Comparative anatomy of root meristem and root cap in some species of Podostemaceae and the evolution of root dorsiventrality

Abstract

In the unusual aquatic Podostemaceae, the root is the leading organ of the plant body and is variously compressed and submerged as it adheres to rock surfaces in rapid water. In an anatomical comparison of the root apical meristems and root caps of 33 species that represent the major lineages of the family, the dorsiventrality of root meristems varied and was classified into four patterns: (1) The root cap is produced outward from a nearly radially symmetrical meristem. (2) The meristem and root cap are markedly dorsiventral; the outermost cells of the hood-shaped cap are acroscopic derivatives from bifacial initials on the ventral side, while the pattern on the dorsal side is similar to pattern 1. (3) Bifacial initials are on both the dorsal and ventral sides. (4) No root cap is present. An evolutionary polarity may be evident from pattern 1 to 2 and then to 3. Pattern 2 arose in the early evolution of the subfamily Podostemoideae and subsequently, pattern 3 arose in species with crustose roots, while the least specialized pattern 1 is retained in Tristichoideae and Weddellinoideae. Pattern 4 characterized by caplessness may have appeared recurrently in Tristichoideae and Podostemoideae. These evolutionary changes in the meristem preceded the specialization of external root morphologies.

Phylogenetic Relationship and Morphology ofDalzellia gracilis(Podostemaceae, Subfamily Tristichoideae) with Proposal of a New Genus

“…There are two likely alternatives for root cap loss: the cap may have been either lost recurrently in ¡ndodalzclliii and Tristicba or lost ¡n the ancestor of the two clades and then regained m /. ramosissima (Koi et al 2006). Similar root cap loss (Kcurred in the subfamily Podostemoideae (Koi et al 2006;MoÜtie et al 2007).…”

“…The histology of the root apical meristem of the capless root of Indodalzellia is nearly the same as that of Tristicba in that the meristem comprises a single dermal layer enclosing inner cells (Koi et al 2006). PhylogeneticaUy, the capless Tristicha is sister to a clade comprising the capless ¡ndodalzellia and the subciade of the capped Indotristicha and the rootless Dalzellia, and the clade of these four genera is sister to the capped Terniopsis ( fig.…”

Neglected features of probable taxonomic value in Podostemaceae: The case of Polypleurum

“…Roots represent most of the plant body and are either thread-like, ribbon-shaped or floating, being randomly or completely attached to the rocks of the river bed, or are crustaceous and creeping over the substratum in a plagiotropic way. A different view, i.e., identification with a shoot, is proposed by Sehgal et al (2007), an opinion that is not accepted here (see also Jäger-Zürn, 2000a;Koi et al, 2006). Numerous rootborne small (mainly) unbranched vegetative and floriferous shoots arise laterally on the flanks or near them on the upper side of the roots and develop in clonal growth.…”

“…stylosum is one of the best studied species of Podostemoideae in various aspects of developmental morphology, embryology and anatomy (Hall, 1972;Kato, 2006;Khosla and Mohan Ram, 1993;Khosla et al, 2000;Koi et al, 2006;Mathew and Satheesh, 1997;Mohan Ram and Sehgal, 1997;Mukkada, 1962Mukkada, , 1964Nagendran, 1975;Nagendran and Arekal, 1981a,b;Nagendran et al, 1977a,b;Nileena, 2001;Rao and Hajra, 1979;Satheesh, 1996;Sehgal et al, 2007;Weddell, 1873;Willis, 1902). Detailed studies made on embryology of P. stylosum prevail (Mukkada, 1962(Mukkada, , 1964Nagendran et al, 1977a,b, as Dicraea stylosa;Khosla et al, 2000;Uniyal and Mohan Ram, 1994), whereas morphological studies mainly concern the outside view of the plants including polymorphism of ecotypes (Nileena, 2001).…”

“…Detailed studies made on embryology of P. stylosum prevail (Mukkada, 1962(Mukkada, , 1964Nagendran et al, 1977a,b, as Dicraea stylosa;Khosla et al, 2000;Uniyal and Mohan Ram, 1994), whereas morphological studies mainly concern the outside view of the plants including polymorphism of ecotypes (Nileena, 2001). Anatomical data of roots of P. elongatum are provided by Koi et al (2006). Polypleurum minus (grammatically incorrectly called P. minor in Mathew et al, 2003), was first described by Weddell (1873, as Dicraea minor), but has not been subject of more detailed studies in the meantime.…”