Members of Podostemaceae are aquatic angiosperms that grow on
water-worn rocks in waterfalls and rapids which are subject
to seasonal fluctuations in water level. The plants grow submerged
during the rainy season and are usually exposed to the air during
the dry season. They flower shortly after exposure to air and
set fruits whilst drying. The family has evolved specialized
morphologies that appear to have adaptive significance. Within
the family, considerable discontinuous morphological variation
is reflected by the current classification in which approx.
270 species are classified into about 47 genera, mostly monotypic
or oligospecific (Cook, 1996
The extensively modified plant body of the Podostemaceae isdifficult to interpret using the ordinary root–shoot concept.A seedling of subfamily Podostemoi deae either has a rudimentaryprimary shoot (plumule) or lacks one, and may or may not havea relatively short-lived primary root (radicle) (Mohan Ram and Sehgal, 1997;Suzuki et al., 2002). The mature plant usuallyconsists of an adventitious (secondary) root, which arises fromvarious parts of the seedling with the exception of the hypocotyltip, and root-borne adventitious shoots. The morphological natureof the diversely structured roots is controversial, and theyhave been variously termed a thallus, root–thallus orcrust, as well as a root (Rutishauser and Huber, 1991). In AsianPodostemoideae species, roots are flattened subcylindrical,ribbon-shaped or foliose (thalloid) (Willis, 1902; Engler, 1930;Troll, 1943; Cusset, 1992; Cook, 1996; Schnell, 1998; Jäger-Zürn, 2000a;Uniyal and Mohan Ram, 2001). In Z. subulatum (Gardn.)C. Cusset and Z. lichenoides (Kurz) Engl., as in many otherPodo stemoideae, the root is subcylindrical or ribbon-shaped,respectively, and alternately branched. Unusually, there isa shoot at every point of root branching (Warming, 1888; Willis, 1902;Mathew and Satheesh, 1997; Rutishauser, 1997; Jäger-Zürn, 2000a).The pattern is as regular as that commonly seen in shootsof most angiosperms where shoots occur in the axil of each subtendingleaf. The shoot emerges on the dorsal surface of the root nearthe lateral edge between the main axis and lateral root. Thisassociation between root branching and shoots is also seen inCladopus, a species of Polypleurum and Asian members of Podostemoideae,whereas there is no such association in other related speciesof Polypleurum (S. Koi et al., unpubl. res.).
The foliose root is chlorophyllous, with extremely reduced vegetativeand reproductive adventitious shoots scattered on the dorsalsurface, and root hairs (adhesive hairs) on the ventral surface.Such roots are seemingly multifunctional. A molecular phylogenyshows that Asian Podostemoideae are monophyletic and dividedinto two clades, one consisting of Zeylanidium and Polypleurum,and the other consisting of Cladopus, Torrenticola, Hydrobryumand Synstylis (Cook and Rutishauser, 2001; Kita and Kato, 2001).This suggests that the foliose roots of Z. olivaceum (Gardn.)Engl. and Z. maheshwarii C. J. Mathew & Satheesh, and ofHydrobryum and Synstylis are derived independently from subcylindricalor ribbon-like roots in the two clades. This proposed parallelevolution is also supported by the developmental morphologyof seedlings (Suzuki et al., 2002). However, no attention hasbeen paid to a possible association between root lobing andshoot formation in Z. olivaceum and Z. maheshwarii in whichthe shoots are scattered irregularly over the dorsal surfaceof the root (Willis, 1902; Mathew and Satheesh, 1997; Jäger-Zürn, 2000b),although they are phylogenetically close to Z. subulatumand Z. lichenoides.
This paper describes the developmental anatomy of roots of Z.subulatum
and Z. lichenoides
, and Z. olivaceum
and Z. maheshwarii
with emphasis on the development of the root meristem, which
is associated with shoot initiation, during the course of root
branching. Anatomical data are compared for the four species
to clarify the evolution from subcylindrical to foliose roots
and the evolution of the developmental relationships of root
branching or lobing with such a regular association with shoots.