Developmental anatomy and branching of roots of four Zeylanidium species (podostemaceae), with implications for evolution of foliose roots.

Podostemaceae have markedly specialized and diverse roots that are adapted to extreme habitats, such as seasonally submerged or exposed rocks in waterfalls and rapids. This paper describes the developmental anatomy of roots of four species of Zeylanidium, with emphasis on the unusual association between root branching and root-borne adventitious shoots. In Z. subulatum and Z. lichenoides with subcylindrical or ribbon-like roots, the apical meristem distal (exterior) to a shoot that is initiated within the meristem area reduces and loses meristematic activity. This results in a splitting into two meristems that separate the parental root and lateral root (anisotomous dichotomy). In Z. olivaceum with lobed foliose roots, shoots are initiated in the innermost zone of the marginal meristem, and similar, but delayed, meristem reduction usually occurs, producing a parenchyma exterior to shoots located between root lobes. In some extreme cases, due to meristem recovery, root lobing does not occur, so the margin is entire. In Z. maheshwarii with foliose roots, shoots are initiated proximal to the marginal meristem and there is no shoot-root lobe association. Results suggest that during evolution from subcylindrical or ribbon-like roots to foliose roots, reduction of meristem exterior to a shoot was delayed and then arrested as a result of inward shifting of the sites of shoot initiation. The evolutionary reappearance of a protective tissue or root cap in Z. olivaceum and Z. maheshwarii in the Zeylanidium clade is implied, taking into account the reported molecular phylogeny and root-cap development in Hydrobryum.

Developmental morphology of ovules and seeds of Nymphaeales.

Ovule and seed development in six species of Nymphaeales was examined. In the Cabombaceae the two species studied resemble some extant basal angiosperms by having a hood-shaped outer integument. A micropyle-hilum complex results when the outer integument and derived testa are lacking between the micropyle and the funiculus, thus the hood-shaped appearance. In the Nymphaeaceae the outer integument is annular at an early stage and then cup-shaped though it is semiannular at initiation in Nupar japonicum and Nymphaea alba. The micropyle and hilum are separated by an intervening testa. Developmental data on the formation of the outer integument, from semiannular to hood-shaped vs. from annular to cup-shaped, are useful for inferring the morphology of the outer integument from the relative position of the micropyle to the hilum in seed fossils. The oldest (early Cretaceous) probable nymphaealean seeds had the micropyle-hilum complex, suggesting that the hood-shaped outer integument may be primitive in the Nymphaeales. This needs to be tested by examination of this feature in other groups of basal angiosperms.

Developmental morphology of the thalloid Hydrobryum japonicum (Podostemaceae).

We describe the unique development and branching of lobed thalli in Hydrobryum japonicum. Lobe formation begins with meristem initiation at random sites near the thallus margin fringed by protective tissues. As the protective tissues are successively peeled off particularly in the growing new lobes, the lobes become naked and then become fringed again by new protective tissues that develop from the marginal part of the new meristems. Subsequently the meristems become less active and are differentiated into parenchymatous ground tissue at maturity. The random pattern of meristem formation during the sporadic development gives rise to a nonorderly branching pattern of the thalli. Some other lobes (∼10%) are regenerated from injured parts of the thalli. The vegetative shoots arise endogenously near the thallus margin and are enclosed by the nonvascular strand nets. The rudimentary shoot apices remain embedded in the thalli. The thalli, though remarkably different from typical roots of other angiosperms, might be extremely transformed roots.

Cell division patterns in the apices of subterranean axis and aerial shoot of Psilotum nudum (Psilotaceae): morphological and phylogenetic implications for the subterranean axis.

The cell division pattern in the apical meristem of Psilotum nudum was examined using epi-illumination microscopy and a paraffin method. In the subterranean axis, about half of the derivative cells of the apical cell produce tetrahedral daughter apical cells by the first three or more oblique divisions. Roughly half of these apical cells give rise to the apical meristems of axes, whereas the other half do not. Various relative activities of the mother and daughter apical cells give rise to disordered branching patterns. In the ill-organized apical meristem as well as the leafless and capless structure, the Psilotum subterranean axis differs from the basic organs of vascular plants such as stem and root and seems to be an independent organ. The cell division pattern characteristic of the subterranean axis persists in the young unbranched aerial shoots, although fewer daughter apical cells are produced. Dichotomous branching of the aerial shoots, as in a variety of organs of pteridophytes, involves loss of the mother apical cell followed by appearance of two daughter apical cells.