Epiphytic orchids on arborescent Velloziaceae and Cyperaceae: Extremes of phorophyte specialisation

Vascular epiphytes are widespread throughout the tropics where they are usually not restricted to particular phorophytes. In contrast to this the trunks of arborescent Cyperaceae and Velloziaceae are regularly colonised by vascular epiphytes with certain orchids showing a remarkably high degree of phorophyte specificity. Arborescent, desiccation-tolerant Cyperaceae (e.g. Afrotrilepis) and Velloziaceae (e.g. Vellozia, Xerophyta) form characteristic elements of rock outcrop vegetation in the Neotropics and Paleotropics where they frequently occur with mat-forming species. Their woody-fibrous stems mainly consist of persistent leaf bases and adventitious roots which possess a velamen radicum. In tropical Africa several species of Polystachya occur in mats and on stems of Cyperaceae (e.g. P. microbambusa on Afrotrilepis pilosa) and Velloziaceae (e.g. P. johnstonii on Xerophyta splendens). In Brazil a higher number of orchid species (e.g. Constantia cipoensis, Pseudolaelia vellozicola, Sophronites brevipedunculata) are largely restricted to the stems of Velloziaceae growing on rock outcrops. Moreover, in Brazil a number of non-phorophyte specific orchids (e.g. Epidendrum saxatile, Prosthechea vespa) grow on the trunks of Velloziaceae. Orchids restricted to the stems of Cyperaceae and Velloziaceae were derived from non-phorophyte specific, epiphytic/lithophytic ancestors and originated independently in South America and Africa. The exact nature of the relationship between arborescent Cyperaceae and Velloziaceae and epiphytic orchids growing exclusively on their trunks is not yet clear. In a number of areas both the highly host specific orchids and the arborescent Cyperaceae/Velloziaceae are seriously threatened by human lit fires and commercial collection.     

Monocot Xylem Revisited: New Information,New Paradigms

Five sources of data force extensive revision of ideas about the nature and evolution of monocot xylem: scanning electron microscopy (SEM) studies of thick sections; availability of molecular phylogenies covering a relatively large number of families and genera; information on ecology and habitat; data concerning habit; and observations from xylem physiology. These five new sources of data, absent from the studies of Cheadle, plus added information from light microscopy, lead to a fresh understanding of how xylem has evolved in monocots. Tracheary elements hitherto recorded as vessel elements with scalariform end walls prove in a number of instances, to retain pit membranes (often porous or reticulate) in the end walls. There is not an inexorable progression from "primitive" to "specialized" xylem in monocots; apparent accelerations or reversions are also possible. The latter include such changes as the result of production of narrower vessel elements; or production of less metaxylem, which is probably heterochronic in nature (an extreme form of juvenilism). Tracheary elements intermediate between vessel elements and tracheids must be recognized for what they are, and not forced into mutually exclusive categories. Original data on tracheids and various types of vessel elements is related here to ecology and habit of groups such as Asteliaceae, Boryaceae, Cyclanthaceae, Orchidaceae, Pandanaceae, Taccaceae, Typhaceae, dracaenoid Asparagaceae, and Zingiberales. Data from palm xylem shows a nearly unique syndrome of features that can be explained with the aid of information from physiology and ecology. Vessellessness of stems and leaves characterizes a large number of monocot species; the physiological and ecological significance of these is highlighted. An understanding of how non-palm arborescent monocots combine an all-tracheid stem xylem with addition of bundles and vegetative modifications is attempted. The effect of the disjunction between xylems of adventitious roots and stems, providing a physiologically demonstrated valve ("rectifier") effect is discussed. "Ecological iteration" has occurred in some monocot lineages, so that early-departing branches in some cases may have more "specialized" xylem because of entry into xeric habitats, whereas nearby crown groups, which may have retained "primitive" xylem, probably represent long occupation of mesic habitats. Cheadle's use of xylem for "negations" of phyletic pathways can no longer be accepted. Symplesiomorphic mesomorphic xylem patterns do characterize many of the earlier-departing branches in the monocots as a whole, however. Cheadle's idea that monocots and non-monocot angiosperms attained vessels independently is improbable in the light of molecular trees for angiosperms. Vessels in roots seem an adaptation to major swings in moisture availability to adventitious roots as compared to taproots. The commonness of all-tracheid plans in stems and leaves in earlier-departing monocot clades is a feature that requires further clarification but is primarily related to the xylem disjunction that adventitious roots have. Secondary vessellessness or something very close to it can be hypothesized for Campynemataceae, Philesiaceae, Taccaceae, and some Orchidaceae. Eleven salient shifts in our conceptual views of monocot xylem are proposed and conclude the paper. Monocot xylem is not a collection of historical information, but a rigorously parsimonious system related to contemporary habits and habitats.     

Regeneration of Sclerenchyma in Wounded Dicotyledon Stems

When the sclerenchyma cylinder that surrounds the vascular cylinderin many dicotyledon stems is interrupted by cutting away oneside of an internode, its continuity becomes restored in somespecies by the differentiation of sclereids within the woundcallus. These sclereids, which may be scattered or arrangedin clumps or in a continuous sheet, lie in a zone within the‘cortical’ parenchyma between the regenerated vascularcylinder and the wound cork. The amount and especially the arrangementof regenerated sclerenchyma tends to reflect that of the originalprimary sclerenchyma cylinder in the unwounded stem, exceptthat longitudinal continuity is poorly developed and all fibresare replaced by sclereids. Syringa vulgaris L, lilac, regeneration, differentiation, fibres, sclereids, sclerenchyma, positional control     

Origins and nature of vessels in Monocotyledons. 14. Vessellessness in Orontioideae (Araceae): adaptation or relictualism?

SEM studies of tracheary elements of subfamily Orontioideae (Lysichiton, Orontium, Symplocarpus) of Araceae show unexpected features. The plants are entirely vesselless. There are small pores in pit membranes of end walls of tracheids in roots and stems, but pit membranes remain intact. End wall pit membranes of stems have a coarse fibrillar texture, somewhat reminiscent of (but different from) those of Nymphaeaceae and Cabombaceae. Acoraceae, which are also vesselless, represent the first branch of the monocot tree, according to phylogenies, and the orontioids form the next branch. Vessellessness is therefore a potentially plesiomorphic feature in monocots, but it may also be related to the highly mesic habitats of Acoraceae and the orontioids. Various other non‐submersed monocots have vesselless or near‐vesselless xylem. Sectioned xylem of Orontioideae is also very suggestive of stages in the development of the pit membranes of both end walls and lateral walls of tracheids: open networks of cellulosic fibrils apparently precede the addition of denser fibrillar meshes, key information in assessing to what extent perforations in scalariform perforation plates of vascular plants may stop formation at the open network stage, and to what extent a thicker pit membrane experiences lysis and disintegration as the vessel element matures.     

A Microautoradiographic Study of Auxin Transport in the Stem of Intact Pea Seedlings (Pisum sativum L.)

Soluble-compound microautoradiography was used to determinethe distribution of radioactivity in transverse sections ofintact dwarf pea stems (Pisum sativum L.) following the applicationof [³H]IAA to the apical bud. Near the transport front labelwas confined to the cambial zone of the axial bundles, includingthe differentiating secondary vascular elements. Fully differentiatedphloem and xylem elements remained unlabelled and no radioactivitywas detected in the leaf or stipule traces. Similar resultswere obtained in experiments with Vicia faba L. plants. Nearerthe labelled apical bud of the pea there was a more generaldistribution of label and evidence was found of free-space transportof radioactive material in the pith. When [³H]IAA was applied to mature foliage leaves the greatestconcentration of label was found in the differentiated phloemelements of the appropriate leaf trace and in the phloem ofthe adjacent axial bundles. Both basipetal and acropetal transportwas detected in this case. These results are consistent with the conclusions drawn fromearlier transport experiments which indicated that in the intactplant the long-distance basipetal transport of auxin from theapical bud takes place in a system which is separated from thephloem transport system and suggests that the vascular cambiumand its immediate derivatives may function as the normal pathwayfor the longdistance movement of auxin in the plant. The physiologicalsignificance of such a transport system for auxin is discussed.     

The Vascular System of Maize Stems Revisited: Implications for Water Transport and Xylem Safety

The plexus of vascular bundles in the nodes of grasses is notoriouslycomplex, where long axial bundles pass through a network oftransverse bundles. The xylem pathways for water in maize stemshave been investigated anatomically and with dye and particulatetracers, revealing some of the details of this complexity. Onlyapprox. 3% of axial vessels pass through nodes without beinginterrupted by end walls. Axial bundles at nodes differ fromthose in internodes in having the metaxylem and protoxylem vesselsconnected by small tracheary elements. So it is only at nodesthat exchange of sap occurs between the large vessels withina bundle. End walls, acting as filters for particles and gasbubbles, always separate axial vessels from vessels in transversebundles. The high redundancy of bundle connections in the nodalplexus is interpreted as providing alternative water pathwaysto bypass embolisms and damaged or diseased sections of thexylem. The pores in the filters at the base of nodes and betweenaxial and transverse vessels within nodes are <20 nm in diameter.Where axial vessels connect to transverse vessels, a varietyof unusual shapes of vessel elements mediate two- and three-wayconnections within the plexus.Copyright 2000 Annals of BotanyCompany Zea mays, cryoSEM, maize, node, pits, pit membranes, vessel ends, vessels, xylem embolism, xylem pathogens     

The unusual vascular structure of the corm of Eriophorum vaginatum: implications for efficient retranslocation of nutrients

Eriophorum spp. are abundant perennial graminoids in the Arctic tundra and boreal peatlands. Because ecological studies indicated that some plants are unusually productive on infertile and cold sites, the anatomy of the overwintering corms of Eriophorum vaginatum (L.) and Eriophorum scheuchzeri (Hoppe) were examined to determine their involvement in nutrient uptake and storage. Components of the long-distance transport pathways were identified within the plants by using histochemical techniques and transport of apoplastic and symplastic dyes. E. scheuchzeri produced a rhizome that consisted mainly of storage parenchyma cells within which collateral vascular bundles were centrally located and arranged in a circle. By contrast, E. vaginatum developed a ring of horizontally arranged xylem and phloem, in addition to axial amphivasal vascular bundles leading to the leaves, all of which were bordered by transfer cells. As shown by the transport of fluorescein in the phloem and Safranine O in the xylem, each axial bundle and adventitious root contacted the horizontal ring of vascular tissues so that solutes from one vascular bundle were translocated into the vascular ring and circulated to another vascular bundle and/or to the roots. In addition, special groups of sclereids that functioned in both phloem and xylem transport were found at the base of the leaf traces and within junctions of senescing roots. These sclereids were named 'vascular sclerenchyma' and it was hypothesized that they provide a moving end for the vascular system because the corm dies progressively from the distal end as it grows upward from the apical meristem. It was concluded that this unusual vascular system of E. vaginatum is efficient in recycling nutrients internally, which may account for its competitive advantage in infertile and cold sites.     

Morphogenesis in Yucca schidigera Roezl. Root Organ Cultures

Root development in suspension cultures of Yucca schidigerawas light-mediated. The green cultures consisted of roots, smalltissue aggregates and suspension cells. Roots possessed an apicalmeristem with a root cap, meristematic region and region ofdifferentiating tissues. Phloem, xylem vessels and tracheidsoccurred in discrete polyarch vascular bundles. Xylary wallthickening was reticulate, and endodermis and pericycle werepresent. Roots of intact Y. schidigera plants had a similardistribution of vascular tissues. Dark-grown cultures were cream-colouredand contained only lobed tissue aggregates and suspension cells. Yucca schidigera Roezl., tissue cultures, morphogenesis, root organ, light/dark     

Tomato prosystemin promoter confers wound-inducible, vascular bundle-specific expression of the β-glucuronidase gene in transgenic tomato plants

Tomato (Lycopersicon esculentum Mill. cv. Better Boy) plants were transformed with a fused gene containing a 2.2-kb promoter fragment of the tomato prosystemin gene and the coding region of the β-glucuronidase (GUS) reporter gene. The transgenic plants exhibited a low constitutive level of prosystemin-β-glucuronidase gene expression, assayed by histochemical staining and GUS enzyme activity, that was associated in the vascular bundles of leaf main veins, petiolules, petioles and stems. The GUS activity in the vascular bundles in each tissue was increased by wounding and by treatment of the plants with methyl jasmonate, similar to the induction of prosystemin in wild-type plants. The increase in GUS activity in the vascular bundles of leaves in response to wounding correlated with the wound-inducible increase in prosystemin mRNA. Tissue printing, using rabbit anti-serum prepared against prosystemin, confirmed that inducible prosystemin protein was localized in vascular bundles of petiolules, petioles and stems of wild-type tomato plants. The evidence indicates that the 2.2-kb promoter region of the tomato prosystemin gene contains elements conferring its correct temporal and spatial expression in the vascular bundles of transgenic tomato plants. Received: 7 January 1997 / Accepted: 2 April 1997     

菰适应湿地环境的解剖和屏障结构特征研究

菰(Zizania latifolia)是一种多年生挺水植物,为了探讨该植物根、茎和叶的解剖结构、组织化学及其质外体屏障的通透性生理。该文利用光学显微镜和荧光显微镜,对菰的根、茎、叶进行了解剖学和组织化学研究。结果表明:(1)菰不定根解剖结构由外而内分别为表皮、外皮层、单层细胞的厚壁机械组织层、皮层、内皮层和维管柱;茎结构由外而内分别为角质层、表皮、周缘厚壁机械组织层、皮层、具维管束的厚壁组织层和髓腔。叶鞘具有表皮和具维管束皮层,叶片具有表皮,叶肉和维管束。(2)不定根具有位于内侧的内皮层及其邻近栓质化细胞和外侧的外皮层组成的屏障结构;茎具内侧厚壁机械组织层,外侧的角质层和周缘厚壁机械组织层组成的屏障结构,屏障结构的细胞壁具凯氏带、木栓质和木质素沉积的组织化学特点,叶表面具有角质层。(3)菰通气组织包括根中通气组织,茎、叶皮层的通气组织和髓腔。(4)菰的屏障结构和解剖结构是其适应湿地环境的重要特征,但其茎周缘厚壁层和厚壁组织层较薄。由此推测,菰适应湿地环境,但在旱生环境中分布有一定的局限性。     

Noteworthy idioblastic sclereids in the stems of Eulychnia (Cactaceae)

Stems of Eulychnia (a genus of six to nine species of candelabriform or arborescent cacti) have a parenchymatic cortex with two distinct regions. The outer chlorenchymatic layer is characterized by a conspicuous parallel striping, whereas the inner cortex region devoid of chlorophyll has a coarsely granular aspect. Stem samples from nine accessions, collected in the field or taken from cultivation, were studied from resin-embedded microtome sections and maceration. Two different forms of lignified sclereids were found dispersed in the cortex and the pith. The sclereids of the outer palisade-like cortex layer are distinctly elongated and strictly oriented at right angle to the stem surface, whereas those of the inner cortex and pith are globular or subglobular and conspicuously enlarged compared with the surrounding parenchyma cells. The ontogeny of the sclereids was studied from stem samples of different ages. Formation of the secondary cell walls starts only after cell growth is completed. A screening of numerous South American cacti for the presence of idioblastic sclereids showed that these structures are unique for the genus Eulychnia. Finally, functional aspects of the sclereids are shortly discussed. It is assumed that the sclereids contribute to the mechanical support and reinforcement of the plants.     

ONTOGENY AND CORRELATIVE RELATIONSHIPS OF THE PRIMARY THICKENING MERISTEM IN FOUR-O'CLOCK PLANTS (NYCTAGINACEAE) MAINTAINED UNDER LONG AND SHORT PHOTOPERIODS

The developmental anatomy of Mirabilis jalapa was investigated during the first 90 days of growth. The primary thickening meristem (PTM) initially differentiates in the pericycle at the top of the cotyledonary node 18 days after germination, then basipetally in the pericycle through the hypocotyl. The PTM differentiates acropetally into the stem and in the pericycle of the primaiy root, commencing 22 days after germination. Endodermis is easily identifiable in hypocotyls as well as in primary roots because of Casparian thickenings in its cells. It has not been definitely identified in stems. There are three rings of primary vascular bundles in the stem. The PTM differentiates as segments of cambium in a layer of cells (probably in the pericycle) on an arc between vascular bundles of the outer bundle ring. Later, arcs of PTM differentiate externally to the phloem of each bundle. Each arc forms a connection between original segments of PTM lying on either side of each vascular bundle. Thus, the PTM becomes a continuous cylinder. The PTM differentiates in the pericycle outside vascular tissue in the hypocotyl and root. Differentiation of the PTM and the mode of secondary thickening is similar in plants exposed to short (8-hr) and to long (18-hr) photoperiods, but some differences were observed. The PTM differentiates closer to the stem apex in all plants over 18 clays of age growing vegetatively under long photoperiods. That is, the diffuse lateral meristem, in whose cells the PTM differentiates in young intemodes, is shorter in nearly all investigated plants growing in long photoperiods. The hypocotyl and base of the primary root of 40-day-old plants in short photoperiods were more enlarged than those of the same age plants in long photoperiods; but, at the end of 64 days, the hypocotyl and primaiy root base were larger in plants growing under short photoperiods. Thirty-four days after seed germination, flower initiation occurs in plants exposed to short photoperiods. One hundred fifty days after seed germination, flowers differentiate on plants exposed to long photoperiods.     

Interactive effects of heat shock and smoke on germination of nine species forming soil seed banks within the Sydney region

Abstract The germination response of seeds from fire‐prone vegetation to fire‐related cues such as heat shock and smoke has usually been studied by applying the cues singly. The few studies that have applied the cues in combination have shown that interactions between the cues are possible. Here, the response of seeds from a number of species to combined heat shock and smoke is reported. Heat shock (25, 50, 75 and 100°C) and aerosol smoke (0, 5, 10 and 20 min) were applied factorially to nine species that form soil seed banks in the Sydney region of south‐eastern Australia. These species were from Epacridaceae (four species), Myrtaceae (four species) and Cyperaceae (one species) and ranged from fire‐sensitive obligate seeders to fire‐tolerant facultative resprouters. Germination of Dracophyllum secundum R. Br and Sprengelia monticola (A. Cunn. ex DC.) Druce was low and did not respond to the germination cues. The positive response of Gahnia sieberiana Kunth and Kunzea ambigua (Sm.) Druce to heat shock and smoke was independent and additive. The positive response of Kunzea capitata Rchb. to the interaction between heat shock and smoke was synergistic, and the response of Baeckea diosmifolia Rudge and Baeckea imbricata (Gaertn.) Druce was unitive, with germination increase only occurring following combined heat and smoke application. Epacris coriacea A. Cunn. ex DC. and Epacris obtusifolia Sm. had low levels of dormancy and hence it was not possible to find a fire response. Gahnia sieberiana and K. capitata responded differently to the combination of heat shock and smoke than has previously been reported. Germination of species from habitats that are infrequently burnt was not affected by heat shock or smoke. Low‐intensity fire or patches within fire may be important for seedling recruitment as the 50°C heat shock stimulated germination in four of the five species that responded to the heat cue, and germination of Baeckea imbricata declined within the 100°C heat shock treatment. Germination of one species, Baeckea imbricata, was only stimulated by a specific combination of cues, indicating that regeneration niches may be narrow for some species and that the application of a range of heat and smoke doses is required to find such responses. Of the species positively responding to heat shock and smoke, a requirement for both cues was prevalent, therefore the response to these cues in isolation cannot be relied upon to give a true indication of the fire response of a species.     

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     

A Partially Permineralized Lepidophloios from the Early Upper Carboniferous of Scotland

A terminal shoot of Lepidophloios with both external morphologyand anatomy preserved is described. It is compared to the compressionspecies L. scoticus and the permineralized species L. scottiiand L. wuenschianus, all from the Lower Carboniferous of Scotland.It is characterized by the leaves having been abscissed rightup to the stem apex and by the occurrence of deflected and overlappingleaf cushions near the apex. These features are in contradictionwith current concepts about the growth of these plants. Lepidophloios, anatomy, arborescent lycopod, Carboniferous, leaf growth, terminal shoot     

Silica and Ash in Tissues of Some Coastal Plants

Ash and silica contents and their depositional patterns in differenttissues of 44 Mississippi coastal plants were determined. Silicacontent of dried plants varied from no more than a trace inChenopodium album L. leaves to 7.37 per cent in Zizanopsis miliacea(Michx) Doell & Aschers leaves. Ash content varied from2.50 per cent in Lythrum lineare L. stems to 28.24 per centin Borrichia frutescens (L.) DC leaves. Plants in the same familytend to be alike in their ability to absorb or not absorb silica.Poaceae and Cyperaceae had consistently high concentrationsof silica. In contrast, the Asteraceae studied had very lowsilica contents but often had high contents of other minerals.Dicotyledonous plants studied had consistently lower silicacontents than the monocotyledons. Plants growing in salt watercontained considerable sodium chloride. Spectra were obtainedfor major elements in four different plants. Energy-dispersiveX-ray analysis shows that distribution of the element siliconis clearly related to certain epidermal structures such as guardcells, ridges, dumb-bells and balls that appear in electronmicrographs. Silica was deposited differently in each type ofplant studied. In many of the plants silica was deposited inrows of irregular-shaped particles running lengthwise of theleaf and in guard cells. In others, like Zizanopsis miliacea(Michx) Doell & Aschers, the deposit was sheet-like. Zizaniaaquatica L. not only had a sheet-like deposit, but the depositwas ridged and there were rows of dumb-bell-shaped silica cells.Related plants had similar structures. Euchlaena mexicana Schrad.,Tripsacum dactyloides (L.) L and Manisuris rugosa (Nutt.) Kuntzeall had irregular phytoliths similar to those in Zea mays L. coastal plants, marsh plants, ash content, silica deposition, scanning electron microscopy, energy-dispersive X-ray analysis, silicon distribution, X-ray diffraction patterns, spectra of elements in plants     

Anatomical studies on Sinofranchetia chinensis (Lardizabalaceae) and their systematic significance

The anatomical structures of the Chinese endemic and monotypic genus Sinofranchetia (Lardizabalaceae) are described. There are reticulate, simple-reticulate, scalariform, simple-scalariform and simple perforations in vessel elements as well as in the fibres in the secondary wood of the roots and the stems. The node is trilacunar. The vascular bundles in the petiole are arranged in a ring. Clustered crystals occur in the parenchymatous cells of stems, petioles and pedicles. Leaf stomata are actinocytic. The nodes of sepals, petals and stamens both in male and female flowers are unilacunar and one-traced. There are three sterile carpels with two to three traces in the male flowers, three fertile carpels with two to three traces, and sometimes three sterile carpels lacking a vascular supply. In morphology, the anther dehiscence mechanism and pollen in the female flowers are the same as in the male flowers, such that the so-called female flowers might be bisexual in morphology. In comparing morphology, the sex of the flowers and the perforations of the vessel elements in Sinofranchetia with Decaisnea and other genera of the Lardizabalaceae, Sinofranchetia is considered a basic group at least as the same evolutionary level in the family as Decaisnea . © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society , 2005, 149 , 271–281.     

药用植物博落回形态与发育解剖学研究

博落回(Macleaya cordata)是罂粟科含有异喹啉生物碱的主要药用植物, 其生物碱提取物具有多种药理活性。目前对博落回的研究主要集中在生物碱的成分、活性和药理等方面, 对与其合成、运输、储藏密切相关的发育解剖结构及生物碱组织化学定位研究还未见报道。该文报道了温室中生长的博落回植株形态特征, 并结合徒手切片和改良碘化铋钾沉淀的方法对主要器官(根、茎、叶)的发育结构和生物碱组织化学染色进行了初步分析。结果表明, 博落回发育基本遵循一般双子叶植物的规律。根中生物碱出现在中柱鞘外的薄壁细胞和导管周围。茎和叶柄中的生物碱主要出现在维管束周围, 偶尔也出现在茎的髓细胞中。总体上, 茎中的生物碱积累量少于根。     

Stem and wood allometric relationships in Cacteae (Cactaceae)

Allometric relationships in organisms are considered a universal phenomenon. A positive scaling has been reported between stem size and cellular size of tracheary elements in wood of different vascular plants, but few studies have been carried out in slow-growing succulent plants. The aim of this study was to evaluate if a relationship exists between size, growth form and wood cell size among individual species of Cacteae. Forty-four species belonging to 16 genera of the tribe Cacteae with differing growth forms and sizes were studied. When analyzing plant size, we found a positive allometric scaling and the larger-sized species showing a higher percentage of succulent tissue and less accumulation of wood tissue. The positive scaling found between plant size (height and diameter) and vessel elements and fiber length support the universality of the allometric relationship proposed for other vascular plants with non-succulent stems. Notably, wide-band tracheids do not scale with plant size or growth form. Succulence associated with narrow vessel elements with distinctive helical secondary walls and wide-band tracheids suggest they are the key adaptations to tolerate drought and provide support to the stems of most taxa in Cacteae. Fibers do not have the primary role of giving mechanical support; therefore, we assume the scarce fibers in clusters represent reaction wood that, along with the fundamental tissue, maintains the vertical position and shape of those species growing in rocky cracks. Our results with species having short succulent stems support the universal theory of positive allometric scaling of vascular plants.