SUMMARYLateral organ growth in seed plants is controlled in part by members of the YABBY (YAB) and class III homeodomain/leucine zipper (HD-ZIPIII) families of transcription factors. HD-ZIPIII genes appear to play a conserved role in such organs, but YAB genes have diversified, with some members of the family having specialized functions in leaves, carpels or ovule integuments. The ancestral expression patterns and timing of divergence of the various classes of YAB genes remain to be established. We isolated and evaluated the expression of one HD-ZIPIII and five YAB genes representing the five major YAB gene classes from Cabomba caroliniana, a member of the earliest-diverging angiosperms. Consistent with observations in eudicots, the FILAMENTOUS FLOWER (FIL) and YABBY5 (YAB5) genes of C. caroliniana were expressed in the abaxial regions of the leaf where new laminar segments arise, and the patterns of expression were mutually exclusive to those of HD-ZIPIII, indicating that these expression patterns are ancestral. Expression of CRABS CLAW (CRC) in the abaxial carpel wall, and of INNER NO OUTER (INO) in the abaxial outer integument of ovules was also conserved between eudicots and C. caroliniana, indicating that these patterns are primitive. However, the CRC gene was also expressed in other floral organs in C. caroliniana, and expression in stamens was also observed in another early-diverging species, Amborella trichopoda, indicating that carpel-specific expression was acquired after divergence of the Nymphaeales. The expression data and phylogeny for YAB genes suggest that the ancestral YAB gene was expressed in proliferating tissues of lateral organs.

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.Key words: developmental morphology; evolution; integument; micropyle-hilum complex; ovules; Nymphaeales; seeds.Bitegmic and anatropous ovules are common in basal angiosperms and are usually considered to be primitive in flowering plants as a whole (Stebbins, 1974;Bouman, 1984;Cronquist, 1988;Takhtajan, 1991;Johri, Ambegaokar, and Srivastava, 1992). Only a few authors regard anatropy as derivative and orthotropy as ancestral (Bocquet and Bersier, 1960). It has been suggested that the outer integuments are cup shaped and asymmetrical (Bouman, 1984). However, recent developmental and morphological studies have shown that in many basal angiosperms the outer integument is sharply asymmetric and hood shaped, that is, it is lacking on the concave (funicular) side of the ovule (Matsui, Imaichi, and Kato, 1993;Umeda, Imaichi, and Kato, 1994;Imaichi, Kato, and Okada, 1995;Igersheim, 1997, 1999;Endress, 1997, 1998; see also Johri, Ambegaokar, and Srivastava, 1992).The origin of bitegmy has been variously interpreted in contrast to the agreement on the origin of the first (inner) integument by fusion of telomes (Herr, 1995). Crane (1985Crane ( , 1986 and Doyle and Donoghue (1986, 1987) postulated that the outer integument is derived from the leafy lamina of a Caytonialike ancestor, while the carpel enclosing ovules is derived by lateral expansion of its leaf axis. It is also hypothesized that the bitegmic ovules have been derived from unitegmic and orthotropous ovules of glossopterid-like gymnosperms by the enclosure of the unitegmic ovules by the ovuliferous leaf to form the outer integument (Stebbins, 1974;Dahlgren, 1983;Kato, 1990;Stewart and Rothwell, 1993;Umeda, Imaichi, and Kato, 1994;Imaichi, Kato, and Okada, 1995;Doyle, 1996). Kato (1991) also speculates that the anatropy is an extreme modification of the hyponastic curvature of leaves (ovuliferous leaf). However, the origin of the outer integument and the origin ...

This study demonstrates with a regional flora that fern gametophytes do not always co-occur with sporophytes of the same species. In particular, noncordiform gametophytes tended to occur independently of conspecific sporophytes. This pattern may be due to the capability for indeterminate growth and vegetative reproduction by gemmae in noncordiform gametophytes.

Vascular plants have evolved shoot apical meristems (SAMs), whose structures differ among plant groups. To clarify the evolutionary course of the different structural types of SAMs, we compared plasmodesmatal networks in the SAMs for 17 families and 24 species of angiosperms, gymnosperms, and pteridophytes, using transmission electron microscopy (TEM). The plasmodesmata (PD) in almost all cell walls in median longitudinal sections of SAMs were counted, and the PD density per unit area was calculated for each cell wall. Angiosperm and gymnosperm SAMs have low densities, with no difference between stratified (tunica-corpus) and unstratified structures. SAMs of ferns, including Psilotum and Equisetum, have average densities that are more than three times higher than those of seed plants. Interestingly, microphyllous lycopods have both the fern and seed-plant types of PD networks; Selaginellaceae SAMs with single apical cells have high PD densities, while SAMs of Lycopodiaceae and Isoetaceae with plural initial cells have low PD densities, equivalent to those of seed plants. In summary, PD networks are strongly correlated to SAM organizations-SAMs with single and plural initial cells have the fern and seed-plant types of PD, respectively. The two SAM organizations may have evolved separately in lycophytes and euphyllophytes and may be associated with gain or loss of the ability to form secondary PD.

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The developmental morphology of seedlings and shoots of Dalzellia zeylanica was examined with reference to the meristem in order to understand the dorsiventral, foliose shoot. In seedlings, no obvious primary shoot and no root are formed. Subsequent to disappearance of the vestigial primary shoot meristem, two shoot meristems are established in the axils of the cotyledons, one of which grows into a secondary shoot. Microtome and SEM examinations of mature plants show that the shoot meristem is complex, comprising three zones along the shoot margin. The organogenetic zone, equivalent to the shoot apical meristem, produces dorsal leaves proximally and much fewer marginal leaves distally. During development, the zone repeatedly changes into a dorsal zone, while a new organogenetic zone is formed in an area between developing marginal leaves, resulting in the alternation of the organogenetic and dorsal zones, which allowed development of the coenosomic structure of the shoot. The dorsal and ventral zones do not produce leaves, but contribute to shoot expansion. The ventral zone also forces the marginal leaves to shift to the lateral side of the shoot. The rosette with tufted leaves might be a modification of the short shoot (ramulus) of other Tristichoideae.

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Root apical meristem (RAM) organization in lycophytes could be a key to understanding the early evolution of roots, but this topic has been insufficiently explored. We examined the RAM organization of lycophytes in terms of cell division activities and anatomies, and compared RAMs among vascular plants. RAMs of 13 species of lycophytes were semi-thin-sectioned and observed under a light microscope. Furthermore, the frequency of cell division in the RAM of species was analyzed using thymidine analogs. RAMs of lycophytes exhibited four organization types: type I (Lycopodium and Diphasiastrum), II (Huperzia and Lycopodiella), III (Isoetes) and RAM with apical cell (Selaginella). The type I RAM found in Lycopodium had a region with a very low cell division frequency, reminiscent of the quiescent center (QC) in angiosperm roots. This is the first clear indication that a QC-like region is present in nonseed plants. At least four types of RAM are present in extant lycophytes, suggesting that RAM organization is more diverse than expected. Our results support the paleobotanical hypothesis that roots evolved several times in lycophytes, as well as in euphyllophytes.