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 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).
Figure 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 μm; (f, i) 5 cm. Arrows show position of radial connections in (m) and (q).
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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.