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Interxylary cork and fission of stems and roots
... Evert 2006) and their references, literature dating back to the middle of the last century concerning similar phenomena within other genera could be found. Like- wise root development has been described for about 40 species of the Dicotyledones within the genera Gentiana, Aconitum, Corydalis, Salvia, Sedum (Jost 1890) and Artemisia (Moss & Gorham 1953). ...
... The parts of roots / rhizomes which affiliate directly to dead parts of the plants, get eliminated by the process of the generation of interxylary peridem. Moss & Gorham (1953) mention the importance of the phenomenon as protection against desiccation and for the adaptation of plants to their habitats, mainly dry and wind-blown soil. The splitting of a plant helps it surviving under difficult environmental circumstances, as each single strand may face another microhabitat with its advantages and disadvantages (Ginzburg 1963). ...
... This principle is well known from many lianas. The holding on of the plants to rocky grounds and crevices may be one reason for the splitting of the roots as well as forces in wind blown soil as already mentioned by Moss & Gorham (1953). ...
The root anatomy of the subalpine to alpine plant species Saussurea discolor (Willd.) DC., and Saussurea pygmaea (Jacq.) Spreng., (Asteraceae) has been investigated by means of light and fluorescence microscopy on specimens of Austrian
provenance. Both species develop a so called interxylary cork which mediates the splitting of the root into various strands.
This phenomenon takes place in the secondary xylem and involves the development of a periderm separating the originally solid
xylem cylinder. Interxylary cork is currently known from approximately 40 species of the Dicotyledones. This is the first
report of this specific anatomical structure from the two studied species.
Key wordsAsteraceae–
Saussurea
–interxylary cork–root anatomy
... Axis splitting has been found in a number of families, including the Aizoaceae, Asteraceae, Boraginaceae, Fabaceae, Frankeniaceae, Fumariaceae, Geraniaceae, Lamiaceae, Onagraceae, Ranunculaceae, Rosaceae, Solanaceae, and Zygophyllaceae (Bowns & West 1976;Fahn 1964;Ginzburg 1963;Jones & Lord 1982;Kutschera & Lichtenegger 1992;Mikhailovskaya 1967;Moss & Gorham 1953;Vasilevskaya & Shokina 1968;Wallace & Romney 1972;Werger 1986;Whalen 1987) (Table 1). It is particularly common in shrubs from arid and semi-arid environments (Fahn 1964;Vasilevskaya & Shokina 1968). ...
... For a discussion of detailed anatomical and morphological characteristics of axis splitting the reader is referred to the literature cited. Briefly, splitting of roots, rhizomes, and stems may occur as a result of interxylary cork formation, including suberization of xylem rays, or simply as a result of differential cambial activity, which leads to the formation of grooves and lobes in the wood (Fahn 1982;Fahn & Cutler 1992;Ginzburg 1963;Jones & Lord 1982;Moss & Gorham 1953;Whalen 1987). This creates a fluted to segmented axis (Figures 3b and 3c). ...
... The possibility that the prevalence of axis splitting in desert shrubs may be partly a phylogenetic effect currently cannot be ruled out. Axis splitting appears to have evolved many times independently (Moss & Gorham 1953), but it may be restricted to certain families and genera and/or certain environments. It is certainly a taxonomically widespread phenomenon (see Table 1). ...
Axis splitting is a widespread phenomenon in desert shrubs, and has been reported for shrubs from several plant families, both in old- and new-world deserts. It is so common in dwarf shrubs of arid environments as to be a defining characteristic of this growth form. Although anatomists described this phenomenon several decades ago, there has been only one ecological study of one species, Ambrosia dumosa. The anatomical nature of the various splitting mechanisms that have been found suggests axis splitting to be an extreme form of hydraulic segmentation. The adaptive advantage of clonal splitting in desert shrubs has yet to be determined, but it appears to be largely a risk-spreading mechanism that enables independent mortality of integrated hydraulic units (IHUs) or ramets. This should be especially advantageous in heterogeneous, water-limited environments, where soil water occurs in pockets too small to support a large shrub-genet. Clonal splitting may cause an increase in intraclonal competition among ramets, but there are also indications that at least some species possess mechanisms to reduce competition by minimizing root system overlap among ramets. Many desert shrub species that undergo clonal splitting maintain a dense clump growth form, possibly because such a growth form has positive effects on water and nutrient status of the soil and long-term effects on other soil properties.
... Thus, one more genus can be added to the list of genera that produce an interxylary phloem cambium which divides bidirectionally, forming secondary xylem centrifugally and secondary phloem centripetally. The occurrence of interxylary cork in the perennial organs of some angiosperms has been reported previously in Epilobium and in several species of Artemisia (Moss, 1934(Moss, , 1936(Moss, , 1940Diettert, 1938;Moss & Gorham, 1953;Ginzburg, 1964). Most of the work performed up to 1940 was treated thoroughly by Moss & Gorham (1953). ...
... The occurrence of interxylary cork in the perennial organs of some angiosperms has been reported previously in Epilobium and in several species of Artemisia (Moss, 1934(Moss, , 1936(Moss, , 1940Diettert, 1938;Moss & Gorham, 1953;Ginzburg, 1964). Most of the work performed up to 1940 was treated thoroughly by Moss & Gorham (1953). This article listed around 40 species with interxylary cork and splitting of the stem, and aimed to correlate various reports with the hope of stimulating further research in this area. ...
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. (C) 2008 The Linnean Society of London.
... The interxylary cork is a protective plant tissue, allowing increased resistance to adversity and improved maintenance of the water balance of plants in arid and water deficient geographical environments for a long time (Hu & Zhang, 1993;Lu, 1993). As early as 1953, Moss and Gorham mention the importance of the interxylary cork as protection against desiccation and for the adaptation of plants to their habitats, mainly dry and wind-blown soil (Moss, 1953). In Artemisia dracunculoides, the interxylary cork may be an important factor in plant adaptation to arid habitats and protection against the invasion of micro-organisms (West, Espeleta, & Donovan, 2003). ...
Programmed cell death (PCD) plays a critical role throughout the lives of plants, it is regarded as a highly regulated and active process of plant cell death during the times of biotic or abiotic stress. This study aims to provide developmental anatomical characteristics of the interxylary cork formation in the roots of Astragalus. membranaceus var. mongholicus, and to subsequently show cytomorphological evidence that PCD is involved in the development of rhytidome and interxylary cork. The developmental anatomy of rhytidome and interxylary cork of the perennial fresh main root of A. membranaceus var. mongholicus was studied using light microscopy, whereas the PCD in the development of rhytidome and interxylary cork was studied using fluorescence microscopy and transmission electron microscopy. Histologically, it was observed that the parenchyma cells of secondary phloem and xylem in roots recovered their meristematic ability, and later developed into rhytidome and interxylary cork. Cytologically, ultrastructural characteristics such as nucleus malformation, vacuole disappearance, mitochondrial degeneration, and vesicle filling were observed. In roots, the nucleus of the phloem parenchyma cells were terminal deoxynucleotidyl transferase‐mediated dUTP nick‐end labeling (TUNEL)‐positive from the pre‐rhytidome stage to the formation of rhytidome stage and 4′,6‐diamidino‐2‐phenylindole dihydrochloride (DAPI)‐negative during the mature rhytidome stage. The TUNEL assay of the xylem parenchyma cells showed positive characteristics from the early stage of interxylary cork formation to the interxylary cork formation stage, whereas DAPI‐negative characteristics were observed in the mature interxylary cork. Gel electrophoresis showed that DNA cleavage was random. Our results indicated that the formation of the rhytidome and interxylary cork involved the PCD process.
... In I. hederifolia, formation of cork in the pith was noticed only in the portion of the stem that possesses intraxylary phloem and lacks secondary xylem. Occurrence of interxylary cork has also been reported in several species of Artemisia and Epilobium by earlier workers (Moss, 1934(Moss, , 1940Moss and Gorham, 1953) but intraxylary cork is so far reported only in I. hederifolia (Rajput et al., 2008). ...
Strands of phloem that are present at the periphery of pith are known as intraxylary phloem. Its presence remains restricted to a small portion of eudicots and considered as a characteristic feature for certain families. In the present study, Ipomoea turbinata Lag. showed development of intraxylary protophloem on adaxial tips of protoxylem from the procambial derivatives. Subsequently, additional intraxylary sieve elements were added from the adjacent parenchyma cells that were morphologically different from the pith cells. In thick stems, thin walled cells located between protoxylem and intraxylary protophloem acquired meristematic characters and formed several small segments of internal cambium. Initially, these cambial segments produced only phloem derivatives. Soon after, these segments became bidirectional and began to produce secondary xylem centrifugally and secondary phloem centripetally. Interestingly, in some of the samples development of secondary phloem and xylem was observed in the same direction. The secondary xylem formed by the internal cambium was composed of wide and fibriform vessels, fibres and axial parenchyma. Phloem possessed sieve tube elements, companion cells and parenchyma cells while rays in both xylem and phloem were mostly uniseriate but multiseriate rays were also observed.
... Similar splitting of the axis and consequently sectorial patterns may be caused by inter-and intraxyllary cork formation. This phenomenon is common in many shrubs (Moss 1934, 1940, Moss and Gorham 1953, Ginzburg 1963. A secondary longitudinal separation of the axis by inter-and intraxyllary cork formation turns adult specimens into groups of independent individual vegetative units and restricts the upward movement of water to one of the splits only. ...
... Axis splitting is a phenomenon in which the main axis of an individual plant segments over the course of development, resulting in physically distinct and functionally independent root-stem-canopy segments (Fig. 1). Common to ecologically dominant desert shrub species worldwide (Diettert, 1938; Fahn, 1977; Ginzburg, 1963; Moss, 1940; Moss and Gorham, 1953; Schenk, 1999 ), no evident phylogenetic relationship links axis-splitting species, although Schenk (1999) contends it may be confined to certain families. Plants achieve axis segmentation through several different developmental mechanisms, e.g., through the development of interxylary cork (Diettert, 1938; Moss, 1940) or unequal activity of the vascular cambium (Fahn, 1977; Ginzburg, 1963; Jones and Lord, 1982). ...
a b s t r a c t Although highly branched from the base, all shrubs have short main axes linking canopies to root systems. Main axes become increasingly segmented into independent canopy/stem/root segments as aridity increases across continents. The resulting hydraulic modularity has been proposed as an adaption to low soil moisture that prevents runaway embolism and minimizes risk of hydraulic failure. Here we test the hypotheses that (1) at a regional scale, the importance of axis-splitting species in communities declines with increasing elevation, as a proxy for precipitation, and (2) that this decline is explained by lower occurrence of low-elevation dominant species. We evaluated all species for axis splitting and determined importance values in plots along an elevational transect in the Mojave Desert. As predicted, as elevation increased, the total importance of axis-splitting species declined from 100% at low-elevation sites to 75% at the highest elevation site. However, this decline was not due solely to the decline of the lower elevation dominant species. At the high elevation site, the influx of new species resulted in a six-fold increase in species richness and almost all of the new high elevation woody eudicotyledonous species exhibited axis splitting; non-splitting species were represented by other growth forms.
... Similar splitting of the axis and consequently sectorial patterns may be caused by inter-and intraxyllary cork formation. This phenomenon is common in many shrubs (Moss 1934, 1940, Moss and Gorham 1953, Ginzburg 1963. A secondary longitudinal separation of the axis by inter-and intraxyllary cork formation turns adult specimens into groups of independent individual vegetative units and restricts the upward movement of water to one of the splits only. ...
Paths of water flow in trees and shrubs of different sizes and chorotypes were investigated with injected dyes. Aged shrubs possessed a sectorial ascent. Young specimens of these species showed a sectorial turning into a ring ascent. Species with large and tall crowns showed either a spiral pattern or sectorial straight pattern. Since independence of branches or trunk segments is characteristic of shrubs, it is suggested that woody species exhibiting a sectorial pattern of water movement should be considered chamaephytes.
Tree-related microhabitats (TreMs) have been identified as key features for forest-dwelling taxa and are often employed as measures for biodiversity conservation in integrative forest management. However, managing forests to ensure an uninterrupted resource supply for TreM-dwelling taxa is challenging since TreMs are structures with a limited availability, some of which are triggered by stochastic events or require a long time to develop. At the tree scale, the role of tree species, diameter at breast height (dbh) and status (i.e. living vs standing dead) for favouring TreM occurrence has been quantified and modelled in several studies, since these tree features are routinely recorded in the field. However, TreM occurrence remains difficult to predict, hampering the elaboration of applicable management strategies that consider TreMs. Using an international database encompassing 110,000 trees, we quantified the explanatory power of tree species, dbh, status, time since last harvest and plot context for predicting TreM occurrence at the tree level. Plot context is so far a “black box” that combines local environmental conditions, past and current management legacies, with local biotic features that have high explanatory power for predicting TreM occurrence. Then, based on the literature, we established a set of 21 factors related to site, stand and tree features for which there is a strong assumption that they play a key role in TreM formation. Finally, we identified a sub-set of nine features that should be recorded in the future to provide additional information to enable better prediction of the occurrence of particular TreMs: (i) at plot level: slope, exposure, altitude and presence of cliffs; and (ii) at tree level: bark features, phyllotaxis and compartmentalization capacity of the tree species, plus ontogenic stage and physiological state of the individual tree sampled.
Results of a comparative wood anatomical survey of the American frankenias are presented. The eleven species examined are woody perennials occurring almost exclusively in arid and semiarid regions and on saline and gypseous soils. The secondary xylem of all species is highly specialized and is characterized by libriform fibers, vessel elements with simple perforation plates, and the absence of rays. Axial elements of all species are quite small. A number of unusual features, e.g., anomalous secondary growth and formation of interxylary cork, were observed in some species. Nonfibrous woods have evolved independently in two species of reduced stature and contrast markedly with the highly fibrous woods of most species. Woods of the American frankenias are compared with those of the Tamaricaceae. The systematic and evolutionary implications of interspecific variation in both qualitative and quantitative features are discussed. There is a general tendency for dimensions of the axial wood elements to be positively associated and to decrease with decreasing plant height. In general, differences in wood anatomy more strongly reflect differences in plant growth form and size than phylogeny.
Axis splitting is a widespread phenomenon in desert shrubs, and has been reported for shrubs from several plant families, both in old- and new-world deserts. It is so common in dwarf shrubs of arid environments as to be a defining characteristic of this growth form. Although anatomists described this phenomenon several decades ago, there has been only one ecological study of one species, Ambrosia dumosa. The anatomical nature of the various splitting mechanisms that have been found suggests axis splitting to be an extreme form of hydraulic segmentation. The adaptive advantage of clonal splitting in desert shrubs has yet to be determined, but it appears to be largely a risk-spreading mechanism that enables independent mortality of integrated hydraulic units (IHUs) or ramets. This should be especially advantageous in heterogeneous, water-limited environments, where soil water occurs in pockets too small to support a large shrub-genet. Clonal splitting may cause an increase in intraclonal competition among ramets, but there are also indications that at least some species possess mechanisms to reduce competition by minimizing root system overlap among ramets. Many desert shrub species that undergo clonal splitting maintain a dense clump growth form, possibly because such a growth form has positive effects on water and nutrient status of the soil and long-term effects on other soil properties.
In 1665, Robert Hooke was examining suberized cork cells from the bark of Quercus suber when he gave the first description of a cell. Von Höhnel was also examining cork cells from Q. suber more than 200 years later when he described the lamellar structure characteristic of suberin (466). It has become quite clear that suberized walls contain an insoluble polymeric material called suberin, which is associated with a complex mixture of nonpolar compounds collectively called waxes. Electron microscopic examination of suberized walls usually reveals a lamellar structure in which the light and dark bands are probably composed of the soluble waxes and the polymer, respectively. Since suberin is laid down as an insoluble polymer in the cell wall it is not possible to isolate suberin as a pure polymer. It can be isolated only as a suberin-enriched wall fraction containing about 50% cell wall carbohydrate material. The insolubility of this material and the complexity of its structure and composition make chemical and biochemical studies of suberin extremely difficult. From the limited number of studies conducted so far we have some knowledge about the location, composition, structure, biosynthesis, biodegradation, and function of suberized cell walls.
Leafy spurge (
Euphorbia
×
pseudovirgata
[Schur]) is an herbaceous perennial and serious weed of European origin that has been accidently introduced into North America. The European anthomyiid flies
Pegomya curticornis
(Stein) and
Pegomya euphorbiae
(Kieffer) are found on several spurge species in Europe and also attack leafy spurge. The two flies induce identical galls on the subterranean stems of their host plants, and the shoots wilt and die. Eggs are laid on the shoot tip, and the larvae bore into the stem by eating pith which is later replaced by callus. This is a rare example of an insect with both boring and gall-inducing feeding strategies. Galls are induced when larvae feed on the ring of vascular tissue. There is no proliferation of nutritive cells but instead thick layers of gall parenchyma are produced. The vascular connections are broken at the gall level and concentric vascular bundles appear in the cortical and gall parenchyma. After pupation an inner periderm differentiates around the chamber surface.
The wood (xylem) in a living tree is protected from microbial attack by the secondary plant surface (periderm and rhytidome), which provides an effective barrier preventing the entry of most potential pathogens, and by constitutive and induced defence mechanisms in the bark (cortex and phloem). Although a few pathogens are able To penetrate these outer tissues directly, most xylem pathogens gain entry through wounds that expose this tissue and render it more vulnerable to attack. In functional sapwood, microbial colonization might be restricted by active defence mechanisms, or by passive microenvironmental restriction consequent upon the high water content and low availability of O 2 in healthy conductive xylem. These factors are not mutually exclusive: indeed, they may operate in concert in many host‐pathogen interactions. Sapwood lesions made by wood‐decaying fungi are characteristically bounded by multiccllular walls OT barrier zones (compartmentalization wall 4 barriers and reaction zones (column boundary layers)), which may function both as inhibitory or degradation‐resistant barriers to further pathogen spread, and as seals to maintain xylem function and prevent the drying and aeration that could predispose to further infection. A range of putative antimicrobial defence mechanisms contributing to the effectiveness of such barriers has been identified in sapwood tissues. This includes cell wall alterations, constitutive and induced antimicrobial compounds, necrotic responses of living cells and the deposition of gummy materials, often resinous or polyphenolic, at the host‐pathogen interface. Nutritional, environmental and anatomical features of living wood might also contribute to pathogen restriction. Although there might be differences in detail, the defences operating in gymnospenns and angiosperms are generally similar. Defence responses against a variety of pathogen categories also have much in common. Dynamic studies are crucial in elucidating the roles of the various components of the host‐pathogen interaction in the wood of living trees. A model for the protection and defence of xylem tissues in woody angiosperms is suggested, based largely upon results from dynamic studies of host‐pathogen interactions in the wood of sycamore ( Acer pseudoplatanus L.).
CONTENTS
Summary
203
I.
Introduction
204
II.
Xylem pathogens
205
III.
Bark the first line of defence
206
IV.
Defence in functional sapwood
207
V.
Defence in heartwood
222
VI.
Dynamic aspects of the host‐pathogen interaction in wood
223
VII.
Genetic control of resistance in xylem tissues
224
VIII.
Antimicrobial defence in the living xylem of trees: towards a consensus model
225
Acknowledgements
227
References
227
Only the terms tree and shrub have been widely recognised for major growth forms of woody gymnosperms
and dicotyledons, However many multi-stemmed woody plants do not conform to the definition
of either category. For these plants it is here proposed that the term woody clump be used and that
communities dominated by woody clumps be referred to as clumpwoods.
Woody clumps are formed as a result of loss of apical dominance of a single stem and its replacement
by several or many stems from tissue at or below ground level. Loss of apical dominance may be a
genetically determined trait which is expressed early in the life of the plant or it may be a consequence
of death, debilitation or physical damage to a mature mainstem.
The stems of woody clumps may arise from morphologically unspecialised stem bases or from
lignotubers, burls, rhizomes, branch and main stem layers, splitting and segmentation of shoot-root axes
or root suckering. Woody clumps may remain as undivided individuals, with varying degrees of surface
spread, or may divide by death or decay of connecting tissue to form clones. Large, undivided clumps
could be called 'pre-clones'. Woody clumps are found in most major plant formations from the arctic
to tropical regions and from deserts to swamplands.
With a few notable exceptions such as North American aspens, the woody clump growth form has
been ignored in ecological studies. We suggest in this paper that the recognition of a special term for
this distinctive growth form would result in a clearer appreciation of its significance in natural and
cultivated plant assemblages. Some of the genetic, ecological and management implications of woody
clumps are discussed.
Partial cambial mortality is a growth form that is characteristic of Pinus aristata trees. To better elucidate their cambial death pattern, tree size and aspect of cambial death data were gathered from three Pinus aristata forests in central Colorado, USA. Stripping frequency tended to be higher for larger diameter classes. Partial cambial mortality exhibits significant directionality within each stand. Furthermore, cambial death was measured to be most frequent on the wind-exposed side of stripped trees in two of the three study sites and appeared to be at the third. Data presented here support the hypothesis that wind plays a role in the occurrence of partial cambial mortality in Pinus aristata. The mechanisms by which wind causes cambial mortality remain unclear.
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