SUCCESSIVE CAMBIA IN THE STEM OF PHYTOLACCA DIOICA

Phytolacca dioica L., an evergreen tree of the Phytolaccaceae, is one of the species of Phytolacca which shows anomalous secondary thickening in its stem. This mode of thickening has been regarded as successive cambial activity or alternatively, in some more recent interpretations, as thickening by unidirectional activity of a cambial zone. The stem thickening of P. dioica is of the former type. The cambium produces fascicular strands, showing centrifugal differentiation of xylem and centripetal differentiation of phloem on opposite sides of the cambial layer, and rays are produced between the fascicular areas. In both xylem and phloem the younger elements are closer to the cambium than the older elements. Succeeding cambia arise periodically by periclinal divisions in a layer of parenchyma cells two or three cells beyond the outermost intact phloem derived from the current cambium. Each cambium forms a few parenchyma cells on both sides before it forms derivatives which mature into lignified xylem elements or conductive elements of the phloem. The parenchyma thus formed toward the outside later becomes the site of the origin of the succeeding cambium. Only one or two layers of this phloem parenchyma go on to form the new cambium; the remaining cells accumulate between the outermost phloem and the cortex. P. weberbaueri shows stem structure similar to P. dioica. P. meziana, a shrub, shows normal stem structure.     

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.     

Developmental anatomy of vascular cambium and periderm ofBotrypus virginianus and its bearing on the systematic position of ophioglossaceae

The developmental anatomy of the vascular cambium and periderm ofBotrypus virginianus was studied, and its bearing on the systematic position of Ophioglossacease is discussed. The cambial zone including cambium is initiated in a procambial ring of the stem before primary vascular tissue is well differentiated. The presumed cambium is composed of fusiform and ray initials. The cambium is extremely unequally bifacial, producing secondary xylem centripetally, and quite a small number of parenchymatous cells but no secondary phloem centrifugally. The cambial activity persists long, although it is very low in the mature part of the stem. It seems that the circumferential increase of the cambium is accommodated by an increase in the number of cambial initials. Secondary xylem is nonstoried and composed of tracheids with circular-bordered pits with evenly thick pit membranes, and uniseriate or partly biseriate radial rays. It makes up the bulk of the stem xylem. Periderm is formed almost entirely around the stem, simultaneous with its increment due to the secondary xylem. The combination of these anatomical features of secondary tissue supports the idea that Ophioglossaceae are living progymnosperms.     

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 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.     

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.     

Efficiency of Agrobacterium rhizogenes–mediated root transformation of Parasponia and Trema is temperature dependent

Parasponia trees are the only non-legume species that form nitrogen-fixing root nodules with rhizobium. Based on its taxonomic position in relation to legumes (Fabaceae), it is most likely that both lineages have gained this symbiotic capacity independently. Therefore, Parasponia forms a bridging species to understand the evolutionary constraints underlying this symbiosis. However, absence of key technologies to genetically modify Parasponia seriously impeded studies on these species. We employed Agrobacterium rhizogenes to create composite Parasponia andersonii plants that harbour transgenic roots. Here, we provide an optimized protocol to infect P. andersonii as well as its non-symbiotic sister species Trema tomentosa with A. rhizogenes. We show that the transformation efficiency is temperature dependent. Whereas the optimal growth temperature for both these species is 28?°C, the transformation is most efficient when co-cultivation with A. rhizogenes occurs at 21?°C. Using this optimized protocol up to 80?% transformation efficiency can be obtained. These robust transformation platforms will provide a strong tool to unravel the Parasponia–rhizobium symbiosis.     

PHLOEM OF SPHENOPHYLLUM

The phloem of most fossil plants, including that of Sphenophyllum, is very poorly known. Sphenophyllum was a relatively small type of fossil arthrophyte with jointed stems bearing whorls of leaves ranging in form from wedge or fan-shaped to bifid, to linear. The aerial stem systems of the plant exhibited determinate growth involving progressive reduction in the dimensions of the stem primary bodies, fewer leaves per whorl, and smaller and simpler leaves distally. The primary phloem occurs in three areas alternating in position with the arms of the triarch centrally placed primary xylem. Cells of the primary phloem, presumably sieve elements, are axially elongate with horizontal to slightly tapered end walls. In larger stems with abundant secondary xylem and secondary cortex or periderm, a zone of secondary phloem occurs whose structure varies in the three areas opposite the arms of the primary xylem, as opposed to the three areas lying opposite the concave sides of the primary xylem. The axial system of the secondary phloem consists of vertical series of sieve elements with horizontal end walls. In the areas opposite the protoxylem the parenchyma is present as a prominent ray system showing dilation peripherally. Sieve elements in the areas opposite the protoxylem arms have relatively small diameters. In the areas between the protoxylem poles the secondary phloem sieve elements have large diameters and are less obviously in radial files, while the parenchyma resembles that of the secondary xylem in these areas in that it consists of strands of cells extending both radially and tangentially. An actively meristematic vascular cambium has not been found, indicating that this layer changed histologically after the cessation of growth in the determinate aerial stem systems and was replaced by a post-meristematic parenchyma sheath made up of axially elongate parenchyma lacking cells indicative of being either fusiform or ray initials. A phellogen arose early in development in a tissue believed to represent pericycle and produced tissue comparable to phellem externally. Normally, derivatives of the phellogen underwent one division prior to the maturation of the cells. Concentric bands of cells with dark contents apparently represent secretory tissue in the periderm and cell arrangements indicate that a single persistent phellogen was present. Sphenophyllum is compared with other arthrophytes as to phloem structure and is at present the best documented example of a plant with a functionally bifacial vascular cambium in any exclusively non-seed group of vascular plants.     

白鲜根的发育解剖学研究

应用半薄切片、常规石蜡切片并结合离析法,对药用植物白鲜(Dictamnus dasycarpus Turcz.)根的发生发育过程进行了研究。结果表明:白鲜根的发生发育过程包括4个阶段,即原分生组织阶段、初生分生组织阶段、初生结构阶段以及次生结构阶段。原分生组织位于根冠内侧及初生分生组织之间,衍生细胞分化为初生分生组织。初生分生组织由原表皮、基本分生组织以及中柱原组成。原表皮分化为表皮,基本分生组织分化为皮层,中柱原分化为维管柱,共同组成根的初生结构;在初生结构中,部分表皮细胞外壁向外延伸形成根毛,皮层中分布有油细胞,内皮层有凯氏带,初生木质部为二原型或偶见三原型,外始式;根初生结构有髓或无。次生结构来源于原形成层起源的维管形成层的活动以及中柱鞘起源的木栓形成层的活动;白鲜次生韧皮部宽广,其中多年生根中可占根横切面积的85%,另外除基本组成分子外,还分布有油细胞;周皮发达,木栓层厚;初生皮层、次生木质部和次生韧皮部薄壁细胞中常充满丰富的淀粉粒。     

白鲜营养器官解剖结构及其与白鲜碱的积累关系

应用植物解剖学、组织化学及植物化学方法对白鲜营养器官根、茎、叶的结构及其生物碱的积累进行了研究。结果显示:(1)白鲜根的次生结构以及茎和叶的结构类似一般双子叶植物;白鲜多年生根主要由周皮、次生韧皮部、维管形成层以及次生木质部组成,根次生韧皮部中可见大量的淀粉、草酸钙簇晶、韧皮纤维以及油细胞;茎由表皮、皮层、维管组织和髓组成;叶由表皮、栅栏组织、海绵组织和叶脉组成;在茎和叶初生韧皮部的位置均分布有韧皮纤维,在叶表皮上分布有头状腺毛和非腺毛;在茎和叶紧贴表皮处分布有分泌囊。(2)组织化学分析结果显示:在白鲜多年生根中,生物碱类物质主要分布在周皮、次生韧皮部、维管形成层和木薄壁细胞中;在茎中,生物碱主要分布在表皮、皮层、韧皮部、木薄壁细胞及髓周围薄壁细胞中;在叶中,生物碱主要分布在表皮细胞、叶肉组织和维管组织的薄壁细胞;此外在分泌囊和头状腺毛中亦含有生物碱类物质。(3)植物化学结果显示,秦岭产白鲜根皮/白鲜皮、根木质部、茎和叶中白鲜碱含量分别为0.041%、0.012%、0.004%和0.002%,其中木质部中白鲜碱含量和其他部分地区白鲜皮中白鲜碱含量类似。研究表明,在秦岭产白鲜营养器官中,除根皮/白鲜皮外,在根木质部亦含有大量的白鲜碱,且在茎和叶中亦含有一定的白鲜碱,具有潜在的开发利用价值。     

Organ regulated expression of the Parasponia andersonii haemoglobin gene in transgenic tobacco plants

Summary Plant haemoglobin genes are known to occur in legume and non-legume families and in both nodulating (e.g. Parasponia andersonii) and non-nodulating species (e.g. Trema tomentosa). Their presence in non-nodulating plants raises the possibility that haemoglobins might serve a function in non-symbiotic tissues distinct from their role in the nitrogen-fixing root nodules induced by micro-organisms. We report here that a P. andersonii haemoglobin promoter can regulate expression of either the P. andersonii haemoglobin gene, or a hybrid construct with the bacterial chloramphenicol acetyltransferase gene (cat), in the nonsymbiotic plant, Nicotiana tabacum. Expression is predominantly in the roots, implying that haemoglobins might have a function in roots of non-nodulated plants. We have also observed a low level of haemoglobin protein in non-nodulated P. andersonii roots, but not leaves, supporting this assertion. The expression in transgenic plants will allow further characterization of the promoter sequences essential for the organ-specific expression of haemoglobins in nonsymbiotic tissues.     

远志根的发育解剖学研究

运用石蜡切片法对远志根的发育过程及1~3年生根的结构进行解剖学研究。结果显示:远志根的原分生组织由3群原始细胞组成,具有典型分生组织的细胞学特征。初生分生组织分化为根冠原、表皮原、皮层原和中柱原;初生结构由表皮、皮层和中柱组成,初生木质部为二原型。次生生长是依靠维管形成层和木栓形成层的活动完成,次生结构从外到内由周皮和次生维管组织组成;远志根次生结构特点为:次生韧皮部在次生维管组织中占主要部分,次生韧皮部中以韧皮薄壁细胞为主且其中储存有丰富的内含物,随着根龄的增加,韧皮薄壁细胞中的内含物也随之增加。3年生的主根中次生韧皮部薄壁细胞中的内含物最丰富;不同年份远志的主根随根龄的增加,周皮、次生韧皮部和次生木质部的面积都呈增加趋势,其中韧皮部和木质部的面积比值随根龄增长呈由小到大的变化,这是远志根的显著特点;根中的周皮发达,具有较厚的木栓层,次生木质部中导管和纤维发达,导管分布频率较高,并具有较大的口径。周皮和次生木质部的结构特征与远志的抗旱特性相适应。     

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.     

Seasonality and growth rings in lianas of Bignoniaceae

Lianas are one of the most important components of tropical forest, and yet one of the most poorly known organisms. Therefore, our paper addresses questions on the environmental and developmental aspects that influence the growth of lianas of Bignoniaceae, tribe Bignonieae. In order to better understand their growth, we studied the stem anatomy, seasonality of formation and differentiation of secondary tissues, and the influence of the cambial variant in xylem development on a selected species: Tynanthus cognatus. Afterwards, we compared the results found in T. cognatus with 31 other species of Bignonieae to identify general patterns of growth in lianas of this tribe. We found that cambial activity starts toward the end of the rainy season and onset of the dry season, in contrast to what is known for tropical trees and shrubs. Moreover, their pattern of xylem formation and differentiation is strongly influenced by the presence of massive wedges of phloem produced by a variant cambium. Thus, the variant cambium is the first to commence its activity and only subsequently does cambial activity progress towards the center of the regular region, leading to the formation of confluent growth rings. In summary, we conclude that: the cambium responds to environmental changes; the xylem growth rings are annual and produced in a brief period of about 2 months, something that may explain why lianas possess narrow stems; and furthermore, phloem wedges greatly influence cambial activity.     

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.     

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.

     

Secondary Growth in Bougainvillea

The anomalous secondary growth was studied in roots and stemsof two species of Bougainvillea. The anomalous cambia arisesuccessively in centrifugal order, each originating among thederivatives of the preceding cambium. Each cambium layer functionsbidirectionally producing xylem towards the inside of the axisand phloem towards the outside. The sequence of production ofvascular cells is the following: (1) conjunctive tissue andxylem fibres towards the inside; (2) phloem towards the outside;(3) additional xylem with vessels towards the inside and additionalphloem towards the outside. The new cambia arise outside theoldest phloem cells of a given increment. This phloem may benonfunctional and crushed at that time. The phloem and the xylemdifferentiate from radially seriated derivatives produced sequentiallyby tangential divisions in the cambium. Divisions among thephloem initials and growth readjustments in the differentiatingxylem obscure the radial seriation to a moderate extent.     

The rise and evolution of the cambial variant in Bignonieae (Bignoniaceae)

SUMMARY Cambial variants represent a form of secondary growth that creates great stem anatomical diversity in lianas. Despite the importance of cambial variants, nothing is known about the developmental mechanisms that may have led to the current diversity seen in these stems. Here, a thorough anatomical analysis of all genera along the phylogeny of Bignonieae (Bignoniaceae) was carried out in order to detect when in their ontogeny and phylogeny there were shifts leading to different stem anatomical patterns. We found that all species depart from a common developmental basis, with a continuous, regularly growing cambium. Initial development is then followed by the modification of four equidistant portions of the cambium that reduce the production of xylem and increase the production of phloem, the former with much larger sieve tubes and an extended lifespan. In most species, the formerly continuous cambium becomes disjunct, with cambial portions within phloem wedges and cambial portions between them. Other anatomical modifications such as the formation of multiples of four phloem wedges, multiple-dissected phloem wedges, and included phloem wedges take place thereafter. The fact that each novel trait raised on the ontogenetic trajectory appeared in subsequently more recent ancestors on the phylogeny suggests a recapitulatory history. This recapitulation is, however, caused by the terminal addition of evolutionary novelties rather than a truly heterochronic process. Truly heterochronic processes were only found in shrubby species, which resemble juveniles of their ancestors, as a result of a decelerated phloem formation by the variant cambia. In addition, the modular evolution of phloem and xylem in Bignonieae seems to indicate that stem anatomical modifications in this group occurred at the level of cambial initials.