Internal pressurization and convective gas flow in some emergent freshwater macrophytes

Internal pressurization and convective through-flow are demonstrated to be common attributes of wetland plants with cylindrical culms or linear leaves. Eight of 14 species tested produced static internal gas pressure differentials of 200-1,300 Pa relative to ambient and internal convective airflows of 0.2 to > 10 cm3 min-’ culm-I, depending on species. Four species produced internal static pressure differentials of < 100 Pa. Two species did not pressurize. The driving forces are gradients in temperature and water vapor between the internal gas spaces of the plants and the ambient atmosphere (thermal transpiration and humidity-induced pressurization). A clear diel variation in pressurization and convective flow was observed; rates were highest in the afternoon and lowest at night, responding to ambient changes in light, temperature, and humidity. The resistance to airflow at the stem-rhizome junction was very high for some species, resulting in a low ability to convert internal pressurization into convective airflow through the rhizomes. Species with a high potential for internal pressurization and a low internal resistance to convective flow seem to have a competitive advantage over species that rely exclusively on diffisive gas transport, which allows them to grow in deeper waters. The rhizomes and roots of many aquatic macrophytes are buried in anaerobic organic sediments. They require oxygen for growth and efficient respiration and have developed lacunar systems of interconnected airspaces to facilitate oxygen transport from shoots to roots (Sculthorpe 1967; Armstrong 1979). Most of these plants release oxygen from root surfaces, creating a rhizosphere that can oxidize phytotoxins such as sulfides and reduced metal ions (Armstrong 1982; Penhale and Wetzel 1983). In many aquatic plants, gas transport is accomplished by simple diflksion (Valiance and Coult 195 1; Armstrong 1979; SandAcknowledgments We thank Jane Roberts for help with species identification, Pino Pistillo for cultivation of plants, and William Armstrong, Jane Roberts, and Gary Jones for critical reviews and comments on the manuscript. This study was partially financed by the Danish Natural Science Research Council, Project 1 l-8694. Additional financial support was provided by the MurrayDarling Freshwater Research Centre and the CSIRO Division of Water Resources. Jensen and Prahl 1982; Sorrel1 and Dromgoole 1987). However, significantly higher gas fluxes are achieved in the lacunar system of some species by a pressurized convective flow of gases (Armstrong et al. 1991). The pressure, which is generated by gradients in temperature and water-vapor pressure between the internal gas spaces and the surrounding atmosphere, is vented through rhizomes and old shoots to the atmosphere (Dacey 198 1; Armstrong and Armstrong 199 1). These convective flows increase the oxygen concentration of gases venting through the rhizome, thus enhancing the diffusive flux of oxygen from rhizomes to roots and into the rhizosphere (Armstrong and Armstrong 1990). Internal pressurization and convective gas flow are important adaptations to growth in an anoxic substrate and may offer a competitive advantage over species relying exclusively on diffusive gas transport. How commonly convective flow occurs as a means of enhanced internal gas transport in wetland plants remains uncertain.

The rhizomes and roots of many aquatic macrophytes are buried in anaerobic organic sediments.They require oxygen for growth and efficient respiration and have developed lacunar systems of interconnected airspaces to facilitate oxygen transport from shoots to roots (Sculthorpe 1967;Armstrong 1979).Most of these plants release oxygen from root surfaces, creating a rhizosphere that can oxidize phytotoxins such as sulfides and reduced metal ions (Armstrong 1982;Penhale and Wetzel 1983).
In many aquatic plants, gas transport is accomplished by simple diflksion (Valiance and Coult 195 1;Armstrong 1979;Sand-Jensen and Prahl 1982;Sorrel1 and Dromgoole 1987).However, significantly higher gas fluxes are achieved in the lacunar system of some species by a pressurized convective flow of gases (Armstrong et al. 1991).The pressure, which is generated by gradients in temperature and water-vapor pressure between the internal gas spaces and the surrounding atmosphere, is vented through rhizomes and old shoots to the atmosphere (Dacey 198 1; Armstrong and Armstrong 199 1).These convective flows increase the oxygen concentration of gases venting through the rhizome, thus enhancing the diffusive flux of oxygen from rhizomes to roots and into the rhizosphere (Armstrong and Armstrong 1990).Internal pressurization and convective gas flow are important adaptations to growth in an anoxic substrate and may offer a competitive advantage over species relying exclusively on diffusive gas transport.
How commonly convective flow occurs as a means of enhanced internal gas transport in wetland plants remains uncertain.
The ability to generate a pressurized internal through-flow has been identified in many floating-leaved species and plants with sim-.ilar leaf morphology (Dacey 198 1;Grosse and Mevi-Schlitz 1987;Grosse et al. 199 1; Mevi-Schiitz and Grosse 1988a).Recently, a similar convective through-flow mechanism has been documented for Phragmites australis, a species with linear aerial leaves (Armstrong and Armstrong 199 1).Too few species have been studied to determine whether the ability to pressurize is restricted mainly to water lilies and plants with a similar morphology or whether it can be related to any functional or ecological plant associations.The relative significance of temperature, water vapor, and light in generating pressure and flow are also not fully resolved, but apparently vary greatly between species (Armstrong and Armstrong 199 1; Mevi-Schiitz and Grosse 1988a; Hwang and Morris 1991).
Our purpose here was to establish the extent of lacunar pressurization and convective flow in a range of native and introduced emergent macrophytes found in southeastern Australia.We also addressed the role of the structure of the gas transport pathway in restricting flow and the response of these processes to diel cycles of light, temperature, and humidity.

Pressurization mechanisms
Internal pressurization in wetland plants can occur as a result of two physical processes: thermal transpiration and humidityinduced pressurization.Both require a porous partition within the plant tissue, ideally with pore sizes in the Knudsen regime, i.e. less than the mean free diffusive path length of the gas molecules (< 0.1 pm), and a consumption of energy in the form of heat.In addition, humidity-induced pressurization requires a constant supply of water inside the plant.
Thermal transpiration is the movement of gases through a porous partition when there is a gradient in temperature across the partition.Thermal transpiration leads to a pressure gradient across the partition, the pressure being higher on the warmer side.In plants, the pressure difference induced by thermal transpiration, AP, (Pa), is related to the absolute temperature of the ambient air, T, (K), and the internal temperature, Ti (Kh as APt = Pa (Ti"e5 Tamoa5 -1) Pa being the ambient pressure (Pa). (1) Humidity-induced pressurization is related to pressure differentials induced by differences in water-vapor pressure across a porous partition.The result of humidityinduced pressurization is that the total pressure will be higher on the more humid side.Under steady state conditions, the pressure differential, AP, (Pa), that can be induced in plants by humidity-induced pressurization is equal to the difference in water-vapor pressure between the two sides of the partition: AP* = Pwi -P*a (2) Pwi and P, being the water-vapor pressure (Pa) inside and outside the plant.
Under most circumstances thermal transpiration and humidity-induced pressurization operate simultaneously and independently.
The potential static pressure differential, APpot (Pa), will therefore be the sum of the two gradients: fqmt = AP, + AP,.
(3) The physical basis and theory of the two processes are described in more detail by Dacey (198 1) and Schriider et al. (1986).

Materials and methods
Plant material-The following fourteen species were tested for lacunar pressurization and convective flow: Baumea articulata (R.Br.) S. T. Blake, Bolboschoenus medianus (V.Cook) Sojak, Cyperus eragrostis Lamk., Cyperus involucratus Rottb., Eleocharis sphacelata R.Br.Experimental -The buildup of static pressure differentials in the lacunae and the accompanying airflow from cut culms or rhizomes were recorded.The prevailing light, temperature, and humidity, as well as the temperature of the gases inside the culms or leaves, were measured.Diel variations in internal pressurization and convective flow in relation to variations in light, temperature, and humidity were evaluated for T. domingensis and J. ingens.
Convective flow and static pressure d$erentials -Convective flow rates and static pressure differentials, AP.,, were measured with a high-sensitivity gas flowmeter (TopTrak, model 822-1, capacity O-l 0 cm" min-l) and an electronic pressure transducer (P-Sensor, type PU-25, capacity 2.5 kPa).The equipment was connected to the internal gas spaces of the plants by 1.6-mm-i.d.PVC tubing.The tube was connected in one of the following ways: to the cut end of a rhizome bearing one to three culms or stems depending on species, to the internal gas spaces of culms or stems through hypodermic needles, or directly to the basal end of cut culms or stems.Care was taken to ensure that the joints were airtight.The other end of the tube was connected to the pressure transducer and gas flowmeter via a timercontrolled, three-way magnetic valve.The valve was set to switch at 3-8-min intervals depending on the ability of the tested plant to pressurize.The time needed to reach a steady pressure differential between the internal gas spaces of the plants and the atmosphere varied from a few seconds to -1 min (Fig. 1).Plants had their root systems submerged in water during measurement to prevent desiccation.
Temperature, water-vapor pressure, and light measurements-Internal temperature, i.e. the temperature of gases inside the culms or leaves, was measured with a needleshaped thermistor (YSI, model 524, Yellow Springs Instr.Co.), and the surface temperature of the leaf or culm (T/) was measured with a thermistor placed on the outside of the leaf (YSI, model 42 1).Air dry-bulb temperature (T,) was measured with a monolithic temperature sensor (National Semiconductor LM-34), and wet-bulb depression was measured with an aspirated psychrometer with a differential type-T thermocouple.All thermistors were calibrated to give similar readings (+O.O5"C) in the temperature interval 5-4O"C.Photon irradiance (PAR) was measured with a quantum sensor (LI-190s) and a quantum/radiometer/photometer (model LI-185B, Lambda Instr.Corp.).
Data collection and treatment -Signals from the gas flowmeter, pressure transducer, timer, temperature probes, and photometer were collected each second by a datalogger (Datataker 500, Data Electronics).Means of lo-30 readings were stored and used for further data treatment.The pressure differentials that could be induced by thermal transpiration and humidity-induced pressurization were calculated with Eq. 1 and 2 with P, set nominally to 101.325 kPa.The water-vapor pressure within the lacunae of the plants was calculated from Tj, assuming saturated conditions, with the Goff-Gratch formulation as published in the Smithsonian Meteorological Tables (1963).The water-vapor pressure of the ambient air (PWa) was calculated from dry-bulb temperature and wet-bulb depression (Smithson.Meteorol.Tables 1963).Internal resistance to gas flow and porosities-To evaluate the effect of the structure of gas transport pathways on convective flow, we measured the distribution of resistance to flow in culms, in junctions between culm and rhizome, and in rhizomes.A length of culm or leaf (usually 100 mm) was connected at one end, in parallel with the pressure transducer, to an air pump and at the other end to the gas flowmeter.The pressure supplied by the air pump (50-2,000 Pa) was regulated to give a flow rate of 2-10 cm3 min-'.For rhizomes, lengths of 20-l 00 mm were used.The junctions between culm and rhizome were the basal part of culms including meristematic tissue and a few millimeters of rhizome (total length always 20 mm).The plant tissue was submerged in water during measurements to ensure there were no surface gas leaks and that joints were airtight.The internal resistance to airflow, R (Pa s mm-2) was calculated from the expression R = PA(FL)-' (4) P being the applied pressure (Pa), F the flow rate (mm3 s-l), A the mean cross-sectional area of the tissue (mm2), and L the length of the tissue (mm).The unit used here for resistance (Pa s mmm2) differs from those (Pa s rnrnp3 or Pa s mme4) usually used to measure resistance (e.g.see Armstrong et al. 1988).This unit is used so we can directly compare the relative tissue resistances for different species with different cross-sectional areas.It is not possible-and not the intention-from the data presented to calculate the effective radius of the gas-space channels with the Poiseuille equation.The resistances given can be used to calculate the resistance (Pa s mm-3) for any length and diameter of organ provided that the porosity of the tissue and the effective radius of the individual gas-space channels are constant.
The porosity (% internal gas volume) was estimated by weighing fresh plant tissue and calculating the volume of tissue from the length and mean cross-sectional area.Crosssectional areas were calculated from the mean diameter of cylindrical culms or by weighing the projected area of a photocopy of a transverse section of linear leaves and noncylindrical culms.The volume of some rhizomes was estimated by submerging the tissue in a measuring cylinder filled with water and recording the volume displaced.
In calculating porosity we assumed the specific weight of the tissue to be equal to one.
Histology -Culm and rhizome materials were collected from the field and plants propagated in the glasshouse.The tissue was cut into blocks < 5 mm long, washed in 0.1 M phosphate buffer (pH 7.6), and then infiltrated three times under vacuum with phosphate-buffered glutaraldehyde (5%).The tissues were held under vacuum for 12 h during the final infiltration treatment.The rinsed tissue blocks were dehydrated by immersion in vials containing 20% acetone, which were placed over CaCl, and 100% acetone in a desiccator.After 24 h the material was critical-point-dried with CO2 and coated with gold/palladium.Specimens were examined in a scanning electron microscope (Jeol JSM-25s) at an accelerating voltage of 25 kV.

Results
Convectivejlow and static pressure d@erentials, AP,-The ability of different species to pressurize and produce convective gas flow was studied outdoors under the pre- vailing weather conditions.Although conditions were relatively stable during the study period (mostly sunny and dry), some variation between experimental runs was unavoidable (Table 1).Internal temperature was generally up to 5°C higher than ambient, although for some species it was lower than ambient.The lower internal temperatures might be an effect of internal evaporative cooling.Variations in the position of the thermistor within the plant tissue in relation to incident radiation might also have influenced this observation.
Of the 14 species tested, six produced a static pressure differential (hp,) of > 500 Pa and two a pressure differential between 200 and 500 Pa (Table 2).Two species, A. donax and B. medianus, did not pressurize.To compensate for the variation in weather conditions between the experimental runs, we calculated the efficiency of pressurization (APS as a % of APpot).For eight species the efficiency was between 8 and 3 1%; in the remainder it was ~2%.The efficiency did vary for individual plants depending on time of day as well as for different specimens of the same species.The efficiency of pressurization, however, clearly discriminates between species that pressurize and those that do not.
Myriophyllum papillosum and L. peploides grow in shallow water with creeping, floating, and emergent stems.Both species produced a slightly elevated internal pressure.Their stems and leaves were very leaky, mainly because of damage by grazing insects.It is not clear whether the slightly elevated internal gas pressure for these species was generated by humidity-induced pressurization and thermal transpiration or by release of photosynthetically produced oxygen directly into the lacunae.
Internal pressurization produced a convective flow of air out of cut rhizomes or culms that varied from 0.2 to 5.3 cm3 min-' culm?
The rates for T. domingensis, T. orientalis, and P. australis were often initially > 10 cm3 min-l (i.e.flows exceeded  t Small specimens had to be.selected or leaves had to be removed to get flow rates within the measuring range (< 10 cm3 min I).
the capacity of the flowmeter).Some leaves static pressure differential of 20-40 Pa and of the Typha species had to be removed a convective flow rate of 0.1-0.2cm3 min-l (leaving three to five intact leaves behind), and a small shoot of Phragmites had to be was recorded at night (Fig. 2~).After sunselected, to reduce the flow rate to within rise, the ambient and internal plant temthe measurement range of the gas flowme-peratures increased, accompanied by an increased static pressure differential and ter.Because C. involucratus has no true rhizome and J. ingens had a high resistance to convective flow rate, reaching maxima in flow in the rhizome, measurements for these the afternoon.As expected there was a linspecies were taken from cut culms only (C.ear relationship between AP, and flow rate involucratus) or through hypodermic nee-(Fig.4a).The pressurization and convective dles inserted directly into the culms (J.in-flow persisted in the late afternoon and early gens).It should be emphasized that the con-evening, even after sunset and after the temvective flow rates listed here are "potential perature differential between the plant tisconvective flow rates," i.e. flow rates from sue and the ambient air had disappeared.
cut plant tissues rather than the convective Plots of AP, and convective flow rate against flow through the closed lacunar system of AP,,, showed higher values in the afternoon an intact plant.compared with the morning, i.e. higher efficiencies for the same AP,,, (Fig. 4b,c).
Diel variation in pressurization and convective flow-The diel variation in internal pressurization and convective flow was evaluated for T. domingensis and J. ingens (Figs. 2 and 3).For T. domingensis a low The data for J. ingens (Figs. 3 and 5) showed a similar pattern, although the weather was more variable during the experimental run.Variations in solar radiation were reflected in the temperature dif-Brix et al. ferentials between the plant tissue and the ambient air, and also in AP,,, and AP,.Thermal transpiration (AP*) was generally low because of the partly cloudy conditions.Pressurization and convective flow were very low or nonexistent at night, and there was a delay in the morning after AP,,, increased before it produced an internal pressurization and convective flow.The plots of APs and flow rate against AP,,, show a similar trend to T. domingensis with higher values for the same AP,,, in the afternoon compared with the morning (Fig. 5b,c).This hysteresis effect might indicate that some other mechanism besides AP,,,, influences internal pressurization and convective flow, although the fact that leaf temperature was measured only at one point and used as an average for the whole leaf could have influenced the calculated magnitude of AP,,,.
Resistance to convectiveflow-The rate of convective flow produced by internal pressurization depends on the resistance of the tissues to gas flow.Resistances in aerial culms and leaves varied from 0.04 Pa s mme2 for P. australis to 6.2 for S. validus (Table 3).The vertical distribution of resistance in the culms is shown in Fig. 6.In most species resistances were highest at the apex and near the basal intercalary meristems.In C. involucratus (Fig. 6b) the resistance increased progressively from the top of the culm toward the base.In J. ingens an additional intercalary me&tern that produces the infloresence at 75 cm had a high resistance to flow (Fig. 6h).This single additional meristem was present in most mature J. ingens culms.
These differences in resistances to convection are evident in the tissue structure.
For example, the pith cavity of S. validus is obstructed by a complex network of diaphragms, partitions, and stretched stellate parenchyma cells (Fig. 7A), while that of C. involucratus has a low porosity and only small aerenchyma canals (Fig. 7B).In J. ingens the pith cavity is irregularly occluded by stellate parenchyma, which is dense near the meristems (Fig. 7C) but relatively sparse elsewhere.In both B. articulata and E. sphacelata the pith cavity is interrupted only by transverse diaphragm plates.The gas spaces in B. articulata diaphragms are long and tortuous (Fig. 7D), whereas those of E. sphacelata are simple pores (Fig. 7E).
The air must pass through the junction between the culm and the rhizome to reach the root system.Most of the species have a basal intercalary meristem, which normally is very compact.The resistance of the junction was very high for Il.articulata and C. involucratus, but very low for P. australis, which has no meristematic tissue occluding its lacunar spaces, and also relatively low for E. sphacelata (Table 3).Resistances in the horizontal rhizomes varied from 0.7 Pa s mm-2 for P. australis to 67 for J. ingens (Table 3).The resistance seems to be related to the porosity and structure of the rhizomes (Fig. 7F), which varies considerably between species.No measurements were taken for C. involucratus, which has a dense, compact rootstock rather than a true rhizome.
To illustrate the effect of the internal resistance on convective flow rate for different species, we calculated flow rates through a portion of culm and through the junction between culm and rhizome for different applied pressure differentials (Fig. 8a).P. australis imposes the least resistance to flow and therefore the highest flow rate, followed  The effect of rhizome length on flow rate is illustrated in Fig. 8b.For species with relatively low resistance to flow within the rhizomes (P.australis, the two Typha species, and E. sphacelata), the flow will be significant even if it has to pass through a long length of rhizome.For S. validus and J. ingens, which have high resistance to flow in the rhizome, the flow rate will decrease significantly if the air has to pass through long rhizomes.This evaluation is made assuming similar cross-sectional areas of culms, junctions, and rhizomes.However, rhizomes typically have cross-sectional areas several times that of the aerial culm; the contribution to resistance by the rhizomes will be less than indicated and therefore the flow higher for the same rhizome length.

Discussion
Our results document for the first time that internal pressurization and convective through-flow of air are common mechanisms of internal gas transport for many wetland species with cylindrical culms and linear leaves.It has already been shown that aeration of the rhizomes of water lilies (Dacey 198 1; Grosse et al. 199 l), the lotus Nelumbo (Dacey 1987;Mevi-Schlitz and Grosse 1988a,b) and the common reed, P. australis (Armstrong and Armstrong 199 1) is enhanced by a similar convective throughflow mechanism.The occurrence of internal pressurization in so many distantly related wetland species suggests it is a common adaptation of these plants to anoxic sediment conditions in wetlands.
The ability to pressurize varies in re- sponse to the gradients in temperature and water-vapor pressure between the internal gas spaces and the ambient atmosphere.In most situations, the pressure differential induced by humidity-induced pressurization is much greater than that induced by thermal transpiration (data not shown).It is not certain which of the two processes is the more important.Measurements with dried leaves of Nelumbo nuc$+ra and with Nymphaea alba in a wind tunnel led Mevi-Schlitz and Grosse (19883) and Grosse et al. (199 1) to conclude that pressurization is mainly induced by thermal transpiration.
On the other hand, Armstrong and Armstrong (199 1) concluded that internal pressurization in P. australis is mainly humidity induced, but also influenced by other plantmediated factors such as stomata1 aperture and photosynthesis.The diel variations in pressurization and convective flow we observed for T. domingensis and J. ingens suggest that humidity-induced pressurization is the dominant driving mechanism in these species.The hysteresis effect observed when plotting AP, and flow rate against APpo, indicates that some other mechanism besides AP,,, influences internal pressurization and convective flow.Some plant-mediated factors, such as stomata1 aperture and photosynthesis, may be responsible for this observation.For E. sphacelata, light per se influences pressurization and convective flow (Brix et al. unpubl. results).
We did not investigate in detail the ability of different plant parts to pressurize.Trials with detached culms and leaves suggested that all live culms of B. articulata, E. sphacelata, S. validus, and J. ingens, and all leaves of T. domingensis and T. orientalis, were able to pressurize as long as they were intact.
Removal of the apical leaves of C. involucratus and subsequent sealing of the cut stem showed that pressurization in this species is initiated in the stem.The lacunae within the stem of C. involucratus are connected to the aerenchyma of the apical leaves, but the resistance to airflow at stem-leaf junc- Internal pressurization in most cases results in a convective through-flow from one plant part to another.In water lilies air enters the youngest leaves, passes down the petioles to the rhizome, and up the petioles of older emergent leaves to the atmosphere (Dacey 198 1).In P. australis (Armstrong and Armstrong 199 1) and in E. sphacelata (Brix et al. unpubl. results), air enters green entire shoots, passes down the pith cavity to the rhizomes, and is vented back to the atmosphere through dead or damaged shoots and stubbles.In contrast to these species, where influx and efflux occur in different leaves or shoots, influx and efflux apparently occur in the same leaf of N. nuczjka (Dacey 1987; Mevi-Schlitz and Grosse 1988a, b).The lacunar system of individual Nelumbo petioles is differentiated into several small canals and two to four large canals.The direction of flow in the lacunar canals is downward in the small canals (influx) and upward in the large canals (efflux).
The plant species we studied did not show any morphological differentiation within the tissues that could suggest the existence of a similar bidirectional gas-transport system.The significance of convective flow for underground organs is evident from the consequent increase in oxygen concentrations within rhizomes (Brix 1988; Armstrong and Armstrong 199 1).Root aeration is enhanced by increased oxygen partial pressure at the rhizome-root junction and the consequently higher diffusive flux into the gas spaces of the root.Oxygen leakage from roots and the ability of plants to main-tain an oxidized rhizosphere are enhanced by convective through-flow (Armstrong and Armstrong 1990).The release of potentially asphyxiating gases produced in the sediment (COZ, CH4) is also enhanced by convective gas flow in emergent plants (Dacey and Klug 1979;Sebacher et al. 1985).
The natural distribution of these species has not been studied in detail in Australian wetlands, making it difficult to relate our results to patterns of species zonation.Nevertheless, the four species that produced significant convective flow through their rhizomes (P.australis, E. sphacelata, and the two Typha species) grow to static water depths of -2 m (Sainty and Jacobs 198 1).In contrast, species that pressurize their culms, but have a high resistance to convective flow (S. validus, J. ingens, B. articu-Zata), are usually more marginal and re- stricted to water < 1 m deep (Sainty and Jacobs 198 1).C. involucratus, C. eragrostis, B. medianus, and Canna sp.do not produce a significant convective through-flow and grow only in very shallow water or on wet soil. A. donax is not a true wetland plant and did not pressurize at all.These observations suggest that species with a high potential for internal pressurization and a low resistance to convective flow may have a competitive advantage over species relying exclusively on diffisive gas transport, allowing them to ventilate their underground tissues and grow in deeper water.
Fig. 1.Plot of data collected during an experimental run with Typha domingensis.Static pressure differentials and convective flow rates were measured alternatively at intervals of 6 min.(PAR-photon irradiance; RHrelatively humidity, calculated from wet-bulb depression; T,-ambient air temperature; T,-internal temperature; T,-leaf surface temperature; T,,,-aspirated wet-bulb temperature.)
Fig. 3.As Fig. 2, but for Juncus ingens.The measurements were taken from three culms in parallel.Thus the convective flow rates represent the flow from three culms.

Fig. 4 .
Fig. 4. Typha domingensis.Relations between (a) convective flow rates of air and static internal pressure differentials; (b) internal static pressure differentials and total pressurization potentials; and (c) convective flow rates and total pressurization potentials during a diel cycle.Arrows indicate the direction of the changes during the diel cycle, starting at midnight at close to zero pressurization and flow.

Table 3 .
Mean resistance to internal airflow and porosity of aerial culms or leaves, rhizomes, and junctions between aerial culms and rhizomes (20-mm length).SD in parentheses; n = 10 for culms, n = 3-5 for rhizomes and junctions.(Not analyzed--a.)compact; not analyzed.t Some junctions had very high resistance (> 50,000 Pa s mm-I); not included in the calculation of the mean.by E. sphacelata, the two Typha species, S. validus, and J. ingens.B. articulata and C. involucratus have very high resistances to flow, especially at the culm-rhizome junction, and therefore allow only very low convective flows.
Fig. 6.Distribution of resistance to convective airflow in the aerial culms or leaves of eight species of emergent macrophytes.Note different scale of the axes.
Fig. 8. [a.] Calculated rates of convective airflow through culms or leaves (length, 300 mm; cross-sectional area, 100 mm2) and culm-rhizome junctions (length, 20 mm; cross-sectional area, 100 mm2) of eight species of emergent macrophytes for different pressure differentials.[b.] Influence of rhizome length (cross-sectional area, 100 mm2) on convective airflow through culm and iunction (dimensions as in panel a) and rhizome applying a pressure differential of 100 Pa.Note log-scale.-

Table 1 .
List of plant species tested, their size (height of aboveground shoots and leaf or culm surface area), mean photon irradiance (PAR), ambient air temperature (T,), internal temperature (Ti), and ambient relative humidity (RH) during the experiments.(Number of pressure-flow measurement cycles-n; SD in parentheses.) Juncaceae Juncus ingens Cannaceae Canna sp.* Surface area of culm or leaf-sheaths only.

Table 2 .
Calculated potential pressure differentials (AP,,) and actual measured static pressure differentials (AP,) and convective flow rates.The effectivities of pressurization, i.e.AP, as a fraction (%) of AP,,, are shown.(Number of pressure-flow measurement cycles-n; SD in parentheses.) CannaceaeCanna sp.