Zeylanidium subulatum and Z. lichenoides
The root in Z. subulatum is somewhat flattened with an oval shape in transverse section (approx. 0·8 mm wide and 0·5 mm thick), whereas that in Z. lichenoides is more flattened and wider, with a ribbon shape (0·8–1·5 mm wide, approx. 0·2 mm thick) (Fig. 1A–D). The root is branched alternately at intervals of 7–9 mm in Z. subulatum and 1–2 mm in Z. lichenoides (Fig. 1A and B). It consists of an epidermis and a parenchymatous ground tissue that is roughly seven to nine cells thick in Z. subulatum and five to seven cells thick in Z. lichenoides. The ground tissue is devoid of intercellular spaces and possesses only a rudimentary vascular bundle (Fig. 1C and D). In both species, the vascular bundle is single and consists of elongated cells. There are no apparent tracheary elements with helical thickenings except in the endogenously developed shoot. Adhesive root hairs are borne densely on the ventral surface, mostly along the vascular bundles (dark bands in Fig. 1A and B).
The root has an apical meristem that is slightly flattened and consists of a single surface layer and small, densely stained inner cells (Fig. 1
E and F). Surface cells are approx. 21 and 14 µm thick in Z. subulatum
and Z. lichenoides
, respectively, whereas the inner cells are approx. 14 and 10 µm thick, respectively. Surface cells are nucleated near the inner walls in both species (Fig. 1
E and F). The surface cells undergo anticlinal divisions exclusively, so the surface layer does not rupture even at root branching but continues to the epidermis in the proximal portion (Fig. 1
E–G). The root apex does not produce a root cap or any similar protective tissue.
Since the shoots occur between main and lateral roots at every branching point, root branching is always associated with shoot formation (Fig. 1A, B, G and H). The root apical meristem widens prior to branching (Fig. 2A and B). Within the further-widened root meristem a shoot primordium is formed accompanied by a group of vacuolated cells exterior (distal) to it (Fig. 2C). Consequently, the meristem splits into two. In a section of Fig. 2C, the heavily stained cells that constitute the smaller meristem are distinguishable from the adjacent, lightly stained cells, although the smaller meristem is less clear than the larger one. The larger daughter meristem will become the apical meristem of a future main root axis, whereas the smaller meristem becomes that of a future lateral root. As the shoot develops, the lightly stained vacuolated cells just above the shoot primordium die, and a void is formed (Fig 2D–F). As the growing shoot emerges, the root tissues covering the shoot expand and eventually rupture (Figs 1H and 2E, F).
Since shoots occur alternately on either side of the root apex, the main root appears zigzagged with alternate lateral branches (Fig. 1
A and B). In Z. subulatum
, the lateral root develops more slowly than the main root, and is rarely branched. In Z. lichenoides
, the young lateral root does not branch due to its very slow growth, but mature roots with floral shoots commonly have branched lateral roots.
The roots of Z. olivaceum are foliose, lobed, and usually 5–8 mm wide and 0·3 mm thick (Fig. 3A–C). The shoots are scattered over the dorsal surface of the root with no obvious pattern (Fig. 3A). The youngest shoots occur near the root margin, whereas the larger and older shoots are further from the outer root margin (Figs 3A–C, 4A and 5A). Most shoots occur interior to shallow or deep incisions in the root margin (Fig. 3A and B), which, in many cases, are deeper in more developed roots, resulting in lobing. However, in some cases, there is no lobe incision exterior to the shoot, resulting in an entire root margin (Figs 3A, C and 5A).
The root is composed of a dorsal and ventral epidermis between which are sandwiched about six parenchymatous layers without intercellular spaces, and an anastomosing provascular strand (Fig. 3
D and E). The strand consists of elongated parenchymatous cells (Fig. 3
E), except in old shoots, which are supplied by tracheary elements with helical thickenings. The unicellular root hairs are borne in patches on the ventral epidermis. Young shoots are supplied by one or two provascular strands from an anastomosing network of strands spreading in the root (Fig. 3
A and C). By contrast, there is no provascular strand in the area exterior to very young shoots, although the area is not incised (Figs 3
A and 5A).
There is a meristem along the margin of the foliose root covered by protective tissue (Fig. 3A and F). In this paper this meristem is referred to as a marginal meristem; its appearance is similar to that found in Hydrobryum japonicum (Ota et al., 2001). The marginal meristem is seven to eight cells wide (in the proximo-distal direction) and four to five cells thick (in the dorso-ventral direction) including the surface layers, and has a moderately layered structure. The protective tissue is composed of large, lightly stained cells that are five to eight cells wide and four to five cells thick. It is produced from the meristem as shown by cell alignment: the inner cells of the protective tissue are younger and smaller while the outer cells are older, larger and more stretched tangentially due to root growth (Fig. 3F). Pieces of protective tissues consisting of very old brown cells are often retained outside the outermost protective cells and are scattered along the margin (Fig. 3B). This suggests that formation of a protective tissue from the marginal meristem is quite discontinuous.
The shoot is initiated endogenously in the innermost zone of the meristem (Fig. 3G). As in Z. subulatum and Z. lichenoides, cells obliquely above a shoot primordium become enlarged, lightly stained, and eventually die to create a void (Fig. 4B and D). Growing shoots protrude from the root after rupturing the dorsal epidermis (Fig. 4F). As the root grows, the shoots shift to the interior from the root margin (Figs 3A–C and 4A). The marginal meristem area exterior to the shoots narrows (three to four cells wide in a proximo-distal direction), and the inner zone of the meristem begins to differentiate into parenchyma (Fig. 4B). In contrast, the neighbouring meristem area, which is not associated with shoots, remains wide (seven to eight cells; Fig. 4C). It seems probable that activity of the marginal meristem decreases under the influence of the initiating shoot. As the meristem gradually becomes less active, the derived root tissue exterior to a shoot becomes thinner, although it is only one cell layer thinner vertically than that in actively growing neighbouring parts (Fig. 4D and E). The epidermal and ground-tissue cells exterior to the shoot are stretched radially, possibly due to tension from the growing neighbouring parts (Fig. 4D and E). Finally, the meristem area exterior to the shoot differentiates into parenchyma, completing the splitting of the meristem (Fig. 4F). No marginal meristem or protective tissue remains although the parenchyma cells at the margin are slightly larger than the meristem cells (Fig. 4F).
In a small number of cases the root margin remains entire (not incised), even exterior to a shoot (Fig. 5A). The exterior part is very elongated and retains the marginal meristem, although this is as thin and stretched as that in incised parts (Fig. 5A and B). This part also lacks a network of strands. There is also another extreme case: once reduced, the marginal meristem may return to a vigorous meristem, resulting in a markedly elongated area with the distal part being supplied by a provascular-strand network (Fig. 3C). Serial longitudinal sections of the elongated area show that the proximal portion is thin and consists of fewer cell layers and stretched cells, while the distal portion is thicker and consists of more layers and small cells (Fig. 5C and D). The distal portion is probably derived from a recovered marginal meristem, which is as vigorous as that in the neighbouring portion (Fig. 5D and E).
The roots of Zeylanidium maheshwarii are foliose and irregularly lobed but the incisions are not associated with shoots (Fig. 6A). The roots examined here are stacked up and grow on other roots with shoots, resulting in irregular lobing. The young root lobe has a proximo-distally narrow marginal meristem and a very narrow layer of protective tissue composed of slightly tangentially elongate cells (Fig. 6B). As in Z. olivaceum, the mature lobe has a well-developed meristem and protective tissue (Fig. 6C and D). The shoot is initiated in young parenchyma just proximal to, and probably derived from, the root marginal meristem (Fig. 6E). As in Z. lichenoides, Z. olivaceum and Z. subulatum, root parenchyma cells distal to the shoot primordium are vacuolated, weakly stained, and eventually die, creating a void (Fig. 6E and F). The meristem distal to the shoot is as vigorous as that prior to shoot formation and in neighbouring parts. It continues to produce root tissues, so that the root is as thick as neighbouring parts (Fig. 6F and G). Consequently, the margin of the foliose root exterior to the shoots is entire (Fig. 6A).