New Phytologist Research Reasons for the presence or absence of convective (pressurized) ventilation in the genus Equisetum Jean Armstrong and William Armstrong Department of Biological Sciences, University of Hull, Kingston upon Hull, HU6 7RX, UK and School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia Summary Author for correspondence: Jean Armstrong Tel: +44 1964 550135 Email: j.armstrong@hull.ac.uk Received: 19 August 2010 Accepted: 24 September 2010 New Phytologist (2011) 190: 387–397 doi: 10.1111/j.1469-8137.2010.03539.x Key words: aerenchyma, anatomy, Equisetum spp., gas flow resistance, humidity-induced convection, intercalary meristem, oxygen, wetland. • The very high rates of convective ventilation reported recently in Equisetum telmateia (up to 120 cm3 min)1; internal wind speed, 10 cm s)1) prompted this study of a further eight species for the presence or absence of convection and the possible reasons for this. • Convection rates were examined in relation to anatomical pathways, internal resistance to applied pressurized gas flow and stomata. • Only species with interconnecting cortical aerenchyma in branches (when present), shoots and rhizomes induced convection. Rapid humidity-induced convection (HIC) occurred in E. palustre (up to 13 cm3 min)1), with slower rates in E. · schaffneri and E. ramosissimum (£ 6 and 3 cm3 min)1, respectively). Excised shoots of E. hyemale and E. fluviatile showed the potential for HIC (£ 0.5 and 0.15 cm3 min)1, respectively), but not into the rhizomes. High rates were linked to low internal gas flow resistance. No convection was detected in E. scirpoides, E. sylvaticum or E. arvense due to the extremely high resistance to pressure flow, for example, from intercalary meristems and, in the last two, to nonaerenchymatous branches. • Of the nine Equisetum species studied so far, four showed through-flow convection; the other species must rely solely on diffusion for underground aeration in wet soils. Introduction For some time, convective (pressurized) ventilation through the internal gas space system has been known to occur in certain aquatic macrophytes: for example, in water-lilies (Dacey, 1980; Grosse, 1996), the common reed, Phragmites australis (Armstrong & Armstrong, 1990; Armstrong et al., 1992; Brix et al., 1992), Eleocharis sphacelata (Sorrell & Boon, 1994) and Typha spp. (Bendix et al., 1994). More recently, in a nonflowering species, the great horsetail, Equisetum telmateia, very high rates of convection have been found; these possibly also occurred in extinct tree horsetails, the Calamites, of the Carboniferous (Armstrong & Armstrong, 2009). Convections, or ‘internal winds’, generated across stomata (or other micropores) by humidity-induced diffusion (HID) and sometimes also by thermal transpiration, which represent types of molecular pump (Armstrong & Armstrong, 1994; Armstrong et al., 1996a,b), pass through shoot and  2010 The Authors New Phytologist  2010 New Phytologist Trust rhizome aerenchyma and vent via shoot parts of small gas flow resistance, for example, senesced shoots or stubble. Under warm conditions and low relative humidity (RH), rates of convection for leaves of the giant water-lily Victoria amazonica (Grosse et al., 1991) and for single shoots of P. australis (J. Armstrong et al., 1996) and E. telmateia (Armstrong & Armstrong, 2009) can be as high as 83, 17 and 120 cm3 min)1, respectively. In E. telmateia, the ‘internal wind’ through the aerenchyma may reach 10 cm s)1. These convection rates are those for excised shoots; depending on resistances in the gas flow pathway, they are lowered by very variable degrees when connected to the rhizome; for example, in E. telmateia, rates can be reduced by 60%. Many emergent macrophytes have roots and rhizomes growing in waterlogged anoxic soils that are nitrate deficient and rich in phytotoxins, such as iron (Fe2+), manganese (Mn2+), sulphides and organic acids. Adequate supplies of atmospheric oxygen to the underground parts and to the soil via the roots are essential for the supply of respiratory O2 to New Phytologist (2011) 190: 387–397 387 www.newphytologist.com New Phytologist 388 Research the roots and rhizomes and for the oxidation of phytotoxins in rhizosphere regions around roots (Armstrong & Drew, 2002; Colmer, 2003). The advantage to emergent aquatic macrophytes of convective over the more usual diffusive ventilation is that convective flows can, over long distances, supply oxygen for rhizome respiration (Armstrong & Armstrong, 1990; Armstrong et al., 1992) and vent waste gases, such as CO2 (Brix et al., 1996), more effectively than diffusion. In addition, by conveying ‘fresh air’ to root–rhizome junctions, convection greatly increases O2 diffusion to roots and radial O2 loss to the rhizosphere (Armstrong & Armstrong, 1990; Armstrong et al., 1992, 1996), where it can oxidize phytotoxins (Begg et al., 1994; Pedersen et al., 2004) and induce nitrification (Kirk & Kronzucker, 2005). Other important advantages of convective aeration are that greater numbers of roots can be supported and the penetration depth for rhizome and root growth can be greatly increased (Vretare-Strand, 2002; Sorrell & Hawes, 2010); convection also increases oxygen supplies to rhizome tips and surrounding phyllospheres (Armstrong et al., 2006). Moreover, the purging properties of convective flows through rhizomes should enhance the removal of ethylene from roots and help to avoid its accumulation to growthinhibiting levels (Visser et al., 1997). All these effects should enhance the competitive advantage of such plants (Vretare-Strand, 2002; Sorrell & Hawes, 2010). It has been reported that the rhizomes of E. telmateia can penetrate to 4 m depth in wet soil (Page, 1997). The Equisetales is an ancient group of nonflowering vascular plants probably dating back 250 million years to the Upper Carboniferous era (Bell & Hemsley, 2000) but, apart from Barber’s (1961) report on E. fluviatile, the aeration system of these plants has been given little attention in the literature. This article explores the pathways of aeration in eight extant species of Equisetum, the possibility that convective flows take place and the reasons for the occurrence or absence of convections. The investigation was conducted in relation to the mechanism of convection, anatomical pathways, stomata and internal resistance to applied pressurized gas flow. Materials and Methods Plant material The sources of Equisetum were as follows: E. · schaffneri Milde, one of the ‘giant horsetails’ (Royal Botanic Gardens, Kew, UK); E. sylvaticum L. (near Finchdale Priory, Co. Durham, UK); E. fluviatile L. (Weedley Springs, South Cave, East Yorkshire, UK); E. ramosissimum Desf. (University of Utrecht Botanic Gardens, the Netherlands and Mazzolla, Tuscany, Italy); E. hyemale L., E. scirpoides Michaux, E. palustre L. and E. arvense L. (Botanic Gardens, University of Hull, UK); and E. telmateia Ehrh. used for New Phytologist (2011) 190: 387–397 www.newphytologist.com comparisons (Warter Farms Estate, Pocklington, East Yorkshire, UK). Plants were collected between May and September 2009 and 2010; freshly excised shoots were used for field experiments; for laboratory experiments and anatomy, plants were transported within Perspex tubes with the bases in water and used within a few hours. Convection rates Rates of convective free flow from whole excised shoots were measured by connecting the basal end by rubber tubing sealed on with silicon rubber to either a digital or soap film flow meter. Convection into the underground rhizome was detected by inserting a flow meter in series between the shoot base and its stubble. Appropriate corrections to flow measurements were made to negate the effects of flow meter resistance. Static pressure (DPs) the maximum pressure developed with the plant connected to the pressure transducer only and the convective flow blocked, was measured by means of a digital pressure transducer (Radio Spares, Corby, Northamptonshire, UK); it is the pressure at which Poisseuille outflow to the atmosphere through stomata is balanced by the inflow of gases and the effect of internal humidification within the plant. RH and temperature were measured using a combined probe (Vaisala, Malmo, Sweden), the photosynthetic photon flux rate using a portable light meter (Li-Cor, Lincoln, NE, USA) and stem diameters by means of digital callipers. Gas space continuity: anatomy The continuity of gas spaces within the plant was investigated anatomically using an Olympus BX40 photomicroscope on fresh material from transverse sections of aerial branches (where present) and stem and rhizome internodes and nodes. Gas space continuity: resistances to applied pressurized through-flow and convection Gas flow resistances and flows were measured at pressures realizable by HID: £ 1000 Pa on excised stems or parts of stems, cut at each end and with aerenchyma channels and ⁄ or pith cavity variously sealed with silicon rubber, as described in Armstrong & Armstrong (2009). In some cases, stepwise excision, by successive node and internode, was used to obtain details on the distribution and magnitude of pressure flow resistance. Pressure flow resistances (RP: Pa s cm)3) were calculated from the relationship, RP = F ⁄ DP, where F (flow rate) is in units of cm3 s)1. The cut ends of convecting shoots (sometimes with rhizomes) were immersed just under the water surface to ascertain the pathway of convection from the pattern of bubbling.  2010 The Authors New Phytologist  2010 New Phytologist Trust New Phytologist Research Results and Discussion To present a complete picture, some relevant previous results for E. telmateia have been included. Convection rates Significant convective flow rates were found for excised stems in E. palustre (up to 13 cm3 min)1), E. · schaffneri (£ 6 cm3 min)1) and E. ramosissimum (£ 2–3 cm3 min)1) (Table 1). These were measured at comparatively low RH (c. 40–50%), T = 20–24C and photosynthetic photon flux rate (700–1700 lmol m)2 s)1); at 30% RH and higher temperatures, values could have been doubled. For example, a rise in temperature from 24C to 32C alone almost doubles the saturated water vapour pressure (3000–4800 kPa), and hence the potential for convection, whereas a decrease in external humidity from just 40% to 30% RH can increase the internal pressurization by a further 20% (Armstrong & Armstrong, 2009). Convection rates were directly related to stem height and total branch length (e.g. for E. palustre, Fig. 1a,b); the static pressure DPs (Fig. 1c), an indicator of convective flow potential, was inversely related to RH, and correlated with previous results for E. telmateia (Armstrong & Armstrong, 2009). We conclude that in these species also the flows are induced by humidity differentials. Flows were extremely low in E. fluviatile (£ 0.15 cm3 min)1). In E. hyemale, there was the potential for flow Table 1 Examples of pressure flow resistance of gas space and convective flow rates for Equisetum spp. Pressure flow resistance of lower stem gas space Subgenera Species Subgenus Hippochaete E. scirpoides E. ramosissimum E. · schaffneri E. hyemale Subgenus Equisetum Section Subvernalia Section Nova E. sylvaticum E. fluviatile E. arvense Section Palustria E. palustre E. telmateia Internodal aerenchyma (Pa s cm)3 mm)1) Nodes (Pa s cm)3) Convection rate (cm)3 min)1) NA c. 1–2 c. 1–2 unless intercalary meristem present 10–15 2 ¥ NA ¥ 10–25k 200–500 1 15–19k 0 £ 3.0 £ 6.0 130–150k 1 £ 1M ¥ 3 0 NA c. 1–2 2 ¥ c. 1–2 <1 NA ¥ £ 0.15 0 70–300 4–50 £ 13 £ 120 0.5 12 10 8 6 4 2 0 0 10 20 30 40 50 60 Shoot length (cm) (b) 14 12 (c) Static pressure (Pa) 14 Convective flow rate (cm3 min–1) (a) Convective flow rate (cm3 min–1) NA, not available. 1 , Radial channels. 2 , Including intercalary meristem. 3 , Potential flow from individual internodes only. 10 8 6 4 2 0 0 350 300 250 200 150 100 50 200 400 600 800 Total branch length + terminal segment (cm) 0 0 20 40 60 80 100 Chamber humidity (% RH) Fig. 1 Equisetum palustre: rates of convective flow (free flow from excised shoots) in relation to shoot length (a) and total branch length (b). (c) Typical example of static pressures of isolated branched node vs external (chamber) relative humidity (RH).  2010 The Authors New Phytologist  2010 New Phytologist Trust New Phytologist (2011) 190: 387–397 www.newphytologist.com 389 New Phytologist (a) 0.6 Convective flow rate (cm3 min–1) 390 Research 0.5 As with E. telmateia, convective flows passed into the rhizomes in E. palustre, E. ramosissimum and E. · schaffneri. There was no evidence of gaseous through-flow into the rhizome system in E. hyemale or E. fluviatile, so we conclude that, although there is the potential for humidity-induced convection (HIC) generated by internodes, it does not take place in these species. 0.4 0.3 0.2 0.1 0.0 Base N IN N IN N IN N IN N IN N IN N IN N IN 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 Apex Gas space continuity from anatomy Node/internode number present at base (b) Stomata Equisetum palustre resembles E. telmateia in that the branch stomata bear, on the external surface of the subsidiary cells, close-fitting elongated pilulae (papillae) which overarch the stomatal pore (Fig. 3a,b; Table 2) (Page, 1972; Armstrong & Armstrong, 2009). In E. sylvaticum and E. arvense, these pilulae are short, sparse and not overarching; stomatal pilulae are absent in E. ramosissimum (Fig. 3d), E. · schaffneri (Fig. 3e), E. hyemale and E. fluviatile (Page, 1972; Table 2). Convective flow rate (cm3 min–1) 0.5 0.4 0.3 0.2 0.1 0.0 0 2 4 6 8 10 12 14 16 18 20 22 Individual internode surface area (cm2) Fig. 2 Equisetum hyemale: (a) convective flow rates (free flow from excised shoots) for stepwise removal, from base, of single node + internodal segments of the same shoot (N, node; IN, internode); (b) convective flow rates from two typical shoots vs surface areas of basal internodes obtained from stepwise shortening of stems (shoot 1, open circles; shoot 2, closed circles). In this species, convective flow is from the basal internode of the stem only; nodes are impermeable to gas flow. from each internode (£ 0.5 cm3 min)1), but flow between internodes was blocked by the intercalary meristems (Fig. 2a). In addition, the flow rate was proportional to the surface area of the connected internode (Fig. 2b). No convective flows were detected in E. scirpoides, E. sylvaticum or E. arvense. Branches Apart from the unbranched E. hyemale and E. scirpoides (Fig. 5f,i) and the inconsistently branched E. ramosissimum (Fig. 4c), stems of all the other species investigated possess a whorl of branches at each node, except for the basal ones (Fig. 4a,e,g,i,l,S3). Only the branches of E. palustre, E. · schaffneri and E. ramosissimum resemble those of E. telmateia in possessing aerenchyma (Fig. 4b,h,d, Table 2) directly interconnecting between the internodes and nodes; the branches of E. sylvaticum, E. fluviatile and E. arvense generally have no aerenchyma channels (Fig. 4f,j,k,m), but those of E. arvense sometimes contain aerenchyma in proximal internodes. Pith cavities are present in the branches of all species examined, except for E. sylvaticum and E. arvense. Prominent substomatal Table 2 Aeration anatomy and branch morphology in Equisetum spp. Stem endodermis Aerenchyma (vallecular canals) Species Stomatal pilulae E. E. E. E. E. E. E. E. E. ) ) ) ) Sparse short Sparse short Sparse short Close long Close long scirpoides ramosissimum · schaffneri hyemale sylvaticum fluviatile arvense palustre telmateia Branch sections (no. sides) Branch internodes 6 8 + + 4 4 2 Cruciform 5 or 6 8 ) ) 3 ) + + Stem internodes Stem nodes Stem nodal radial channels Stem internodes Stem nodes ) + + + + + lso + + + ) + + +disc. +disc. +disc. lso +disc. + + ) + + 1 ) ) 1 ) lso ) ) ) Single circum. Double Double Double Single circum. Single vb Single circum. Single circum. Single circum. Single circum. Single vb Single vb Single vb Single circum. Single vb Single circum. Single circum. Single circum. +, Present; ), absent; circum., circumferential, encompassing all the vascular bundles; disc., discontinuous because of intercalary meristem; lso, lower stem only; vb, around each vascular bundle. 1 , Radial gas spaces for diffusion present. 2 , Apart from basal internode which is hexagonal. 3 , Apart from proximal internode which may possess aerenchyma. New Phytologist (2011) 190: 387–397 www.newphytologist.com  2010 The Authors New Phytologist  2010 New Phytologist Trust New Phytologist Research E. telmateia (a) E. ramosissimum (d) E. palustre (b) E. x schaffneri (e) (c) Fig. 3 Branch stomata in species of Equisetum that show convective flow. (a, b, d) Scanning electron micrographs of outer surfaces of stomata of E. telmateia, E. palustre and E. ramosissimum, respectively. Note pilulae in (a, b). (c) Transverse section of branch of E. palustre showing substomatal cavities. (e) Outer surface of stoma of E. · schaffneri by light microscopy. Bars: (a, b, d, e) 10 lm; (c) 50 lm. (a, b, d) After Page (1972); reproduced with permission of the New Phytologist Trust. cavities are visible in all branches (e.g. Fig. 3c) and in the stems of E. scirpoides and E. hyemale; these cavities connect with aerenchyma channels, when present, via intercellular spaces. Stems Apart from E. scirpoides, the stems of all species possess a central pith cavity (Fig. 5d,g,k,o,r,t) traversed by a diaphragm at each node (Fig. 5b,h,m,p). Diaphragms may be quite porous (E. · schaffneri: Fig. S3, E. fluviatile and E. ramosissimum) or relatively lacking in gas space (E. telmateia, E. palustre and E. arvense). In all the species studied here, apart from E. scirpoides and E. fluviatile, cortical aerenchyma channels (vallecular canals) are present in all stem internodes (Fig. 5a,g,k,o,r,t, Table 2). We found them to be absent in E. scirpoides (Fig. 5j); however, Johnson (1933) reported their presence. In E. fluviatile, aerenchyma channels are present only in the lower unbranched  2010 The Authors New Phytologist  2010 New Phytologist Trust half of the stem (Fig. 5d) and absent in the upper branched part (Fig. 5c). Within the genus, aerenchyma channel discontinuity in stems can be a result of nonaerenchymatous blockages in the region of the intercalary meristem level with the top edges of the leaf sheath, as in E. fluviatile, E. · schaffneri, E. arvense and E. sylvaticum (Fig. 5e,l,s,u, Supporting Information, Fig. S1). In E. hyemale (Fig. 5h), there can also be a blockage close to the node. However, in E. · schaffneri (lower part of stem), E. palustre and E. ramosissimum (Fig. 5b,n,p, Table 2), successive internodal cortical aerenchyma channels connect directly across the nodes; similar connections have also been found previously in E. telmateia (Armstrong & Armstrong, 2009). In E. · schaffneri and E. ramosissimum stems, near the lower edge of the pith diaphragm, there are radial channels of sufficient porosity for pressure flow between cortical aerenchyma channels and the pith cavity (Fig. 5m,q, S3, Table 2); these are similar to the radial channels of Phragmites (Armstrong & Armstrong, 1988) and the infranodal canals of the extinct Calamites (Williamson, 1871; Boureau, 1964; Armstrong & Armstrong, 2009). Examples of internodal anatomy for some of these species can also be found in Brune et al. (2008). It was interesting to note the form of the endodermises in the various species. It seems certain that the Casparian bands will prevent apoplastic gas space connection, and this is apparent from microscopy as well as applied pressurized through-flow examination. E. fluviatile has an endodermis around each vascular bundle; elsewhere internodal endodermises are circumferential (around all the vascular bundles collectively), with either an internal and external endodermis, as in E. ramosissimum, E. · schaffneri and E. hyemale (Fig. S2), or single external as in E. sylvaticum, E. arvense, E. palustre and E. telmateia (Table 2). These endodermises should prevent internodal apoplastic gas connection between aerenchyma and the pith. However, at the nodes in E. ramosissimum, E. · schaffneri and E. hyemale, the two endodermises are lost and each vascular bundle is surrounded by its own endodermis, allowing apoplastic gas space connections between aerenchyma and pith cavity, which are visible microscopically; in the first two species, there are radial channels of low porosity enabling some convective flow in this direction. However, in E. arvense, E. palustre and E. telmateia, there are no radial gas connections, the nodal endodermises are circumferential and convection is via the aerenchyma channels only in the last two species. Rhizomes The rhizomes of E. arvense, E. fluviatile, E. hyemale, E. ramosissimum and E. · schaffneri resembled stems in containing a pith cavity and surrounding cortical aerenchyma channels; the rhizomes of the other species contained only aerenchyma channels. New Phytologist (2011) 190: 387–397 www.newphytologist.com 391 New Phytologist 392 Research (a) E. palustre (b) E. x schaffneri (g) (h) E. ramosissimum (i) E. fluviatile (j) (c) (d) (k) (l) (e) E. sylvaticum (f) E. arvense (m) Fig. 4 Habits and branch anatomy for species of Equisetum. (a, c, e, g, i, l) Habit; bars, 5 cm. (b, d, f, h, j, k, m) Transverse sections of branch internodes; bars, 200 lm. Gas space continuity from applied pressure flows and convection As in E. telmateia, it was possible to blow gas through stem aerenchyma channels of successive internodes and nodes in E. palustre and E. ramosissimum, and through the lower parts of the stem in E. · schaffneri; this was consistent with New Phytologist (2011) 190: 387–397 www.newphytologist.com the nodal aerenchyma connections described above. There was more resistance to flow through the aerenchyma channels of the nodes than the internodal cortex in E. telmateia, and E. palustre (Fig. 6), E x schaffneri and E. ramosissimum (Table 1) because of a narrowing of the channels at the nodes (Fig. 5a,b,k,m,o,p). Gas could only be blown through aerenchyma of excised stem internodes, but not through that of  2010 The Authors New Phytologist  2010 New Phytologist Trust New Phytologist Research E. x schaffneri E. palustre (a) (b) (k) (l) (m) (n) E. fluviatile E. ramosissimum (c) (o) (p) (q) (d) (e) (f) E. hyemale E. arvense (g) (r) (s) (h) E. scirpoides (i) (j) E. sylvaticum (t) (u) Fig. 5 Stem aeration anatomy and habits for species of Equisetum. (a, c–e, g, j–l, o, r–u) Transverse sections of stem internodes: from upper branched part of stem (c), lower unbranched part of stem (d, e), near intercalary meristem at top of leaf sheath (e, l, s, u), and mid-internode (a, d, g, j, k, o, r, t). (b, h, m, n, p, q) Transverse sections of stem nodes, that is, near diaphragms, and insertion of branches when present. (f, i) Habits. Bars: (a–e, g, h, j–u) 400 lm; (f, i) 5 cm. Arrows show position of radial connections in (m) and (q). successive internodes and nodes, in E. arvense, E. hyemale and E. fluviatile, because of virtually infinite resistance to through-flow through the intercalary meristem (e.g. for E. arvense, Fig. 7), and in E. hyemale because of a lack of  2010 The Authors New Phytologist  2010 New Phytologist Trust aerenchyma in the nodes. The pith cavities of E. fluviatile, E. ramosissimum and E. · schaffneri were permeable to gas flow, but not those of E. palustre, E. telmateia or E. arvense. As found previously for E. telmateia, aerenchyma channels of New Phytologist (2011) 190: 387–397 www.newphytologist.com 393 New Phytologist 394 Research (a) 200 (c) IN 160 4000 N 140 IN 120 3500 100 N 80 IN 60 40 N 20 IN 0 0 20 40 60 80 100 120 140 160 Distance along segment (mm) (b) 300 Accumulated pressure-flow resistance (Pa s cm–3 s–1) IN 250 200 Accumulated pressure-flow resistance (Pa s cm–3) Accumulated pressure-flow resistance (Pa s cm–3 s–1) 180 3000 2500 2000 1500 1000 500 150 N 0 100 0 50 0 IN 0 10 2 Base 20 30 40 50 60 4 6 Node 8 10 Apex 70 Distance from basal end (mm) Fig. 6 Typical examples of accumulated resistances to applied gaseous pressure flow in stems ⁄ stem segments as detected from: (a) sequential removal of single nodes (N) and intermodal (IN) material in Equisetum telmateia stem; (b) sequential shortening of a single node with part of internode on each side for E. palustre; (c) sequential removal of individual node + internodal segments in E. palustre (compare with Fig. 7). branches, stems and rhizomes were interconnected in E. palustre, E. · schaffneri and E. ramosissimum, and pith cavities of stems and rhizomes in E. · schaffneri and E. ramosissimum. When the cut ends of excised shoots were just immersed in water, bubbles were observed from the aerenchyma channels only in E. palustre and E. telmateia, and from both aerenchyma and pith cavity in E. · schaffneri and E. ramosissimum, giving some indication of the convective flow pathways. The same applied when parts of the rhizomes were attached. Final comments The type of convection found so far among the Equiseta appears to be induced by humidity differentials (Fig. 1c; Armstrong & Armstrong, 2009). This relies on HID, a kind of ‘molecular-pump’ involving the inward diffusion of O2 New Phytologist (2011) 190: 387–397 www.newphytologist.com and N2 through narrow (< 1 lm) stomatal pores highly resistant to pressurized Poiseuille backflow. Details on the mechanism are given in Armstrong & Armstrong (1994) and Armstrong et al. (1996a,b). In summary, diffusion gradients are induced by humidification within the plant, which reduces O2 and N2 concentrations to below atmospheric levels. The inwardly diffusing gases, together with water vapour continually produced from substomatal cells, cause a pressurization which drives convective gas flows (‘internal winds’) along the path of least resistance through stem and rhizome with venting through older, more porous parts of the plant (Dacey, 1980; Armstrong et al., 1996). HIC is powered by the latent heat of evaporation, to maintain the humidity of the gas spaces, and is most rapid under warm, dry (low RH), sunny conditions (Armstrong et al., 1996; Brix et al., 1996; Steinberg, 1996). Under conditions of 100% RH and at night, rates of HIC are normally very low or zero, so that the plants must then rely on diffusive  2010 The Authors New Phytologist  2010 New Phytologist Trust New Phytologist Research 100 >106 Pa s cm–3 at 58 mm Segment pressure-flow resistance (Pa s cm–3) 90 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 Distance along segment (mm) Fig. 7 Typical example of accumulated resistances to applied gaseous pressure flow by sequential removal of individual node + internodal segments in Equisetum arvense stem (compare with Fig. 6b). ventilation. Living shoots are necessary for HIC; for species which die back, this only occurs during the growing season. The pathways of convection vary between species and are summarized in Fig. 8. In E. telmateia, E. palustre, E. · schaffneri and E. ramosissimum, features that appear to correlate with the ability to induce significant convective flows are the continuity of the aerenchymatous gas space between shoot and rhizome (Figs 3, 4) and the internal resistance to gas flow, which can be overcome by pressures generated by HIC (Fig. 6; Tables 1,2). Rapid convection in E. telmateia and E. palustre is linked to the low resistances to gas flow, numerous aerenchymatous branches which provide large stomatal areas and, possibly, rows of closely interlocking stomatal pilulae (Fig. 3a,b) close to the optimal pore width (0.2–0.3 lm), helping to curtail backflow. In E. · schaffneri and E. ramosissimum, where convection rates are slower (£ 6 and 3 cm3 min)1, respectively), branches are sparser (Fig. 4c,g) and stomatal pilulae are absent (Fig. 3d, e). However, relative to the sizes of the plants, convection rates are greater in the latter. Potential flow rates in E. hyemale and E. fluviatile are very slow and of little significance because they cannot be transmitted to the rhizomes. Blockages in the stem cortical aerenchyma are not bypassed in E. hyemale and E. fluviatile, whereas they are bypassed to some extent via radial channels into the pith cavity in E. · schaffneri and E. ramosissimum. As mentioned in the Introduction, convective ventilation has been considered to be advantageous for a number of wetland plants and, apart from E. arvense, the species studied grow in damp to wet habitats (Clapham et al., 1952; Husby, 2003). Vretare-Strand (2002) concluded that species with pressurized ventilation have a competitive advantage in deep water, resulting in long-term competitive exclusion of species lacking pressurized ventilation, and that this will affect species’ zonation patterns along water depth gradients, with a more pronounced effect in substrates with low redox potentials. Similarly, Sorrell & Hawes (2010) concluded that the development of convective flow appears to be essential for the dominance of helophyte species in > 0.5 m depth, especially under eutrophic conditions, although exposure, sediment characteristics and light attenuation frequently constrain them to a shallower depth than their flow capacity permits. We suggest that, also in Equisetum, HIC may help roots and rhizomes to grow to greater depths and extent, and contribute to an increased competitive advantage. Page (1997) reported that rhizomes of E. telmateia can penetrate to 4 m depth in wet soil, but E. fluviatile is the only Equisetum species that we examined which commonly grows in 30–60 cm depth of standing water, whereas E. hyemale can also grow partly submerged; surprisingly, these species have no effective convection. However, they appear to be very well adapted for diffusive ventilation E. telmateia and E. palustre Branch stomata Substomatal cavities Branch aerenchyma E. ramosissimum and E. x schaffneri Branch Branch Substomata aerenchyma stomatal cavities Stem nodal aerenchyma Stem nodal aerenchyma Via radial channels † Stem internodal aerenchyma Rhizome aerenchyma Stem internodal aerenchyma Rhizome aerenchyma Stem pith cavity Venting via stubble or damaged shoots † † Rhizome pith cavity Venting via stubble or damaged shoots Fig. 8 Pathways of convective flow in some species of Equisetum. †Convective flow will enhance O2 diffusion into (i) roots and radial oxygen loss to the rhizospheres and (ii) into rhizome apices and ROL to the phyllospheres.  2010 The Authors New Phytologist  2010 New Phytologist Trust New Phytologist (2011) 190: 387–397 www.newphytologist.com 395 396 Research (Barber, 1961; Hyvonen et al., 1998): they possess thinwalled stems (Fig. 5d,g), with probably low overall respiration rates, and a wide pith cavity in stem and rhizome with relatively porous diaphragms. Equisetum sylvaticum, E. scirpoides and E. arvense lack continuity of aerenchymatous connections between branches (where present), stem and rhizome (Figs 4, 5, S1); the last two species show infinite resistance to applied pressurized gas flow through the stem (Table 1) and it is therefore not surprising that these three species are nonconvecting. 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Williamson WC. 1871. On the organisation of the fossil plants the coal-measures. – Part I. Calamites. Philosophical Transactions of the Royal Society, London. Series B, Biological Sciences 161: 477–509. Supporting Information Additional supporting information may be found in the online version of this article. Research below the intercalary meristem and their lost continuity across the meristem. Fig. S2 Equisetum hyemale: transverse sections of internode showing an internal and external endodermis with Casparian bands. Fig. S3 Equisetum · schaffneri: slides of habit and higher definition slides of stem anatomy showing gas transport features. Please note: Wiley-Blackwell are not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing material) should be directed to the New Phytologist Central Office. Fig. S1 Equisetum arvense: transverse sections of stem internode showing aerenchyma (vallecular canals) above and  2010 The Authors New Phytologist  2010 New Phytologist Trust New Phytologist (2011) 190: 387–397 www.newphytologist.com 397