathogen movement at the xylem level. The distribution of infection severity of cuttings across the range of genotype's susceptibility suggests that a risk-based mechanism is involved. We used this experimental data to calibrate a mechanistic stochastic model of the pathogen spread and we provide evidence that the efficiency of the compartmentalization process within a given xylem vessel is a function of its diameter.     

The Populus homeobox gene ARBORKNOX1 reveals overlapping mechanisms regulating the shoot apical meristem and the vascular cambium

Secondary growth is supported by a dividing population of meristematic cells within the vascular cambium whose daughter cells are recruited to differentiate within secondary phloem and xylem tissues. We cloned a Populus Class 1 KNOX homeobox gene, ARBORKNOX1 (ARK1), which is orthologous to Arabidopsis SHOOT MERISTEMLESS (STM). ARK1 is expressed in the shoot apical meristem (SAM) and the vascular cambium, and is down-regulated in the terminally differentiated cells of leaves and secondary vascular tissues that are derived from these meristems. Transformation of Populus with either ARK1 or STM over-expression constructs results in similar morphological phenotypes characterized by inhibition of the differentiation of leaves, internode elongation, and secondary vascular cell types in stems. Microarray analysis showed that 41% of genes up-regulated in the stems of ARK1 over-expressing plants encode proteins involved in extracellular matrix synthesis or modification, including proteins involved in cell identity and signaling, cell adhesion, or cell differentiation. These gene expression differences are reflected in alterations of cell wall biochemistry and lignin composition in ARK1 over-expressing plants. Our results suggest that ARK1 has a complex mode of action that may include regulating cell fates through modification of the extracellular matrix. Our findings support the hypothesis that the SAM and vascular cambium are regulated by overlapping genetic programs. Electronic Supplementary Material Supplementary material is available for this article at This work was supported by the USDA Forest Service and USDA NRI Grant 2003-00664 to AG, and a grants from the U.S. Department of Energy, Office of Science, Biological and Environmental Research Carbon Sequestration Program to AG and SD.     

Anatomy of stem‐node‐leaf continuum in Aconitum (Ranunculaceae) in the Eastern Carpathians

Studies of the anatomical structure of the stem and leaf, with special emphasis on the organization of the vascular system, has been carried out on 13 Aconitum species from Aconitum subgenera Aconitum, Anthora and Lycoctonum. All investigated species show a more or less mesomorphic anatomical structure, typical for other Ranunculaceae. Hence, these species have similar trilacunar three‐trace organization of the nodal vascular system. In the stem the vascular system is open (with weakly developed cambium) or closed collateral, and incomplete (consisting of the bundles were represented), large complete, middle‐size complete and small incomplete or with weakly developed xylem. The number of vascular bundles in petioles appear to have no taxonomical value. Nevertheless, it was found that the spatial organization of these vascular bundles in the petiole are of taxonomic importance. As a result, the investigated species can be divided into four main groups congruent with the current sectional and subgeneric division of the genus. The only exception was A. × cammarum (A. sect. Acomarum) in which the is identical to that of A. sect. Cammarum. The most primitive vascularization is found in A. anthora, while the most advanced one is found in A. variegatum. The highly differentiated and distinct nodal anatomy of A. anthora suggests a high, plausibly subgeneric, taxonomical rank of this species.     

Continuity of Procambium and Anomalous Cambium During Formation of Successive Cambia in Celosia argentea

The development of woody plants is related to the continuity of the procambium and cambium. Whether such a continuity is present in plants with successive cambia, especially in those, where the first cambium is formed outside the primary vascular bundles, has not been analyzed so far. Therefore, we studied the development of vascular meristem in Celosia argentea, in which the first and successive cambial cylinders arise outside the primary bundles and, intriguingly, in the literature are interpreted as developmentally independent structures. Our results showed that in C. argentea, the outermost procambial cells maintain their meristematic characteristics during differentiation of vascular bundles and divide periclinally, forming the zone of procambium-derived cells outside the primary bundles. This zone comprises parenchyma cells bordering the bundles, and a continuous ring of the incipient cambial cells neighboring the primary cortex. Later in the development, the ability to preserve the outermost cells in the cambium undifferentiated is repeated during the formation of successive cylinders of cambia. Together, our results clearly point to the developmental continuity of the procambium and successive cambia in C. argentea, despite their seemingly spatial distinctiveness. We postulate that the mechanism demonstrated in C. argentea is universal and orchestrates the development of successive cambia in other plant species.

     

Anomalous secondary vascular tissue inHelminthostachys zeylanica (Ophioglossaceae)

The vascular anatomy ofHelminthostachys zeylanica was examined with special reference to anomalous secondary tissue. Primary xylem development gradually takes place centrifugally. In branched rhizomes with destroyed apices, the vascular cylinder apical to the insertion of branch traces is generally composed of primary xylem, accessory xylem, inner parenchyma of radially arranged cells, outer parenchyma of irregularly arranged cells, and partly crushed phloem, listed in order going outwards. The accessory xylem as well as the inner parenchyma ofHelminthostachys zeylanica is probably secondarily produced, partly to contribute to the branch traces, in a position corresponding to that of secondary vascular tissue developed from a normal cambium inBotrychium sensu lato. It is suggested that although a cambium is lacking inHelminthostachys zeylanica, the secondary vascular tissues are comparable between the genera. The phylogenetic implication of this tissue is discussed.     

TYPES OF CAMBIAL ACTIVITY AND WOOD ANATOMY OF STYLIDIUM (STYLIDIACEAE)

Three types of cambial activity, two hitherto unreported, are described for Stylidium. The four species of sect. Rhynchangium of subgenus Nitrangium have woody cylinders in upright stems. In these a cambium formed beneath the endodermis produces a determinate quantity of fibers, vessel elements, and interxylary phloem strands toward the inside but no derivatives toward the outside; this was correctly reported by Van Tieghem and Morot (1884a) but doubted by subsequent workers. The same species have lignotubers in which a cambium produces contorted xylem (mostly vessels) to the inside, phellem toward the outside. In S. glandulosum and S. laricifolium a cambium formed beneath the endodermis produces an indeterminate quantity of xylem (fibers and vessel elements) and interxylary phloem toward the inside, nothing toward the outside. The xylem is rayless and lacks axial xylem parenchyma. These three modes of cambial activity represent innovations within Stylidiaceae. The family has a wholly herbaceous ancestry if one can judge from the total lack of cambial activity in vascular bundles.     

不同水稻品种叶片显微结构和超微结构的比较研究

以江苏省常规粳稻品种武育粳3号、武运粳7号和日本优质粳稻品种关东194为材料,利用光学显微镜和透射电子显微镜,研究了抽穗期叶片主脉的显微结构和超微结构。结果表明武育粳3号和武运粳7号叶片主脉的维管束数目和气腔数目均较多,叶绿体基粒片层排列紧密,中间无空隙。而关东194叶片主脉的维管束数目和气腔数目较少,叶绿体基粒片层排列疏松,中间有空隙。武育粳3号和武运粳7号的这种结构特点可能与其高产特性有关。     

欧美杂种山杨微扦插不定根发生过程的解剖学研究

采用石蜡切片技术,以欧美杂种山杨插穗基部茎段为实验材料,连续解剖观察插穗不定根发生发育过程,分析根原基发生部位与扦插生根的关系。结果显示:欧美杂种山杨插穗不定根的发生过程分为4个时期,为根原基诱导期,不定根起始期、表达期和伸长生长期。根原基诱导期维管形成层产生具有分生组织特点的薄壁细胞;不定根起始期,维管形成层及附近的薄壁细胞脱分化,形成不定根原基发端细胞;不定根表达期,根原基发端细胞不断分裂成具有方向性的根原基,根原基穿过韧皮射线和皮层,向皮孔方向发展;不定根伸长生长期,根原基从皮孔伸出,其内部的维管系统开始发育,形成不定根。研究认为,欧美杂种山杨为皮部诱导生根类型,不定根原基起源于维管形成层区,起源部位单一,扦插难生根。