Fig. 1. Flattened-cylindrical roots of Zeylanidium. A, C, E, G and H, Z. subulatum. B, D and F, Z. lichenoides. A and B, Surface views of alternately branched roots. Note that shoots always occur between main and lateral roots. There are dark bands of root hairs along vascular strands. C and D, Transverse sections of relatively young portions of roots. There is a rudimentary vascular bundle at the centre of the root. E and F, Longitudinal sections of apical portions of roots, showing capless apical meristem composed of surface and inner cells. G and H, SEM micrographs of branched root tips at early (G) and late (H) stages of branching, associated with developing shoots between roots. LR, Lateral root; M, apical meristem; RH, root hair; S, shoot. Bars = 2 mm (A and B), 100 µm (C–F) and 200 µm (G and H).
Fig. 2. Paradermal (frontal) sections of root tips at successive stages of root branching in Zeylanidium subulatum (A–E) and Z. lichenoides (F), showing splitting of meristem and exogenous origin of lateral roots associated with endogenous shoot initiation. A, Apex before branching. B, Widened apex at earliest stage of branching. C, Increasingly wide apex in which the shoot primordium forms. D, Young main and lateral roots and intervening young shoot at similar stage to that shown in Fig. 1G. Lightly stained large cells are present above the shoot primordium. E, Developing main and lateral roots and intervening shoot. F, Young main and lateral roots and intervening young shoot at similar stage to that shown in D. LA, Lateral root apical meristem; MA, main root apical meristem; S, shoot primordium. Bars = 100 µm.
Fig. 3. Foliose roots of Zeylanidium olivaceum. A, Transmitted light micrograph showing root incisions associated with shoots and the absence of strands in areas exterior to shoots. Note that a root exterior to a shoot (bottom right) is entire at the margin and has no provascular strand. B, SEM micrograph of dorsal side of root. Arrows indicate pieces of old protective tissue that remain on top of the current protective tissue. C, Transmitted light micrograph showing entire margin of lobe exterior to shoot. Arrow indicates provascular strands formed by recovered marginal meristem, and also indicates plane of section shown in Fig. 5C. D, Radial section showing marginal meristem and protective tissue covering it. E, Paradermal section showing branched provascular strand of elongate cells. F, Paradermal section showing protective tissue covering the marginal meristem. The inner protective cells are small, whereas the outer cells are larger and stretched tangentially. G, Radial section showing newly initiated shoot primordium in innermost zone of marginal meristem. M, marginal meristem; P, protective tissue; S, shoot. Bars = 1 mm (A and C), 500 µm (B), 100 µm (D and F) and 50 µm (E and G).
Fig. 4. Incision in foliose roots of Zeylanidium olivaceum. A, Transmitted light micrograph. B–F, Radial sections. A, Dorsal view of root lobe showing shoot primordium near margin. Arrow and double arrow indicate the plane of sections shown in B and C, respectively. B and C, Sections of root including a more developed shoot primordium than shown in Fig. 3G, and of neighbouring portions of root with no initiating shoot. The marginal meristem exterior to a shoot primordium is somewhat reduced compared with that in the neighbouring part, and the shoot primordium is embedded in root parenchyma. D and E, Same root at a later stage of incision. The marginal meristem is almost differentiated in a thin root exterior to a more developed shoot primordium (D) compared with the neighbouring area with a vigorous marginal meristem (E). F, Root at a later stage when incision is nearly complete. The marginal meristem is fully differentiated and there is no protective tissue at the end of a thin incised root exterior to an emerging shoot. M, Marginal meristem; P, protective tissue; S, shoot. Bars = 1 mm (A) and 50 µm (B–F).
Fig. 5. Entire (non-incised) roots of Zeylanidium olivaceum. A, Transmitted light micrograph. B–E, Radial sections. A, Dorsal view showing entire root lobe exterior to shoot (right). B, Section in plane indicated by arrow in A showing recovered marginal meristem at nearly the same stage of shoot development as shown in Fig. 4D. C, Section in plane indicated by arrow in Fig. 3C. Note that the proximal portion of a root exterior to a shoot is thin and the distal portion is thick. D, Magnification of distal portion of root shown in C showing vigorous marginal meristem (cf. Fig. 5E). E, Neighbouring section of root shown in D. M, Marginal meristem; P, protective tissue; S, shoot. Bars = 1 mm (A), 200 µm (C) and 50 µm (B, D and E).
Fig. 6. Foliose roots of Zeylanidium maheshwarii. A, SEM micrograph of stacked roots showing no regular shoot–root lobe association. B and C, Paradermal sections showing young marginal meristem covered by tangentially short protective cells (B) and well-developed meristem covered by tangentially stretched protective cells (C). D–G, Radial sections. D, Root with well-developed marginal meristem. E, Root with a shoot primordium that has just initiated in young parenchyma proximal to marginal meristem. F, Root with a slightly more developed shoot primordium with a void above it. G, Root showing thickened area distal to the shoot. A portion of marginal meristem has broken away due to mechanical damage. M, Marginal meristem; P, protective tissue; S, shoot. Bars = 500 µm (A), 50 µm (B–F) and 100 µm (G).
Fig. 7. Patterns of root branching and lobing in four Zeylanidium species, and hypothesized evolution of foliose roots. A shoot is initiated in the middle of an apical meristem in Z. subulatum and Z. lichenoides (A), in the innermost zone of a marginal meristem in Z. olivaceum (B) and in parenchyma proximal to a meristem in Z. maheshwari (C). Root branching (A) or lobing (B, left-hand side) is due to the meristem splitting after shoot formation, while non-lobing (B, right-hand side) occasionally occurs as a result of recovery of a meristem once it has become reduced during development. Non-lobing (C) may also be due to the lack of interruption of a meristem by shoot formation. The suggested evolution of foliose roots involves meristem widening (horizontally) and shifting of the shoot initiation site to the innermost meristem zone and then further to parenchyma proximal to a meristem. Shaded areas indicate meristems and solid circles indicate initiating shoots. The protective tissue is not shown in Z. olivaceum (B) and Z. maheshwari (C).