Radial secondary growth and formation of successive cambia and their products in Ipomoea hederifolia L. (Convolvulaceae)

Ipomoea hederifolia stems increase in thickness using a combination of different types of cambial variant, such as the discontinuous concentric rings of cambia, the development of included phloem, the reverse orientation of discontinuous cambial segments, the internal phloem, the formation of secondary xylem and phloem from the internal cambium, and differentiation of cork in the pith. After primary growth, the first ring of cambium arises between the external primary phloem and primary xylem, producing secondary phloem centrifugally and secondary xylem centripetally. The stem becomes lobed, flat, undulating, or irregular in shape as a result of the formation of both discontinuous and continuous concentric rings of cambia. As the formation of secondary xylem is greater in one region than in another, this results in the formation of a grooved stem. Successive cambia formed after the first ring are of two distinct functional types: (1) functionally normal successive cambia that divide to form secondary xylem centripetally and secondary phloem centrifugally, like other dicotyledons that show successive rings, and (2) abnormal cambia with reverse orientation. The former type of successive rings originates from the parenchyma cells located outside the phloem produced by previous cambium. The latter type of cambium develops from the conjunctive tissue located at the base of the secondary xylem formed by functionally normal cambia. This cambium is functionally inverted, producing secondary xylem centrifugally and secondary phloem centripetally. In later secondary growth, xylem parenchyma situated deep inside the secondary xylem undergoes de‐differentiation, and re‐differentiates into included phloem islands in secondary xylem. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 30–40.     

Stem anatomy and development of inter- and intraxylary phloem in Leptadenia pyrotechnica (Forssk.) Decne. (Asclepiadaceae)

Modification of external morphology and internal structure of plants is a key feature of their successful survival in extreme habitats. They adapt to arid habitats not only by modifying their leaves, but also show several modifications in their conducting system. Therefore, the present study is aimed to investigate the pattern of secondary growth in Leptadenia pyrotechnica (Forssk.) Decne., (Asclepiadaceae), one such species growing in Kachchh district, an arid region of Gujarat State. A single ring of vascular cambium, responsible for radial growth, divided bidirectionally and formed the secondary xylem centripetally and the phloem centrifugally. After a short period of secondary xylem differentiation, small arcs of cambium began to form secondary phloem centripetally instead of secondary xylem. After a short duration of such secondary phloem formation, these segments of cambium resumed their normal function to produce secondary xylem internally. Thus, the phloem strands became embedded within the secondary xylem and formed interxylary phloem islands. Such a recurrent behavior of the vascular cambium resulted in the formation of several patches of interxylary phloem islands. In thick stems the earlier formed non-conducting interxylary phloem showed heavy accumulation of callose on the sieve plates followed by their crushing in response to the addition of new sieve elements. Development of intraxylary phloem is also observed from the cells situated on the pith margin. As secondary growth progresses further, small arcs of internal cambium get initiated between the protoxylem and intraxylary phloem. In the secondary xylem, some of the vessels are exceptionally thick-walled, which may be associated with dry habitats in order to protect the vessel from collapsing during the dryer part of the year. The inter- and intraxylary phloem may also be an adaptive feature to prevent the sieve elements to become non-conducting during summer when the temperature is much higher.     

Wood Anatomy and the Development of Interxylary Phloem of Ipomoea hederifolia Linn. (Convolvulaceae)

In Ipomoea hederifolia Linn., stems increase in thickness by forming successive rings of cambia. With the increase in stem diameter, the first ring of cambium also gives rise to thin-walled parenchymatous islands along with thick-walled xylem derivatives to its inner side. The size of these islands increases (both radially and tangentially) gradually with the increase in stem diameter. In pencil-thick stems, that is, before the differentiation of a second ring of cambium, some of the parenchyma cells within these islands differentiate into interxylary phloem. Although all successive cambia forms secondary phloem continuously, simultaneous development of interxylary phloem was observed in the innermost successive ring of xylem. In the mature stems, thick-walled parenchyma cells formed at the beginning of secondary growth underwent dedifferentiation and led to the formation of phloem derivatives. Structurally, sieve tube elements showed both simple sieve plates on transverse to slightly oblique end walls and compound sieve plates on the oblique end walls with poorly developed lateral sieve areas. Isolated or groups of two to three sieve elements were noticed in the rays of secondary phloem. They possessed simple sieve plates with distinct companion cells at their corners. The length of these elements was more or less similar to that of ray parenchyma cells but their diameter was slightly less. Similarly, in the secondary xylem, perforated ray cells were noticed in the innermost xylem ring. They were larger than the adjacent ray cells and possessed oval to circular simple perforation plates. The structures of interxylary phloem, perforated ray cells, and ray sieve elements are described in detail.     

Vascular Continuity in the Primary and Secondary Stem Tissues of Bougainvillea (Nyctaginaceae)

An investigation of stem structure of Bougainvillea by serialsections and cine-photography shows that the medullary systemof the inner area of young stems is the sole vascular systemdirectly continuous into the lateral appendages (leaves, axillarybuds and axillary thorns) via complex nodal anastomoses. Thevascular system at the periphery of the primary bundles is notdirectly continuous into these appendages. In secondary growth,there is direct continuity between vascular bundles within asingle ring, in a tangential direction via either xylem aloneor both xylem and phloem, and between rings in a radial directionalways via xylem and phloem, even though the rings are derivativesof successive cambia. Bougainvillea, vascular system, phloem, xylem, anomalous secondary thickening     

Stem anatomy of Dolichos lablab Linn (Fabaceae): Origin of cambium and reverse orientation of vascular bundles

Secondary growth in the stem of Dolichos lablab is achieved by the formation of eccentric successive rings of vascular bundles. The stem is composed of parenchymatous ground tissue and xylem and phloem confined to portions of small cambial segments. However, development of new cambial segments can be observed from the obliterating ray parenchyma, the outermost phloem parenchyma and the secondary cortical parenchyma. Initially cambium develops as small segments, which latter become joined to form a complete cylinder of vascular cambium. Each cambial ring is functionally divided into two distinct regions. The one segment of cambium produces thick-walled lignified xylem derivatives in centripetal direction and phloem elements centrifugally. The other segment produces only thin-walled parenchyma on both xylem and phloem side. In mature stems, some of the axial parenchyma embedded deep inside the xylem acquires meristematic activity and leads to the formation of thick-walled xylem derivatives centrifugally and phloem elements centripetally. The secondary xylem comprises vessel elements, tracheids, fibres and axial parenchyma. Rays are uni-multiseriate in the region of cambium that produces xylem and phloem derivatives, while in some of the regions of cambium large multiseriate, compound, aggregate and polycentric rays can be noticed.     

Stem anatomy of the dwarf subshrub Cressa cretica L. (Convolvulaceae)

Stem anatomy and development of medullary phloem are studied in the dwarf subshrub Cressa cretica L. (Convolvulaceae). The family Convolvulaceae is dominated by vines or woody climbers, which are characterized by the presence of successive cambia, medullary- and included phloem, internal cambium and presence of fibriform vessels. The main stems of the not winding C. cretica shows presence of medullary (internal) phloem, internal cambium and fibriform vessels, whereas successive cambia and included phloem are lacking. However, presence of fibriform vessels is an unique feature which so far has been reported only in climbing members of the family. Medullary phloem develops from peri-medullary cells after the initiation of secondary growth and completely occupies the pith region in fully grown mature plants. In young stems, the cortex is wide and formed of radial files of tightly packed small and large cells without intercellular air spaces. In thick stems, cortical cells become compressed due to the pressure developed by the radial expansion of secondary xylem, a feature actually common to halophytes. The stem diameter increases by the activity of a single ring of vascular cambium. The secondary xylem is composed of vessels (both wide and fibriform), fibres, axial parenchyma cells and uni-seriate rays. The secondary phloem consists of sieve elements, companion cells, axial and ray parenchyma cells. In consequence, Cressa shares anatomical characteristics of both climbing and non-climbing members. The structure of the secondary xylem is correlated with the habit and comparable with that of other climbing members of Convolvulaceae.     

Seasonal dynamics of phloem and xylem formation in silver fir and Norway spruce as affected by drought

The dynamics of phloem growth ring formation in silver fir (Abies alba Mill.) and Norway spruce (Picea abies Karst.) at different sites in Slovenia during the droughty growing season of 2003 was studied. We also determined the timing of cambial activity, xylem and phloem formation, and counted the number of cells in the completed phloem and xylem growth rings. Light microscopy of cross-sections revealed that cambial activity started on the phloem and xylem side simultaneously at all three plots. However, prior to this, 1–2 layers of phloem derivatives near the cambium were differentiated without previous divisions. The structure of the early phloem was similar in silver fir and Norway spruce. Differences in the number of late phloem cells were found among sites. Phloem growth rings were the widest in Norway spruce growing at the lowland site. In all investigated trees, the cambium produced 5–12 times more xylem cells than phloem ones. In addition, the variability in the number of cells in the 2003 growth ring around the stem circumference of the same tree and among different trees was higher on the xylem side than on the phloem side. Phloem formation is presumably less dependent on environmental factors but is more internally driven than xylem formation.     

Spatial relation between xylem and phloem in the stem of Hibiscus cannabinus L. (Malvaceae)

The distribution of the phloem in relation to the xylem was examined in the stem of Hibiscus cannabinus L. with reference to a report in the literature that this species has internal (intraxylary) phloem, a feature not previously observed in the Malvaceae. In the present study, the stem was found to have phloem only outside the xylem (external or extraxylary phloem). In the protophloem, the sieve tubes are obliterated while the internode elongates and the associated cells become fibres with thick secondary walls. Fibres occur in the secondary phloem also. As seen in transections of stems, the secondary xylem forms a continuous ring. The primary xylem extends in the form of arcs into the pith. The tracheary cells of the protoxylem become crushed or completely obliterated in elongating internodes. The associated parenchyma cells either retain thin walls or develop secondary thickenings.     

西洋参根的发育解剖学研究

西洋主根顶端的原分生组织由三群原始细胞组成。初生木质部为三原型。维管形成层产生的次生维管组织中薄壁细胞占主导地位;维管分子量少、聚集成群,分散在薄壁组织中。周皮加、周皮发生较迟,其木栓形成层由紧靠内皮层的皮层细胞产生。不同年龄西洋参主根随着龄龄的增加,周皮、次生真心皮部和木质部面积均呈增加趋势,但韧皮部与木质部面积比值自５：１下降至１：１。一年生根由中柱鞘产生初生分泌道,由维管形成层产生一圈次生分     

Stem anatomy and development of successive cambia in Hebanthe eriantha (Poir.) Pedersen: a neotropical climbing species of the Amaranthaceae

Hebanthe eriantha (Poir.) Pedersen, a climbing species of the Amaranthaceae increases in stem thickness by forming successive cambia. The family is dominated by herbaceous species and is constantly under discussion due to its disputed nature of the meristem. In the young stem small alternate segments of vascular cambium cease to divide and new arc of cambium initiates outside to it. The newly formed arcs connect with pre-existing alternate segments of cambium to complete the ring. On the contrary, in thick stems, instead of small segments, complete ring of cambium is replaced by new one. These new alternate segments/cambia originate from the parenchyma cells located outside to the phloem produced by previous cambium. Cambium is storied and exclusively composed of fusiform initials while ray cells remain absent at least in the early part of the secondary growth. However, large heterocellular rays are observed in 15-mm diameter stems but their frequency is much lower. In some of the rays, ray cells become meristematic and differentiate into radially arranged xylem and phloem elements. In fully grown plants, stems are composed of several successive rings of secondary xylem alternating with secondary phloem. Secondary xylem is diffuse-porous and composed of vessels, fibres, axial parenchyma while exceptionally large rays are observed only in the outermost regions of thick stems. Vessel diameter increases progressively from the centre towards the periphery of stems. Although the origin of successive cambia and composition of secondary xylem of H. eriantha remains similar to other herbaceous members of Amaranthaceae, the occurrence of relatively wider and thick-walled vessels and large rays in fully grown plants is characteristic to climbing habit.     

CO2浓度加倍对辽东栎维管组织结构的影响

 以辽东栎(Quercus liaotungensis)的13年生幼树为材料,分别培养在大气CO2浓度加倍(700μl·L-1)与对照(350μl·L-1)的开顶式熏气室中,研究CO2浓度升高对其茎次生木质部和次生韧皮部结构的影响。结果表明：经CO2浓度加倍处理的两个生长季内,辽东栎的年轮宽度明显增加,为对照的300％～370％,其中晚材宽度的增加更为显著,为对照的750％～830％。另外,晚材中导管的密度和径向直径分别比对照增加50％和20％;木纤维细胞的比例约为对照的170％。但早材的导管分子和木纤维细胞与对照相比均无显著变化。在CO2浓度加倍条件下,辽东栎的次生韧皮部中含晶韧皮薄壁细胞的数目,每条韧皮纤维切向带中韧皮纤维细胞的数目,以及韧皮纤维长度均有显著增加(p≤0.05)。相反地,韧皮纤维细胞的直径和筛管分子长度却无明显变化。值得提出的是,在CO2浓度加倍的条件下,次生韧皮部的宽度、筛管分子的直径、以及每年形成的韧皮部细胞总数分别为对照的82％、87％和80％。综上所述,大气CO2浓度加倍对辽东栎次生木质部的生长发育具明显的正效应,而对次生韧皮部的细胞总数与筛管分子的影响则呈负效应。     

Effects of Growth Hormone Application on the Secondary Growth of Roots and Stems in Picea sitchensis (Bong.) Carr

Indol-3-ylacetic acid (IAA), gibberellin A3 (GA) and 6 benzylaminopurine(BAP) were applied factorially each at 3x10^–2 M in lanolinto the roots and stems of Sitka spruce seedlings and the activityof the two secondary meristems, the vascular cambium and phellogen,and of the parenchymatous tissues between them, was examined.All the treatments, with the exception of GA produced a localizedstimulation of radial growth at the point of application andthere was a similarity in the response of the various tissuesin both the root and stem. Radial growth of the xylem was notsignificantly affected in the roots whereas in the stems BAPand IAA stimulated growth. In the phloem BAP produced significantstimulation in both roots and stems and IAA stimulated growthin the roots. Growth of the parenchyma and periderm externalto the phloem was also strongly stimulated by both BAP, andIAA in roots and stems. In roots and stems the application of BAP altered the derivativesproduced by the vascular cambium, resulting in the productionof large multiseriate rays in the xylem, and giving rise toan overall increase in the proportion of ray tissue. Picea sitchensis (Bong.) Carr, Sitka spruce, secondary growth, xylem, phloem, periderm, wood rays, Indol-3-ylacetic acid, gibberellin A3, 6 benzylaminopurine, growth hormones     

Fluctuations of cambial activity in relation to precipitation result in annual rings and intra-annual growth zones of xylem and phloem in teak (Tectona grandis) in Ivory Coast

Background and Aims Teak forms xylem rings that potentially carry records of carbon sequestration and climate in the tropics. These records are only useful when the structural variations of tree rings and their periodicity of formation are known. Methods The seasonality of ring formation in mature teak trees was examined via correlative analysis of cambial activity, xylem and phloem formation, and climate throughout 1·5 years. Xylem and phloem differentiation were visualized by light microscopy and scanning electron microscopy. Key Results A 3 month dry season resulted in semi-deciduousness, cambial dormancy and formation of annual xylem growth rings (AXGRs). Intra-annual xylem and phloem growth was characterized by variable intensity. Morphometric features of cambium such as cambium thickness and differentiating xylem layers were positively correlated. Cambium thickness was strongly correlated with monthly rainfall (R(2) = 0·7535). In all sampled trees, xylem growth zones (XGZs) were formed within the AXGRs during the seasonal development of new foliage. When trees achieved full leaf, the xylem in the new XGZs appeared completely differentiated and functional for water transport. Two phloem growth rings were formed in one growing season. Conclusions The seasonal formation pattern and microstructure of teak xylem suggest that AXGRs and XGZs can be used as proxies for analyses of the tree history and climate at annual and intra-annual resolution.     

杨树新梢积累营养贮藏蛋白质的细胞学研究

采用光学显微镜和电子显微镜技术,对杨树新梢中的营养贮藏蛋白质进行了细胞学鉴定。在用戊二醛固定的标本中,营养贮藏蛋白质呈颗粒状,积累在中央大液泡里。在新梢伸长生长时期,新梢茎的基部已积累了营养贮藏蛋白质,在伸长生长刚停止,中上部的叶片近成熟时,整个新梢的茎都有营养贮藏蛋白质的积累,其中,以新梢基部的茎最为丰富。营养贮藏蛋白质优先在次生韧皮部的韧皮薄壁细胞和韧皮射线薄壁细胞中积累,在新梢伸长生长停止后,新梢基部茎的木质部中也积累了相当数量的营养贮藏蛋白质,主要分布在初生木质部和内侧次生木质部的各种生活的薄壁细胞中。新梢较早地积累营养贮藏蛋白质是热带树木和温带树木的一个共同特点,对于树木的氮代谢和树木当年的生长发育可能具有重要的调控作用。     

Distribution of an Indoleacetic Acid-oxidase-inhibitor in the Storage Root of Daucus carota

Indoleacetic acid (IAA)-oxidase from both secondary phloem and xylem was dependent on 2,4-dichlorophenol for activity, and was enhanced by addition of Mn²⁺. The pH optimum was 6.0 from both tissues. IAA-oxidase and its inhibitors were distributed differently in the secondary phloem and secondary xylem of carrot root. In the phloem a high IAA-oxidase activity was distributed uniformly along the radius but in the xylem a somewhat lower concentration decreased from the cambium. IAA-oxidase inhibitor in the phloem increased exponentially from a very low concentration near the cambium, whereas in the xylem an appreciable concentration was present near the cambium, decreasing linearly with distance from the cambium. Longitudinal gradients in the xylem parallel studies by other workers with the greatest IAA-destroying capacity present in older tissues. In the xylem inhibitor decreased and IAA-oxidase increased from the root apex. In the phloem IAA-oxidase was uniform, whereas the inhibitor increased in older tissue.

The IAA-oxidase inhibitors in phloem and xylem may be different. In the xylem the IAA-oxidase inhibitor may be a lignin precursor present in young cells which disappears as lignification proceeds. In the phloem IAA-oxidase reacting with endogenous IAA appears to form a physiologically active product.

     

Anatomical Studies on Secondary Xylem and Secondary Phloem of Dipentodon sinicus

The anatomy of secondary xylem and secondary phloem in Dipentodon sinicus Dunn a precious and protected plant in China was studied, and compared with the wood anatomy of 8 other genera in Celastraceae. The main characteristics of this genus were described as follows: vessel members length were relatively long with scalariform perforations in oblique end walls, which were formed by 14 (9--28) bars. Intervessel pits possessed scalariform pattern. Libriform fibres and tracheid-fibres were septate. The sieve tube elements of the secondary phloem were relatively shorter, with simple sieve plates in the slightly inclined to almost horizontal end walls. There was no obvious boundary between the functional and non-functional phloem. In the Baileyan sense, the secondary xylem possessed a lower level of specialization, and displayed an obviously primitive and conservative character. In contrast, the secondary phloem possessed a higher level of specialization and displayed advanced characters. Therefore, the phylogenetic evolution between xylem and phloem was not synchronous in this genus. The result provided a novel pattern in the development of xylem and phloem structure which has not been noticed before.     

五叶木通营养器官解剖结构与三萜皂苷的积累关系

采用显微制片技术、组织化学定位和三萜皂苷定量分析方法,对五叶木通的营养器官分别进行了三萜皂苷的组织化学定位、含量和解剖结构特征研究。结果表明：三萜皂苷在五叶木通不同营养器官中的含量不同,含量测定与组织化学定位推测的结果吻合,叶＞根＞茎;三萜皂苷在同一营养器官的不同发育时期含量不同,总的规律是：老叶＞幼茎＞幼根＞幼叶﹥老根﹥老茎。三萜皂苷在不同器官中具有明确的积累位置：根中主要是次生韧皮部和周皮;皮层和次生韧皮部是茎中三萜皂苷积累部位;叶内则以栅栏组织为主。五叶木通根中初生木质部二原型,次生木质部内导管多为单管孔,少数为管孔链,纤维含量丰富。茎中韧皮纤维丰富,在幼茎中纤维成束状罩在初生韧皮部的外方,在次生韧皮部中纤维呈带状环绕在外方。叶为异面叶。     

掌叶大黄根多糖的积累分布特征

采用组织化学方法和苯酚硫酸比色法研究了掌叶大黄（Rheum palmatum）根中大黄多糖的贮藏分布特征和含量变化规律。结果表明：大黄多糖在根内的贮藏是多位点的,在根周皮的栓内层、次生维管组织的薄壁细胞内不同程度地贮藏和积累了一定数量的大黄多糖,次生木质部的木薄壁细胞和次生韧皮部的韧皮薄壁细胞是主要贮藏和积累的部位;不同发育时期根中大黄多糖含量的变化规律为,随着植物的不断成熟,根及其各组织中大黄多糖的总含量表现为逐渐增高的趋势,但在发育的后期略有下降;韧皮薄壁细胞与木薄壁细胞相比,贮藏大黄多糖的含量相对较多,大黄多糖的贮藏积累方式为逐渐累积的方式。     

A novel insight into the structure of amphivasal secondary bundles on the example of Dracaena draco L. stem

Key message

Pattern of tracheids found along the bundles extends understanding of their cross - sectional anatomy and sheds a new light on the issue of radial transport in monocotyledons with secondary growth.

Abstract

Secondary growth of Dracaena draco L. stem is connected with the formation of amphivasal vascular bundles in which a centrally located phloem is surrounded by a ring of xylem cells (tracheids). However, as visible in a single transverse section, there is a tendency towards variation among the secondary bundles from such with a xylem ring to ones in which the tracheids do not completely surround the phloem, i.e., are separated by vascular parenchyma cells. We aimed to elucidate the cross-sectional anatomy of amphivasal secondary bundles using the method of serial sectioning (with sections 3 μm thick), which allowed us to follow very precisely the bundle structure along its length. The analysis revealed that the xylem arrangement in these bundles depends on the position of a section in the bundle path. Each amphivasal bundle is composed of sectors where tracheids form a ring, as well as of such where tracheids are separated by vascular parenchyma cells. We hypothesize that this structure of amphivasal vascular bundles facilitates radial transport of assimilates to the sink tissues. The result of the anatomical analysis is discussed in a physiological context.