Stem surface injuries of 20 species of succulent Euphorbia (Euphorbiacae) from South Africa

Cactus and succulent Euphorbia plants have green stems that are exposed to sunlight over long periods. Previous research demonstrated that barking injuries occur to stem surfaces of 20 species of cacti in the Americas. For these 20 cactus species the amounts of injuries were related to annual amounts of sunlight exposure on surfaces. Cacti with many injuries had high mortality rates. Data herein show that stem surfaces of 20 species of Euphorbia in South Africa have identical surface injuries as for cacti. Euphorbia plants at 30°S show a mean ratio of scale and bark injuries of 3.5:1 of equatorial-facing to polar-facing surfaces, a ratio is similar to the 4:1 of annual sunlight exposure ratio at that latitude. Results show that Euphorbia stems have injuries on younger tissues than cactus species. Thus, Euphorbia stems are more sensitive than cactus stems. Euphorbia species with most stem injuries had smooth (non-undulate) surfaces, a one-celled epidermal cell layer only, cortex cells that abutted the epidermis, and initial injuries that involved only epidermal cells. This is the first report of identical stem surface injuries to Euphorbia species that are identical to surface injuries to cactus species worldwide. Moreover, the amounts on stem injuries on Euphorbia and long-lived, columnar cactus species are highly correlated with relative amounts of sunlight on stem surfaces over the annual cycle. These results suggest that these species are good bio-indicators of annual sunlight exposure.     

Distribution and impact of exotic gall flies (Lipara sp.) on native and exotic Phragmites australis

Two exotic gall fly species infest stems of native and exotic Phragmites australis (Cav.) Trin. ex Steudel in northeastern North America. In this study, we determined the distribution of Lipara similis Schiner and L. rufitarsis Loew in native and exotic P. australis in Rhode Island. We also studied the within-stand distributions of each fly species and their effects on flowering of native and exotic P. australis. We collected stems from populations throughout southern Rhode Island and measured stem length and diameter, and percent flowering. Stems were then dissected to determine Lipara infestation. L. similis and L. rufitarsis were found throughout Rhode Island infesting both native and exotic P. australis, but their presence and abundance varied among sites. Within stands, L. similis infests the taller, thicker interior stems and L. rufitarsis infests the shorter, thinner exterior stems. Lipara similis reduces stem length by 6%; L. rufitarsis infestation reduces stem length by 37%. The flowering rate of uninfested stems is significantly lower in native P. australis stems than in exotic stems. Both Lipara species prevent infested stems from flowering. In adjacent stands of native and exotic P. australis, L. rufitarsis infests significantly more native stems than exotic stems, possibly further reducing the reproductive potential of the native plants relative to the exotic. Lipara species may play a role in facilitating the displacement of native P. australis by the exotic genotype.     

长期增温对树线交错带岷江冷杉幼苗异龄叶大小与出叶强度关系的影响

全球气候变暖强烈影响树线交错带植物的生活史策略,异龄叶大小-出叶强度权衡关系是常绿植物生活史策略的重要内容。以川西树线交错带的岷江冷杉（Abies faxoniana）幼苗为例,研究气候变暖对异龄叶大小与出叶强度关系的影响。通过开顶箱（Open-top chamber, OTC）对川西王朗自然保护区树线交错带的岷江冷杉进行模拟增温,采用标准化主轴估计（Standardized major axis estimation, SMA）方法研究了叶大小（单叶质量、单叶面积）与出叶强度（基于茎生物量、茎体积）间异速生长关系对长期增温的响应及其年际变化。结果表明：使用不同参数表征叶大小与出叶强度得到的结果存在差异;多年生小枝上存在单叶质量-出叶强度的负等速权衡关系,共同主轴随小枝年龄增加而向下漂移;长期增温并不影响单叶质量与出叶强度的异速生长关系,不同年龄小枝的异速生长常数对增温具有差异性响应。增温处理中当年生小枝在相同单叶质量下的出叶强度更低,以换取叶片总数的增加,使小枝具有更大的可塑性而适应增温。本研究提供了岷江冷杉幼苗协调异龄叶大小与出叶强度从而适应长期增温的证据,为评估树木生长随气候变化而加速提供了理论参考。     

Nickel localization in tissues of different hyperaccumulator species of Euphorbiaceae from ultramafic areas of Cuba

Pantropical species of the genera Phyllanthus and Euphorbia and the Cuban endemic genus Leucocroton from the Euphorbiaceae family, were selected for nickel localization microanalysis. Scanning Electron Microscopy coupled with Energy Dispersive X-ray Microanalysis (SEM-EDX) was used for qualitative detection of nickel in the selected Ni-hyperaccumulator species: Euphorbia helenae, Leucocroton linearifolius, L. flavicans Phyllanthus orbicularis, P. discolor and P. xpallidus, all collected from Cuban ultramafic soils. The leaves and stems from the Euphorbiaceae species analyzed were the organs with higher nickel accumulation. Elemental mapping of leaves and stem tissues from these species have been compared. The highest Ni concentrations were found in the laticifer tubes of stems and the epidermis tissues of leaves in all the analyzed species, suggesting a general pattern of the Euphorbiaceae family for nickel accumulation. The high nickel concentrations and its rather homogeneous distribution found in leaves of these Ni-hyperaccumulating plants suggest a possible role in protection mechanisms against environmental stress, such as UV irradiation.     

COMPARATIVE GAS EXCHANGE OF LEAVES AND BARK IN THREE STEM SUCCULENTS OF BAJA CALIFORNIA

Arborescent stem succulents in tropical and subtropical deserts depend on scarce and uncertain rainfall. Gas exchange and the diurnal acidity fluctuation of bark and ephemeral leaves were measured under both dry and moist soil conditions in Fouquieria columnaris (cirio or boojum tree) and Pachycormus discolor (torote bianco or elephant tree) and in stems of the columnar cactus Pachycereus pringlei (cardon) in the Central Desert of Baja California, Mexico. Results demonstrated that ephemeral leaves were the only site of exogenous CO2 assimilation in F. columnaris and P. discolor; there was no measurable gas exchange across the green photosynthetic bark. The pattern of gas exchange in F. columnaris and P. discolor was consistent with that of C3 plants. P. pringlei was shown to be a typical Crassulacean acid metabolism plant on the basis of acid fluctuations and gas exchange. Chlorophyll fluorescence studies of the green bark of F. columnaris and P. discolor indicated that this tissue is photosynthetically functional, and that CO2 assimilation can rise above the compensation point under high CO2 concentrations, such as may occur within the plant. The green photosynthetic bark of these species may be an adaptation for surviving prolonged drought and may function to recycle endogenous respiratory CO2, thus maintaining the plant's energy reserves and permitting rapid production of leaves in response to infrequent rains.     

Biomass allocation between extension- and leaf display-oriented shoots in relation to habitat differentiation among five deciduous liana species in a Japanese cool-temperate forest

Liana species have a variety of habitat preferences. Although morphological traits connected to resource acquisition may vary by habitat preference, few studies have investigated such associations in lianas. In previous work on temperate lianas, we observed (1) free standing leafy shoots and (2) climbing shoots that clung to host plants; we examined relationships between habitat preference and shoot production patterns in five liana species. Among the five species, two were more frequent at the forest edges (forest-edge species), and two were more common within the forests (forest-interior species). The proportion of climbing shoots in current-year shoot mass of young plants (3–8 m in height) was greater in the forest-edge species (45–60%) than in the forest-interior species (6–30%). In consequence, there was a greater leaf mass ratio in the total current-year shoots of forest-interior species. This, combined with a greater specific leaf area, endows forest-interior species with a leaf area per unit shoot mass double that of forest-edge species. Forest-edge species had longer individual climbing shoots whose length per unit stem mass was smaller than in forest-interior lianas. Extension efficiency, measured as the sum of the climbing stem length per unit current-year shoot mass, was thus similar between forest-edge and interior species. In conclusion, liana shoot production patterns were related to species habitat preferences. A trade-off between current potential productivity (leaves) and the ability to search for hosts and/or well-lit environments (climbing stems) may underpin these relationships.     

Wood and bark anatomy of lianoid Indomalesian and Asiatic species of Gnetum

Quantitative and qualitative data on wood and bark are offered for 11 species of lianoid Indomalesian and Asiatic species of Gnetum section Cylindrostachys. Material of roots was studied for two species, material of an underground stem for one species, and stem material was studied for all species; wood from inside and outside of a large stem of G. montanum was analysed (quantitative data do not change with age in this species). Roots have shorter, narrower vessel elements, more numerous per mm², compared with those of stems; these trends run counter to those in dicotyledons. Roots and underground stems have more abundant parenchyma and less abundant sclerenchyma than do stems. Parenchyma of both roots and stems is rich in starch. All of the species studied here have both fibre-tracheids and tracheids, but tracheids are not distributed vasicentrically as they are in dicotyledons. Tori are reported for tracheary elements of three species studied here. Vasicentric axial parenchyma (which usually is thick-walled) is present in all species; thick or thin-walled diffuse or diffuse-in-aggregates apotracheal parenchyma is present in almost all of the species studied. Rays are mostly dimorphic in size, but show various conditions with respect to wall thickness, sclerenchyma presence, and crystal presence. As in other lianoid species of Gnetum, the species of the present study show origin of lateral meristem activity in parenchyma of the innermost cortex. Cortex and bark of the species studied here are relatively uniform in distribution of gelatinous fibres, nests of sclereids, the cylinder of brachysclereids that extends around the stem, and sclerenchymatous phelloderm. Laticifers were observed in bark of only two species studied. Although a few species characters are evident, the species that comprise Section cylindrostachys differ from each other mostly in degree rather than in presence or absence character state distributions. Secretory cavities are newly reported for the genus.     

Herbivore-mediated facilitation alters composition and increases richness and diversity in ruderal communities

Little is known about positive interactions among members of herbaceous plant communities initiating secondary succession (i.e., ruderal communities). Here, we explored the possibility that Euphorbia schickendantzii (Euphorbia), a latex-containing herb, facilitates other ruderals by protecting them from herbivores in recently plowed and overgrazed sites in central Argentina. To test this hypothesis, we compared plant number, height, reproductive output, and herbivore damage for four species when associated with Euphorbia versus in adjacent open zones without Euphorbia. Additionally, we classified species in the community according to their palatability, and compared community composition, richness, and diversity between Euphorbia and open zones. Dominant (66 % relative abundance) and highly palatable species exhibited increased plant number, size, and fecundity, and decreased herbivory when associated with Euphorbia relative to non-Euphorbia zones. In contrast, a physically and chemically well-defended species showed greater number of individuals in the open and no differences in herbivory between sampling zones. In detrended correspondence analysis, ordination scores of most palatable species were closer to Euphorbia, while those of most unpalatable species were closer to the open. Community composition differed between areas, with six species (25 % of the community) occurring exclusively with Euphorbia and three other species occurring only in open zones. Additionally, richness and diversity were greater in communities associated with Euphorbia than in those associated with non-Euphorbia zones. These results support our hypothesis, highlight the importance of facilitation in altering community-level responses, and indicate that positive interactions can play a more significant role in organizing terrestrial ruderal communities than previously recognized.     

Steroids from Euphorbia and other latex-bearing plants

Steroids in latices isolated from 15 Euphorbia species and four other latex-bearing plants were analysed (in terms of their chemotaxonomy) using computerized GC-MS. Steroid patterns divided these plants into six groupings.     

Photosynthetic Assimilation of 14CO2 by Mature Brown Stems of the Tea Plant (Camellia sinensis L.)

The fixation of ¹⁴CO₂ by mature brown stems of the tea plantwas studied by supplying ¹⁴CO₂ to selected stems of pruned andintact plants for 24 h under field conditions. Utilization of¹⁴C assimilates for the production of new shoots was also examined.The photosynthetic nature of the fixation of ¹⁴CO₂ is demonstrated.The efficiency of this fixation was very low compared with thattaking place in leaves. The movement of labelled assimilatesfrom stem bark to the roots was inappreciable, whereas newlyemerged shoots on the pruned frame drew labelled assimilatesfrom stem bark.     

Diversity,evolution, and function of stomata bearing structures in Cuscuta (dodders,Convolvulaceae): From extrafloral nectar secretion to transpiration in arid conditions

Cuscuta includes ca. 200 species of functionally holoparasitic plants grouped in four subgenera: Monogynella, Cuscuta, Pachystigma, and Grammica. Multicellular structures with stomata in Cuscuta are represented by extrafloral nectaries (ENs), reported from the stems of one Monogynella species, and stomatiferous protuberances (SPs), which are non-secretory. These latter structures had been noted on the stems of three Grammica species more than a century ago but entirely forgotten until recently when similar, non-secretory SPs were reported on the flowers of several new Grammica species. Here we study for the first time: (1) the extent of occurrence, diversity and evolution of secretory (ENs) and non-secretory (SPs) multicellular structures in Cuscuta, and (2) the function of SPs. We undertook a character evolution study of ENs and SPs on the stems and flowers of 136 Cuscuta taxa, and examined the structure/ultrastructure of SPs. ENs are inferred as primitive and characterize subg. Monogynella. SPs are derived in the remaining subgenera; they are ubiquitous on the flowers of Cuscuta and Pachystigma, but absent on their stems. Subgenus Grammica species develop two functional types of stems during their life cycle: vegetative, exploratory stems with very low stomatal densities (and no SPs), and reproductive, haustorial stems with numerous SPs. Moreover, 24 species from nine clades of subg. Grammica have evolved morphologically diverse floral SPs with systematic significance. To preliminarily ascertain SP function, we determined in the field the water uptake of Tithonia tubiformis plants parasitized or not by Cuscuta costaricensis, a species with both stem and floral SPs, and the stomatal conductance of dodder stems and flowers, as well as host leaves. Water uptake of parasitized hosts was significantly higher compared to non-parasitized plants, even after host leaves were removed, both during the day and night. The increased water uptake of parasitized hosts and stomatal conductance values suggest a transpiration role for the SPs, which is also confirmed by their lacunar structure. Grammica species with floral SPs grow in arid areas or characterized by a pronounced dry season during flowering/fruiting, which suggests that SPs may have evolved to stimulate the host water uptake during these phenophases.     

Discrimination of Kalopanax pictus from its varieties and other ‘Kalopanacis Cortex’ plants by multiplex polymerase chain reaction (PCR)

‘Kalopanacis Cortex’ (KC) is an important medicinal material prescribed in Korea for the treatment of various ailments such as paralysis, arthritis, and rheumatism. In Korea, KC is defined as the dried stem bark of Kalopanax pictus. However, the stem barks of Zanthoxylum ailanthoides and Erythrina plants such as E. variegata have also been described as KC and prescribed for the same diseases. Since the pharmacological activities and contents of compounds isolated from the stem barks of these KC plants are obviously different, KC from K. pictus should be clearly discriminated from other stem bark in the best interest of public health. This study sequenced the internal transcribed spacers (ITS) of 52 samples of the KC or KC plants collected from Korea, China, and Vietnam. On the basis of different nucleotide sequences of specific ITS regions among the KC plants, the primer set KP F2/KC R1 was designed to amplify a 398-bp DNA marker for discriminating K. pictus from its varieties and from other KC plants. This primer set, along with the primer sets ZR F1/ZR R4, KP F1/EV R2, and KPF1/KC R1, was successfully amplified with the DNA markers of the Z. ailanthoides, E. variegata, and K. pictus taxa, respectively. The multiplex polymerase chain reaction method developed here not only discriminates the stem bark of K. pictus from stem barks of other KC plants but also identifies KC plants that supply KC in a single process.     

Phosphorus nutrition of jarrah (Eucalyptus marginata) seedlings

Summary Effects of calcium phosphate supply on plant dry matter and phosphorus concentrations of parts of jarrah (Eucalyptus marginata) seedlings grown in a lateritic topsoil from the jarrah forest were examined in two glasshouse trials. Phosphorus deficiency depressed root and shoot dry weights and severely deficient leaves were smal and purple with prominent red major veins. Phosphorus deficiency severely reduced stem phosphorus levels (0.5% to 0.02%, experiment 1). Phosphorus concentrations were higher in bark than wood and the amount of phosphorus in the bark was sensitive to stem age and phosphate supply. Phosphorus adequate plants had bark phosphorus concentrations in the range 0.2–0.9% compared to <0.1% in deficient plants (experiment 2). Jarrah leaves accumulated dry matter up to 80 days after expansion and some leaves exported phosphorus during this period. Bark analysis may therefore be preferable to leaf analysis for detecting phosphorus deficiency in this species.     

Anti-influenza (H1N1) potential of leaf and stem bark extracts of selected medicinal plants of South India

Variations in antioxidant and anti-viral activities (against Influenza AP/R/8 (H1N1) virus) between the leaves and stem bark of selected medicinal plants were studied. Malin Darby canine kidney (MDCK) cells were used for the viral infection and the antiviral activity of the extracts was studied using sulphorhodamine B (SRB) assay. The stem bark of the plants including Strychnos minor, Diotacanthus albiflorus, Strychnos nux-vomica and Chloroxylon swietenia showed higher flavonoid contents as well as 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) scavenging activity when compared with their leaves. In case of 1,1-diphenyl-2-picrylhydrazyl (DPPH) activity, the stem bark of S. nux-vomica and leaf extract of C. swietenia showed the highest activity. Based on the IC₅₀ values, the stem bark extracts of Cayratia pedata (20.5 μg/mL) and S. minor (22.4 μg/mL) showed high antiviral activity. In the mean-time S. nux-vomica, C. swietenia and C. swietenia bark extracts showed cytotoxicity to the MDCK cells. When comparing the stem bark and leaves the content of gallic acid, ferulic acid, o-coumaric acid, total flavonoids (TFC) and total phenols (TPC) was higher in stem bark and hence their anti-viral activity was high. Further study based on the metabolites against H1N1 can reveal the potential of therapeutic compounds against the viral disease.     

Repeated range expansion and niche shift in a volcanic hotspot archipelago: Radiation of C4 Hawaiian Euphorbia subgenus Chamaesyce (Euphorbiaceae)

Woody perennial plants on islands have repeatedly evolved from herbaceous mainland ancestors. Although the majority of species in Euphorbia subgenus Chamaesyce section Anisophyllum (Euphorbiaceae) are small and herbaceous, a clade of 16 woody species diversified on the Hawaiian Islands. They are found in a broad range of habitats, including the only known C₄ plants adapted to wet forest understories. We investigate the history of island colonization and habitat shift in this group. We sampled 153 individuals in 15 of the 16 native species of Hawaiian Euphorbia on six major Hawaiian Islands, plus 11 New World close relatives, to elucidate the biogeographic movement of this lineage within the Hawaiian island chain. We used a concatenated chloroplast DNA data set of more than eight kilobases in aligned length and applied maximum likelihood and Bayesian inference for phylogenetic reconstruction. Age and phylogeographic patterns were co‐estimated using BEAST. In addition, we used nuclear ribosomal ITS and the low‐copy genes LEAFY and G3pdhC to investigate the reticulate relationships within this radiation. Hawaiian Euphorbia first arrived on Kaua`i or Ni`ihau ca. 5 million years ago and subsequently diverged into 16 named species with extensive reticulation. During this process Hawaiian Euphorbia dispersed from older to younger islands through open vegetation that is disturbance‐prone. Species that occur under closed vegetation evolved in situ from open vegetation of the same island and are only found on the two oldest islands of Kaua`i and O`ahu. The biogeographic history of Hawaiian Euphorbia supports a progression rule with within‐island shifts from open to closed vegetation.     

陕西分布10种大戟属植物茎叶形态结构比较

利用石蜡切片法、叶表皮装片法对陕西分布10种大戟属植物茎叶的形态结构进行了比较观察。结果表明:(1)10种植物叶的上下表皮细胞形态、气孔分布及大小等指标存在明显差异,茎的结构在各种之间也存在一定差异。(2)用Statistica 6.0分析软件对所测的性状特征值进行了主成分分析和聚类分析,结果发现,在10种植物中,甘遂与其他种的亲缘关系最远,南大戟与华北大戟的亲缘关系较近,斑地锦与地锦草亲缘关系最近。该研究结果为大戟属植物的分类研究奠定了理论依据。     

How is light interception efficiency related to shoot structure in tall canopy species?

Coexistence of multiple species is a fundamental aspect of plant and forest ecology. Although spatial arrangement of leaves within crowns is an important determinant of light interception and productivity, shoot structure varies considerably among coexisting canopy species. We investigated the relative importance of structural traits in determining the light availability of leaves (I) and light interception efficiency at the current-year shoot level (LIE_CS; the total light interception of leaves divided by shoot biomass) at the top of crowns of 11 canopy species in a cool-temperate forest in Japan. In accordance with Corner’s rules, the total mass, stem mass, total mass of leaf laminae, individual leaf area, and stem cross-sectional area of current-year shoot were positively correlated with each other, and branching intensity (the number of current-year shoots per branch unit of 1-m length) was inversely correlated with these traits across species. In contrast, I was correlated not with these traits, but with leaf elevation angle (a _L). Moreover, variation in LIE_CS across species was caused by variation in I (thus in a _L). Thus, a _L is a key parameter for the leaf light interception of canopy shoots in this cool-temperate forest. Differences in a _L across species might be related to different physiological strategies that developed in the high light and water-limited environment of forest canopies. Small variation in the length of current-year shoots among species implies that variations in I and LIE_CS would be important for the coexistence of these canopy species.     

江西常绿阔叶林木本植物不同冠层高度当年生小枝茎构型对叶生物量的影响

以江西阳际峰自然保区69种木本植物为对象,采用标准化主轴回归(SMA)的方法,对不同冠层高度当年生小枝的构件生物量(叶生物量、茎生物量、小枝生物量)与茎构型特征(茎直径、茎长度、茎宽长比、茎体积和茎密度)进行了分析,研究当年生小枝茎构型对叶生物量的影响.结果表明:不同冠层高度及生活型间叶生物量、茎生物量、小枝生物量、茎直径、茎长度、茎宽长比及茎体积均无显著差异,而茎密度差异显著.不同冠层高度及生活型间,叶生物量与茎生物量及总生物量之间均呈显著等速生长关系.小枝叶生物量分别与茎直径、茎体积呈显著异速生长关系,且该异速生长指数在不同冠层高度无显著差异.茎长度、茎宽长比及茎密度对当年生小枝叶生物量变异的解释力较小(<24%).冠层高度和生活型对小枝叶-茎生物量的等速分配关系影响不显著.小枝茎构型中,相比于茎长度、茎宽长比以及茎密度,茎的直径与体积对当年生小枝叶生物量的影响更大,冠层高度对当年生小枝茎构型与叶生物量间的异速分配模式无显著影响.     

Developmental plasticity and biomechanics of treelets and lianas in Manihot aff. quinquepartita (Euphorbiaceae): a branch-angle climber of French Guiana

Background and Aims

Most tropical lianas have specialized organs of attachment such as twining stems, hooks or tendrils but some do not. Many climbers also have an early self-supporting phase of growth and in some species this can produce treelet-sized individuals. This study focuses on how a liana can climb without specialized attachment organs and how biomechanical properties of the stem are modulated between self-supporting treelets and canopy-climbing lianas.

Methods

Biomechanics and stem development were investigated in self-supporting to climbing individuals of Manihot aff. quinquepartita (Euphorbiaceae) from tropical rain forest at Saül, central French Guiana. Bending tests were carried out close to the site of growth. Mechanical properties, including Young''s elastic modulus, were observed with reference to habit type and changes in stem anatomy during development.

Key Results

This liana species can show a remarkably long phase of self-supporting growth as treelets with stiff, juvenile wood characterizing the branches and main stem. During the early phase of climbing, stiff but unstable stem segments are loosely held in a vertical position to host plants via petiole bases. The stiffest stems – those having the highest values of Young''s modulus measured in bending – belonged to young, leaning and climbing stems. Only when climbing stems are securely anchored into the surrounding vegetation by a system of wide-angled branches, does the plant develop highly flexible stem properties. As in many specialized lianas, the change in stiffness is linked to the development of wood with numerous large vessels and thin-walled fibres.

Conclusions

Some angiosperms can develop highly effective climbing behaviour and specialized flexible stems without highly specialized organs of attachment. This is linked to a high degree of developmental plasticity in early stages of growth. Young individuals in either open or closed marginal forest conditions can grow as substantial treelets or as leaning/climbing plants, depending on the availability of host supports. The species of liana studied differs both in terms of development and biomechanics from many other lianas that climb via twining, tendrils or other specialized attachment organs.Key words: Biomechanics, bending, developmental plasticity, French Guiana, liana, Manihot aff. quinquepartita (Euphorbiaceae), treelet, branch angle climber, Young''s modulus     

On the allometry of biomass partitioning and light harvesting for plants with leafless stems

Prior explicit allometric models are extended to predict the scaling relationship between the ability of plants with leafless stems to harvest sunlight H and total standing plant biomass M(T) (which equals the sum of standing stem and root biomass, M(S) and M(R)). Provided that H scales in a directly proportional manner (isometrically) with respect to either stem surface area (i.e.H proportional, variant SA(S) ) or total stem biomass (i.e. H proportional, variant M(S)), the allometric model presented here predicts that SA(S) proportional, variant M(T)(3/4) or M(S) proportional, variant M(T)(3/4), respectively. These alternative predictions are tested empirically using data for standing stem and root biomass gathered for the large columnar cactus species Pachycereus pringlei. Statistical comparisons between observed and predicted scaling relationships indicate that SA(S) proportional, variant M(T)(3/4), whereas M(S) proportional, variant M(T)(3/4) is mathematically inconsistent with the observation that stem biomass scales nearly isometrically with respect to root biomass. The contention that the H of leafless stems scales isometrically with respect to stem surface area is thus reasonable both theoretically and empirically.