Analysis of circular bordered pit function II. Gymnosperm tracheids with torus-margo pit membranes

A model of xylem conduit function was applied to gymnosperm tracheids with torus-margo pit membranes for comparison with angiosperm vessels. Tracheids from 17 gymnosperm tree species with circular bordered pits and air-seed pressures from 0.8 to 11.8 MPa were analyzed. Tracheids were more reinforced against implosion than vessels, consistent with their double function in transport and support. Tracheid pits were 3.3 to 44 times higher in hydraulic conductivity than vessel pits because of greater membrane conductivity of the torus-margo configuration. Tight scaling between torus and pit size maximized pit conductivity. Higher pit conductivity allowed tracheids to be 1.7-3.4 times shorter than vessels and still achieve 95% of their lumen-limited maximum conductivity. Predicted tracheid lengths were consistent with measured lengths. The torus-margo structure is important for maximizing the conductivity of the inherently length-limited tracheid: replacing the torus-margo membrane with a vessel membrane caused stem tracheid conductivity to drop by 41%. Tracheids were no less hydraulically efficient than vessels if they were long enough to reach their lumen-limiting conductivity. However, this may only be possible for lumen diameters below approximately 60-70 μm.     

Testing hypotheses that link wood anatomy to cavitation resistance and hydraulic conductivity in the genus Acer

? Vulnerability to cavitation and conductive efficiency depend on xylem anatomy. We tested a large range of structure-function hypotheses, some for the first time, within a single genus to minimize phylogenetic 'noise' and maximize detection of functionally relevant variation. ? This integrative study combined in-depth anatomical observations using light, scanning and transmission electron microscopy of seven Acer taxa, and compared these observations with empirical measures of xylem hydraulics. ? Our results reveal a 2 MPa range in species' mean cavitation pressure (MCP). MCP was strongly correlated with intervessel pit structure (membrane thickness and porosity, chamber depth), weakly correlated with pit number per vessel, and not related to pit area per vessel. At the tissue level, there was a strong correlation between MCP and mechanical strength parameters, and some of the first evidence is provided for the functional significance of vessel grouping and thickenings on inner vessel walls. In addition, a strong trade-off was observed between xylem-specific conductivity and MCP. Vessel length and intervessel wall characteristics were implicated in this safety-efficiency trade-off. ? Cavitation resistance and hydraulic conductivity in Acer appear to be controlled by a very complex interaction between tissue, vessel network and pit characteristics.     

Do xylem fibers affect vessel cavitation resistance?

Possible mechanical and hydraulic costs to increased cavitation resistance were examined among six co-occurring species of chaparral shrubs in southern California. We measured cavitation resistance (xylem pressure at 50% loss of hydraulic conductivity), seasonal low pressure potential (P(min)), xylem conductive efficiency (specific conductivity), mechanical strength of stems (modulus of elasticity and modulus of rupture), and xylem density. At the cellular level, we measured vessel and fiber wall thickness and lumen diameter, transverse fiber wall and total lumen area, and estimated vessel implosion resistance using (t/b)(h)(2), where t is the thickness of adjoining vessel walls and b is the vessel lumen diameter. Increased cavitation resistance was correlated with increased mechanical strength (r(2) = 0.74 and 0.76 for modulus of elasticity and modulus of rupture, respectively), xylem density (r(2) = 0.88), and P(min) (r(2) = 0.96). In contrast, cavitation resistance and P(min) were not correlated with decreased specific conductivity, suggesting no tradeoff between these traits. At the cellular level, increased cavitation resistance was correlated with increased (t/b)(h)(2) (r(2) = 0.95), increased transverse fiber wall area (r(2) = 0.89), and decreased fiber lumen area (r(2) = 0.76). To our knowledge, the correlation between cavitation resistance and fiber wall area has not been shown previously and suggests a mechanical role for fibers in cavitation resistance. Fiber efficacy in prevention of vessel implosion, defined as inward bending or collapse of vessels, is discussed.     

Rare pits, large vessels and extreme vulnerability to cavitation in a ring-porous tree species

? The rare pit hypothesis predicts that the extensive inter-vessel pitting in large early-wood vessels of ring-porous trees should render many of these vessels extremely vulnerable to cavitation by air-seeding. This prediction was tested in Quercus gambelii. ? Cavitation was assessed from native hydraulic conductivity at field sap tension and in dehydrated branches. Single-vessel air injections gave air-seeding pressures through vessel files; these data were used to estimate air-seeding pressures for inter-vessel walls and pits. ? Extensive cavitation occurred at xylem sap tensions below 1 MPa. Refilling occurred below 0.5 MPa and was inhibited by phloem girdling. Remaining vessels cavitated over a wide range to above 4 MPa. Similarly, 40% of injected vessel files air-seeded below 1.0 MPa, whereas the remainder seeded over a wide range exceeding 5 MPa. Inter-vessel walls averaged 1.02 MPa air-seeding pressure, similar and opposite to the mean cavitation tension of 1.22 MPa. Consistent with the rare pit hypothesis, only 7% of inter-vessel pits were estimated to air-seed by 1.22 MPa. ? The results confirm the rare pit prediction that a significant fraction of large vessels in Q. gambelii experience high probability of failure by air-seeding.     

Direct measurements of intervessel pit membrane hydraulic resistance in two angiosperm tree species

The hydraulic resistance of pit membranes was measured directly in earlywood vessels of Fraxinus americana and Ulmus americana. The area-specific resistance of pit membranes (r(mem)) was higher than modeled or measured values obtained previously for hardwood species, with r(mem) of 5.24 × 10(3) MPa·s·m(-1) for Fraxinus and 2.56 × 10(3) MPa·s·m(-1) for Ulmus. The calculated resistance of pit canals was three orders of magnitude below total pit resistance indicating that pit membranes contributed the majority of resistance. Scanning electron microscopy indicated that pit membranes of Ulmus were thinner and more porous than those of Fraxinus, consistent with the difference in r(mem) between the species. Measurements of average vessel diameter and length and area of wall overlap with neighboring vessels were used to partition the vascular resistance between vessel lumen and pit membrane components. Pit membrane resistance accounted for 80% of the total resistance in Fraxinus and 87% in Ulmus in 2-yr-old branch sections. However, measurements of vessel dimensions in the trunk suggest that the division of resistance between pit membrane and lumen components would be closer to co-limiting in older regions of the tree. Thus, pit membrane resistance may be of greater relative importance in small branches than in older regions of mature trees.     

Bordered pit structure and vessel wall surface properties. Implications for embolism repair

The idea that embolized xylem vessels can be refilled while adjacent vessels remain under tension is difficult to accept if the cavitated vessels remain hydraulically connected to vessels under tension. A mechanism by which embolized conduits could be hydraulically isolated from adjacent conduits requires the existence of a non-zero contact angle and a flared opening into the bordered pit chamber such that a convex air-water interface forms at the entrance into the pit chamber. We measured the contact angle and pit chamber geometry for six species. The contact angle measured in the vessel lumen ranged between 42 degrees to 55 degrees, whereas the opening into the pit chamber ranged between 144 degrees and 157 degrees. If the surface properties within the pit chamber are similar to those in the lumen, a convex meniscus will form at the flared opening into the pit chamber. The maximum pressure difference between water in the lumen and gas in the pit chamber that could be stabilized by this interface was calculated to be within the range of 0.07 to 0.30 MPa.     

From the sap's perspective: The nature of vessel surfaces in angiosperm xylem

Premise of the Study

Xylem sap in angiosperms moves under negative pressure in conduits and cell wall pores that are nanometers to micrometers in diameter, so sap is always very close to surfaces. Surfaces matter for water transport because hydrophobic ones favor nucleation of bubbles, and surface chemistry can have strong effects on flow. Vessel walls contain cellulose, hemicellulose, lignin, pectins, proteins, and possibly lipids, but what is the nature of the inner, lumen‐facing surface that is in contact with sap?

Methods

Vessel lumen surfaces of five angiosperms from different lineages were examined via transmission electron microscopy and confocal and fluorescence microscopy, using fluorophores and autofluorescence to detect cell wall components. Elemental composition was studied by energy‐dispersive X‐ray spectroscopy, and treatments with phospholipase C (PLC) were used to test for phospholipids.

Key Results

Vessel surfaces consisted mainly of lignin, with strong cellulose signals confined to pit membranes. Proteins were found mainly in inter‐vessel pits and pectins only on outer rims of pit membranes and in vessel‐parenchyma pits. Continuous layers of lipids were detected on most vessel surfaces and on most pit membranes and were shown by PLC treatment to consist at least partly of phospholipids.

Conclusions

Vessel surfaces appear to be wettable because lignin is not strongly hydrophobic and a coating with amphiphilic lipids would render any surface hydrophilic. New questions arise about these lipids and their possible origins from living xylem cells, especially about their effects on surface tension, surface bubble nucleation, and pit membrane function.     

Inter-vessel pitting and cavitation in woody Rosaceae and other vesselled plants: a basis for a safety versus efficiency trade-off in xylem transport

The hypothesis that greater safety from cavitation by air-seeding through inter-vessel pits comes at the cost of less porous pit membranes with greater flow resistance was tested . Sixteen vessel-bearing species were compared: 11 from the Rosaceae, four from other angiosperm families, and one fern. Unexpectedly, there was no relationship between pit resistance (and hence the prevailing membrane porosity) and cavitation pressure. There was, however, an inverse relationship between pit area per vessel and vulnerability to cavitation (r² = 0.75). This suggests that cavitation is caused by the rare largest membrane pore per vessel, the average size of which increases with total pit area per vessel. If safety from cavitation constrains pit membrane surface area, it also limits vessel surface area and the minimum vessel resistivity. This trade-off was consistent with an approximately three-fold increase in vessel resistivity with cavitation pressure dropping from −0.8 to −6.6 MPa. The trade-off was compensated for by a reduction in the percentage of vessel wall pitted: from 10–16% in vulnerable species to 2–4% in resistant species. Across species, end-wall pitting accounted for 53 ± 3% of the total xylem resistivity. This corresponded to vessels achieving on average 94 ± 2% of their maximum possible conductivity if vessel surface area is constrained.     

As human pial arteries (internal diameter 200-1000 microm) get smaller, their wall thickness and capacity to develop tension relative to their diameter increase.

Pial arteries play a key role in the regulation of human cerebral blood flow. However, many of the features and mechanisms that regulate the tone and diameters of these vessels cannot be studied in situ. One approach is to study in vitro segments of arteries obtained during neurosurgical procedures. The ratios of arterial media thickness to lumen diameter and of the capacity to develop wall force to lumen diameter have important functional consequences and are known to change in disease. Experiments were carried out on pial arteries from normotensive humans to determine the way in which these parameters vary with vessel size. Vessel dimensions--media thickness and lumen diameter were derived from fixed sections using quantitative morphometry. Wall force was measured using a resistance artery myograph. The ratio of media thickness to lumen diameter and of maximum tension developed to lumen diameter both increased as vessel diameter decreased. These ratios do not change over the age range of 15-75 years. These findings show that although in vivo intralumenal pressure falls as human pial arteries become smaller, their media thickness and capacity to develop tone increase.     

Hydraulic properties of individual xylem vessels of Fraxinus americana

Studies of the hydraulic properties of xylem vessels have been limited to measurements of whole plant or whole stem segments. This approach allows the longitudinal transport properties of the ensemble of vessels within a stem to be determined, but provides little information on radial transport. Here the xylem of Fraxinus americana L. has been examined using a new method that allows the transport properties of individual vessels to be examined. One goal of this study was to quantify transport parameters relevant to embolism repair. The longitudinal conductivity of vessel segments open at both ends (i.e. no end walls) agreed with values predicted by the Poiseuille equation. Radial specific conductance (conductance per unit area) was approximately six orders of magnitude lower than the longitudinal conductance of the vessel segment normalized by the cross-sectional area of the vessel lumen. There was a step increase in the radial specific conductance of previously gas-filled vessels when the delivery pressure exceeded 0.4 MPa. This is consistent with the idea that positive pressure, required for embolism repair, can be compartmentalized within a vessel if the bordered pit chambers are gas-filled. The diffusion coefficient for the movement of gas from a pressurized air-filled vessel was of the same order of magnitude as that for air diffusing through water (1.95 e(-9) m(2) s(-1)). Estimates of the time needed to displace all of the gas from an air-filled vessel were in the order of 20 min, suggesting that gas removal may not be a major limitation in embolism repair.     

A comparison of two methods for measuring vessel length in woody plants

Vessel lengths are important to plant hydraulic studies, but are not often reported because of the time required to obtain measurements. This paper compares the fast dynamic method (air injection method) with the slower but traditional static method (rubber injection method). Our hypothesis was that the dynamic method should yield a larger mean vessel length than the static method. Vessel length was measured by both methods in current year stems of Acer, Populus, Vitis and Quercus representing short‐ to long‐vessel species. The hypothesis was verified. The reason for the consistently larger values of vessel length is because the dynamic method measures air flow rates in cut open vessels. The Hagen–Poiseuille law predicts that the air flow rate should depend on the product of number of cut open vessels times the fourth power of vessel diameter. An argument is advanced that the dynamic method is more appropriate because it measures the length of the vessels that contribute most to hydraulic flow. If all vessels had the same vessel length distribution regardless of diameter, then both methods should yield the same average length. This supports the hypothesis that large‐diameter vessels might be longer than short‐diameter vessels in most species.     

Air method measurements of apple vessel length distributions with improved apparatus and theory

Studies showing that rootstock dwarfing potential is related to plant hydraulic conductance led to the hypothesis that xylem properties are also related. Vessel length distribution and other properties of apple wood from a series of varieties were measured using the 'air method' in order to test this hypothesis. Apparatus was built to measure and monitor conductivity to air of fresh wood segments of different lengths. Theory for determining vessel length distribution was improved to give a single parameter uni-modal vessel length probability density function. The function, derived by combining the exponential extinction (with extinction coefficient k) of wood conductivity to air (C) as wood length (x) increases (i.e. C=Co exp (-kx)) with the differential double difference formula, is Px=xxk2 exp (-kx), where Px is the fraction of vessels of length x. The main parameter of the distribution, k, was found to be the inverse of the mode of the distribution, i.e. the most common vessel length, Lo. Lo for ten apple rootstock and scion varieties varied from 5.6+/-0.1 cm (+/-SE) for MM.111 to 9.0+/-1.0 for Prunifolia (P <0.05). Average maximum vessel length was approximately 50 cm, and differences were not significant. Effective vessel radii ranged from 14 for Prunifolia to 24.3+/-0.7 micro m for M.26, with standard errors less than 12% of the mean. Specific conductivity of a 15 cm wood segment ranged from 2x109-4) to 1.6+/-0.2x10(-2) dm3 s(-1) kPa(-1) m(-1) for maruba and M.26, respectively, with standard errors up to 63% of the mean. Vessel density at the air entry point ranged from 18+/-3 to 42+/-6 vessels mm-2 for M.26 and MM.106, respectively, with standard errors as high as 89% of the mean. It was concluded that there is no general relationship between the wood properties investigated and rootstock size class, and that plasticity increases from vessel lengths to radii to specific conductivity and vessel densities.     

Vulnerability of xylem to embolism in a mangrove vs an inland species of Rhizophoraceae

Vulnerability of xylem conduits to cavitation and embolism was compared in two species of Rhizophoraceae, the mangrove Rhizophora mangle L. and the tropical moist-forest Cassipourea elliptica (Sw.) Poir. Cavitation (water column breakage preceeding embolism) was monitored by ultrasonic detection; embolism was quantified by its reduction of xylem hydraulic conductivity. Acoustic data were not predictive of loss in hydraulic conductivity, probably because signals from cavitating vessels were swamped by more numerous ones from cavitating fibers. Rhizophora mangle was the less vulnerable to embolism of the two species, losing 80% of its hydraulic conductivity between – 6.0 and – 7.0 MPa. Cassipourea elliptica lost conductivity in linear proportion to decreasing xylem pressure from – 0.5 to – 7.0 MPa. Species vulnerability correlated closely with physiological demands of habitat; the mangrove Rhizophora mangle had field xylem pressures between – 2.5 and – 4.0 MPa. whereas the minimum for Cassipourea elliptica was – 1.6 MPa. Differences in vulnerability between species could be accounted for by differences in the measured air permeability of intervessel pit membranes. According to this explanation, embolism occurs when air enters a water-filled vessel from a neighboring air-filled one via pores in shared pit membranes.     

Intervessel pit membrane thickness best explains variation in embolism resistance amongst stems of Arabidopsis thaliana accessions

Background and AimsThe ability to avoid drought-induced embolisms in the xylem is one of the essential traits for plants to survive periods of water shortage. Over the past three decades, hydraulic studies have been focusing on trees, which limits our ability to understand how herbs tolerate drought. Here we investigate the embolism resistance in inflorescence stems of four Arabidopsis thaliana accessions that differ in growth form and drought response. We assess functional traits underlying the variation in embolism resistance amongst the accessions studied using detailed anatomical observations.MethodsVulnerability to xylem embolism was evaluated via vulnerability curves using the centrifuge technique and linked with detailed anatomical observations in stems using light microscopy and transmission electron microscopy.Key ResultsThe data show significant differences in stem P_50, varying 2-fold from −1.58 MPa in the Cape Verde Island accession to −3.07 MPa in the woody soc1 ful double mutant. Out of all the anatomical traits measured, intervessel pit membrane thickness (T_PM) best explains the differences in P₅₀, as well as P₁₂ and P₈₈. The association between embolism resistance and T_PM can be functionally explained by the air-seeding hypothesis. There is no evidence that the correlation between increased woodiness and increased embolism resistance is directly related to functional aspects. However, we found that increased woodiness is strongly linked to other lignification characters, explaining why mechanical stem reinforcement is indirectly related to increased embolism resistance.ConclusionsThe woodier or more lignified accessions are more resistant to embolism than the herbaceous accessions, confirming the link between increased stem lignification and increased embolism resistance, as also observed in other lineages. Intervessel pit membrane thickness and, to a lesser extent, theoretical vessel implosion resistance and vessel wall thickness are the missing functional links between stem lignification and embolism resistance.     

Chronic airway infection leads to angiogenesis in the pulmonary circulation.

In both pulmonary and systemic hypertension, the walls of the arteriolar vessels are thickened and the lumen size is reduced, leading to increased total vascular resistance. It has been reported previously that chronic airway infection and inflammation lead to increased wall thickness in the pulmonary vasculature, without the development of pulmonary hypertension. The aim of the present study was to examine quantitatively the remodeling of intra-acinar blood vessels in chronically infected rat lungs. Adult rats were anesthetized and inoculated intratracheally with Pseudomonas aeruginosa (n = 10) incorporated into agar beads to induce chronic airway infection. Control groups included rats inoculated with sterile agar beads (n = 8) and rats that were not inoculated (n = 6). Chronic infection caused vascular wall thickening without reduction in mean lumen radius. Furthermore, chronic infection led to increased total length of intra-acinar vessels and increased numbers of branch points, demonstrating that angiogenesis had occurred. Preservation of lumen size and formation of new parallel pathways in the vasculature of chronically infected lungs account for the maintenance of normal PVR despite vessel wall remodeling.     

Possibilities and limitations of histomorphometric studies of arterial vessels of the microcirculation

The choice of a procedure for measurements of blood vessels (b.v.) should depend on the type of b.v. investigated: For vessels defined anatomically and in respect to their function of organ supply the method of SUWA et al. (1961) is the most adequate one, provided that b.v. dilatations developing during lifetime and persisting after death as a result of special methods of fixation or shock-freezing or being generated by postmortal perfusion can be excluded. Changes in wall thickness of defined b.v. sectioned transversally can also be determined by measurement of the wall area and/or the maximal chord length. At undefined vessels procedures which measure the wall-to-lumen ratio are useless, because lumen changes are not quantifiable. Thus lumen changes can mask of mimick changes in wall thickness. The especially interesting b.v. either with a pathologically changed or without an elastic membrane are not measurable. The unsable method to detect changes in wall thickness at undefined b.v. is founded on the determination of the vessel wall area and/or the number of all the b.v. classified according to their diameter of a certain organ region. But the investigation of the smallest precapillary vessels presupposes their visualization by a special modification of the ATPase reaction.     

Comparative vessel anatomy of arctic deciduous and evergreen dicots

Arctic tundra plant species exhibit striking variation in leaf character and growth form. Both are likely related to differences in vessel anatomy, and all may affect responses to climate changes in the Arctic. To investigate the relationships among leaf character, growth form, vessel anatomy, and susceptibility to freeze-thaw-induced xylem cavitation, xylem vessel characteristics were compared among six deciduous and six evergreen arctic dicot species of erect and prostrate growth forms. We hypothesized that deciduous and erect species would have larger and longer vessels than evergreen and cushion/mat-forming species. Vessel lengths, diameters, and densities were measured for each species. Theoretical vessel flow rates were calculated using Poiseuille's law for ideal capillaries. Flow rates were used to determine the susceptibility of vessels to cavitation induced by freeze-thaw events that may become more frequent with global warming. Vessel diameters were larger in deciduous species compared to evergreens, and in shrubs/trees vs. cushion/mat-forming plants. Vessel length distributions, however, did not differ for growth form or leaf character. Vessel density was greater in cushion/mat-forming species than in shrub/tree species. Deciduous plants showed a greater contribution to total conductivity by relatively larger vessels than evergreens. One of the deciduous species, Vaccinium uliginosum, is predicted to be susceptible to freeze-thaw-induced cavitation. These results have important implications for future arctic species composition and plant community structure.     

Polar auxin transport is implicated in vessel differentiation and spatial patterning during secondary growth in Populus

Premise of the Study

Dimensions and spatial distribution of vessels are critically important features of woody stems, allowing for adaptation to different environments through their effects on hydraulic efficiency and vulnerability to embolism. Although our understanding of vessel development is poor, basipetal transport of auxin through the cambial zone may play an important role.

Methods

Stems of Populus tremula ×alba were treated with the auxin transport inhibitor N‐1‐naphthylphthalamic acid (NPA) in a longitudinal strip along the length of the lower stem. Vessel lumen diameter, circularity, and length; xylem growth; tension wood area; and hydraulic conductivity before and after a high pressure flush were determined on both NPA‐treated and control plants.

Key Results

NPA‐treated stems formed aberrant vessels that were short, small in diameter, highly clustered, and angular in cross section, whereas xylem formed on the untreated side of the stem contained typical vessels that were similar to those of controls. NPA‐treated stems had reduced specific conductivity relative to controls, but this difference was eliminated by the high‐pressure flush. The control treatment (lanolin + dimethyl sulfoxide) reduced xylem growth and increased tension wood formation, but never produced the aberrant vessel patterning seen in NPA‐treated stems.

Conclusions

These results are consistent with a model of vessel development in which basipetal polar auxin transport through the xylem‐side cambial derivatives is required for proper expansion and patterning of vessels and demonstrate that reduced auxin transport can produce stems with altered stem hydraulic properties.     

Contrasting hydraulic architecture and function in deep and shallow roots of tree species from a semi-arid habitat

Background and Aims

Despite the importance of vessels in angiosperm roots for plant water transport, there is little research on the microanatomy of woody plant roots. Vessels in roots can be interconnected networks or nearly solitary, with few vessel–vessel connections. Species with few connections are common in arid habitats, presumably to isolate embolisms. In this study, measurements were made of root vessel pit sizes, vessel air-seeding pressures, pit membrane thicknesses and the degree of vessel interconnectedness in deep (approx. 20 m) and shallow (<10 cm) roots of two co-occurring species, Sideroxylon lanuginosum and Quercus fusiformis.

Methods

Scanning electron microscopy was used to image pit dimensions and to measure the distance between connected vessels. The number of connected vessels in larger samples was determined by using high-resolution computed tomography and three-dimensional (3-D) image analysis. Individual vessel air-seeding pressures were measured using a microcapillary method. The thickness of pit membranes was measured using transmission electron microscopy.

Key Results

Vessel pit size varied across both species and rooting depths. Deep Q. fusiformis roots had the largest pits overall (>500 µm) and more large pits than either shallow Q. fusiformis roots or S. lanuginosum roots. Vessel air-seeding pressures were approximately four times greater in Q. fusiformis than in S. lanuginosum and 1·3–1·9 times greater in shallow roots than in deep roots. Sideroxylon lanuginosum had 34–44 % of its vessels interconnected, whereas Q. fusiformis only had 1–6 % of its vessels connected. Vessel air-seeding pressures were unrelated to pit membrane thickness but showed a positive relationship with vessel interconnectedness.

Conclusions

These data support the hypothesis that species with more vessel–vessel integration are often less resistant to embolism than species with isolated vessels. This study also highlights the usefulness of tomography for vessel network analysis and the important role of 3-D xylem organization in plant hydraulic function.     

SEM studies on vessels in ferns. 11. Ophioglossum

With scanning electron microscopy (SEM), the nature of metaxylem vessel elements and tracheids was examined in Ophioglossum crotalophomides, 0. pendulum subsp. falcatum , and 0. vulgatum roots and rhizomes. Vessels were identified in all species. End walls of vessel elements, which bear perforations, are like lateral wall pitting of those elements in the secondary wall framework and differ only in absence of pit membranes or presence of pit membrane remnants. Some of the perforations contain pit membrane remnants that have large pores, small porosities, or are threadlike or weblike in structure. Dimorphic perforations were found in some vessel elements of rhizomes of 0. pendulum subsp. falcatum. Tracheids are very likely present in addition to vessels in all three species. The secondary wall framework of both tracheids and vessels is basically scalariform, although deviations in pattern are present. Vessel elements of Ophiglossum are entirely comparable to those of leptosporangiate ferns.