Sympodial structure of spikelets in the tribe Schoeneae (Cyperaceae)

Spikelet morphology of 250 specimens of 47 species of Schoeneae was examined using light microscopy and scanning electron microscopy. We confirmed that spikelet structure in Schoeneae is cymose with a sympodial "rachilla." Monopodial spikelets, as described by most current literature, were not found in Schoeneae. Prophylls are not always present in sympodial spikelets and cannot be used to determine whether a spikelet is sympodial or monopodial. Spikelets of Schoeneae develop acropetally, and the uppermost glume may or may not produce a flower. The last feature may be variable within species and within individual plants, so presence or absence of this flower is not an indication of sympodial or monopodial spikelet structure in Schoeneae. Relative position of flower, glume, and axis is a reliable criterion to judge whether spikelets are sympodial or monopodial. In some species of Schoenus and in Ptilothrix, formation of the arch-shaped base of the fertile glume relates to the shape of the inclined nodes on which the glume grows. This study highlights the need to reinvestigate spikelet structure in other tribes of Cyperaceae.     

DEVELOPMENT OF THE EPIPHYLLOUS INFLORESCENCE OF HELWINGIA JAPONICA (HELWINGIACEAE)

The inflorescence of Helwingia japonica (Thunb.) Dietr. is initiated adjacent to the leaf axil on the adaxial side of the base of a leaf primordium during its second plastochron. The inflorescence which develops from the resulting primordium comes to be situated on the midrib of the mature fertile leaf, through the action of a basal, intercalary meristem. In fertile leaves this meristem develops beneath, as well as above, the insertion of the inflorescence primordium on the leaf primordium. The same meristem is present in sterile leaves as well. A separate, adaxial vascular bundle departs from the leaf trace in the base of the petiole and leads to the inflorescence, in the mature fertile leaf. This adaxial vascular bundle is absent in sterile leaves. It is argued that the vascular anatomy does not conclusively confirm the hypothesis that the epiphyllous inflorescence is the congenital fusion product of a leaf and an axillary inflorescence. Instead, it is suggested that the interplay of changes in the position of primordium initiation, and intercalary growth, offers an ontogenetic explanation of the situation, which in turn may be related to the phylogeny of the species in question. It appears to be misguided and futile to look for homologies (i.e., 1:1 correspondences) between fertile and sterile leaves, since 1:1 correspondences do not exist in this case.     

LEAF-OPPOSED BUDS IN MUSA: THEIR DEVELOPMENT AND A COMPARISON WITH ALLIED MONOCOTYLEDONS

A single, lateral, vegetative bud which is positioned 180° from the axil of a leaf is a generic feature of Musa (Musaceae). Such leaf-opposed buds occur in all ten species and five cultivars examined, representing all four sections of the genus and all groups of cultivated bananas and plantains. The bud arises relatively late and is first visible as a vascular-free “clear zone” in the axis directly below the future bud meristem site. It is first associated with the fifth or sixth leaf primordium from the apex. A defined superficial meristem develops on the stem directly above the insertion of the leaf margins one or more plastochrons later. Normal, basically axillary, vegetative buds occur in the closely related genera: Orchidantha (Lowiaceae), Heliconia (Heliconiaceae), Strelitzia, and Ravenala (Strelitziaceae). These buds arise in the axil of the first to the third leaf primordium in a manner similar to most other monocotyledons. Axillary vegetative buds also occur in the remaining families of the Zingiberales: Cannaceae, Costaceae, Marantaceae, and Zingiberaceae.     

Development of the mixed inflorescence in Zea diploperennis litis, Doebley & Guzman (Poaceae)

Development of the mixed inflorescence in Zea diploperennis Iltis, Doebley & Guzman (Poaceae) Mixed inflorescences of diploperennial teosinte, which terminate the main branches of the plant, arise in the same fashion as tassel spikes. The apical meristem produces bracts in a decussate arrangement. A single axillary bud primordium is initiated in the axil of each bract. Growth of the bract is retarded as the bud enlarges and divides longitudinally into two separate spikelet primordia. The paired spikelets running in two ranks on either side of the inflorescence primordium produce the four-rowed condition typical of teosinte tasselS. In the transition region between male and female portions of the inflorescence, development of the pedicellate spikelet of each spikelet pair is arrested at an early ontogenetic stage. Continued growth of the sessile spikelet and associated rachis flaps destroy the remnants of the arrested spikelet in basal portions of the inflorescence. A similar abortion of the lower floret of the sessile spikelet results in a single pistillate floret per node at anthesis. These results provide further support for the hypothesis that a tassel-like mixed inflorescence of teosinte is ancestral to the maize ear.     

Flower primordium formation at the Arabidopsis shoot apex: quantitative analysis of surface geometry and growth

Geometry changes, especially surface expansion, accompanying flower primordium formation are investigated at the reproductive shoot apex of Arabidopsis with the aid of a non-invasive replica method and a 3-D reconstruction algorithm. The observed changes are characteristic enough to differentiate the early development of flower primordium in Arabidopsis into distinct stages. Primordium formation starts from the fast and anisotropic growth at the periphery of the shoot apical meristem, with the maximum extension in the meridional direction. Surprisingly, the primordium first becomes a shallow crease, and it is only later that this shape changes into a bulge. The bulge is formed from the shallow crease due to slower and less anisotropic growth than at the onset of primordium formation. It is proposed that the shallow crease is the first axil, i.e. the axil of a putative rudimentary bract subtending the flower primordium proper, while the flower primordium proper is the bulge formed at the bottom of this axil. At the adaxial side of the bulge, the second axil (a narrow and deep crease) is formed setting the boundary between the flower primordium proper and the shoot apical meristem. Surface growth, leading to the formation of the second axil, is slow and anisotropic. This is similar to the previously described growth pattern at the boundary of the leaf primordium in Anagallis.     

TENDRILS OF PASSIFLORA FOETIDA: HISTOGENESIS AND MORPHOLOGY

Passiflora foetida bears an unbranched tendril, one or two laterally situated flowers, and one accessory vegetative bud in the axil of each leaf. The vegetative shoot apex has a single-layered tunica and an inner corpus. The degree of stratification in the peripheral meristem, the discreteness of the central meristem, and its centric and acentric position in the shoot apex are important plastochronic features. The procambium of the lateral leaf trace is close to the site of stipule initiation. The main axillary bud differentiates at the second node below the shoot apex. Adaxial to the bud 1–3 layers of cells form a shell-zone delimiting the bud meristem from the surrounding cells. A group of cells of the bud meristem adjacent to the axis later differentiates as an accessory bud. A second accessory bud also develops from the main bud opposite the previous one. A bud complex then consists of two laterally placed accessory bud primordia and a centrally-situated tendril bud primordium. The two accessory bud primordia differentiate into floral branches. During this development the initiation of a third vegetative accessory bud occurs on the axis just above the insertion of the tendril. This accessory bud develops into a vegetative branch and does not arise from the tissue of the tendril and adjacent two floral buds. The trace of the tendril bud consists of two procambial strands. There is a single strand for the floral branch trace. The tendril primordium grows by marked meristematic activity of its apical region and general intercalary growth.     

Flowering and apical meristem growth dynamics

The shoot apical meristem generates stem, leaves, and lateralshoot meristems during the entire shoot ontogeny. Vegetativeleaves are generated by the meristem in the vegetative developmentalphase, while in the reproductive phase either bracts subtendinglateral flower primordia (or paraclades), or perianth and strictlyreproductive organs are formed. Meristem growth is fully characterizedby the principal growth rates, directions, volumetric, and arealgrowth rates. Growth modelling or sequential in vivo methodsof meristem observation complemented by growth quantificationallow the above growth variables to be estimated. Indirectly,growth is assessed by cell division rates and other cell cycleparameters. Temporal and spatial changes of growth and geometrytake place at the meristem during the transition from the vegetativeto the reproductive phase. During the vegetative phase, meristemgrowth is generally indeterminate. In the reproductive phaseit is almost always determinate, but the extent of determinacydepends on the inflorescence architecture. In the vegetativephase the central meristem zone is the slowest growing region.The transition from the vegetative to the reproductive phaseis accompanied by an increase in mitotic activity in this zone.The more determinate is the meristem growth, the stronger isthis mitotic activation. However, regardless of the extent ofthe activation, in angiosperms the tunica/corpus structure ofthe meristem is preserved and therefore the mitotic activityof germ line cells remains relatively low. In the case of thethoroughly studied model angiosperm plant Arabidopsis thaliana,it is important to recognize that the flower primordium developsin the axil of a rudimentary bract. Another important featureof growth of the inflorescence shoot apical meristem is theheterogeneity of the peripheral zone. Finally, the role of mechanicalfactors in growth and functioning of the meristem needs furtherinvestigation. Key words: Flower primordium, geometry, growth, inflorescence, shoot apical meristem, transition from vegetative to reproductive phase Received 4 October 2007; Revised 5 November 2007 Accepted 6 November 2007     

Initiation of axillary and floral meristems in Arabidopsis

Shoot development is reiterative: shoot apical meristems (SAMs) give rise to branches made of repeating leaf and stem units with new SAMs in turn formed in the axils of the leaves. Thus, new axes of growth are established on preexisting axes. Here we describe the formation of axillary meristems and floral meristems in Arabidopsis by monitoring the expression of the SHOOT MERISTEMLESS and AINTEGUMENTA genes. Expression of these genes is associated with SAMs and organ primordia, respectively. Four stages of axillary meristem development and previously undefined substages of floral meristem development are described. We find parallels between the development of axillary meristems and the development of floral meristems. Although Arabidopsis flowers develop in the apparent absence of a subtending leaf, the expression patterns of AINTEGUMENTA and SHOOT MERISTEMLESS RNAs during flower development suggest the presence of a highly reduced, "cryptic" leaf subtending the flower in Arabidopsis. We hypothesize that the STM-negative region that develops on the flanks of the inflorescence meristem is a bract primordium and that the floral meristem proper develops in the "axil" of this bract primordium. The bract primordium, although initially specified, becomes repressed in its growth.     

Flower or spikelet? Understanding the morphology and development of reproductive structures in Exocarya (Cyperaceae, Mapanioideae, Chrysitricheae)

Fundamental questions of floral morphology remain unresolved in the grasslike monocots in order Poales, including what constitutes a flower and what constitutes a spikelet. The mapaniid sedges have particularly complex spikeletlike structures, variously interpreted as clusters of flowers or spikelets. Recent phylogenetic studies of Cyperaceae have identified the mapaniid clade as sister to the rest of the family, but the homology of mapaniid reproductive units (RUs) and spikeletlike units (SLUs) to other sedge flowers and spikelets is unclear. We examined reproductive development in the mapaniid Exocarya sclerioides. Inflorescence branches terminated in a SLU with bracts and 1-4 RUs. RUs had four small leaflike structures (LLSs): two lateral LLSs, each associated with a stamen, an abaxial LLS associated with a stamen, and an adaxial LLS. The gynoecium terminated the RU. All RUs were axillary to bracts, and unexpanded bracts and RUs were produced beyond expanded RUs, so SLUs were racemose. RUs developed from a single primordium that initiated two lateral LLSs, then two lateral stamens, then the gynoecium. Initiation of the abaxial LLS and stamen and the adaxial LLS followed. We hypothesize that the RU is a sympodial branch that terminates in a hermaphroditic flower with two stamens and a gynoecium; the two lateral LLSs are halves of a deeply divided prophyll.     

SYMMETRY AND DEVELOPMENT OF BUTOMUS UMBELLATUS (BUTOMACEAE) AND LIMNOCHARIS FLAVA (LIMNOCHARITACEAE)

The prostrate rhizome of Butomus umbellatus produces branch primordia of two sorts, inflorescence primordia and nonprecocious vegetative lateral buds. The inflorescence primordia form precociously by the bifurcation of the apical meristem of the rhizome, whereas the non-precocious vegetative buds are formed away from the apical meristem. The rhizome normally produces a branch in the axial of each foliage leaf. However, it is unclear whether the rhizome is a monopodial or a sympodial structure. Lateral buds are produced on the inflorescence of B. umbellatus either by the bifurcation or trifurcation of apical meristems. The inflorescence consists of monochasial units as well as units of greater complexity, and certain of the flower buds lack subtending bracts. The upright vegetative axis of Limnocharis flava has sympodial growth and produces evicted branch primordia solely by meristematic bifurcation. Only certain leaves of the axis are associated with evicted branch primordia and each such primordium gives rise to an inflorescence. The flowers of L. flava are borne in a cincinnus and, although the inflorescence is simpler than that of Butomus umbellatus, the two inflorescences appear to conform to a fundamental body plan. The ultimate bud on the inflorescence of Limnocharis flava always forms a vegetative shoot, and the inflorescence may also produce supernumerary vegetative buds. Butomus umbellatus and Limnocharis flava exhibit a high degree of mirror image symmetry.     

FLORAL ORGANOGENESIS IN FIVE GENERA OF THE MARANTACEAE AND IN CANNA (CANNACEAE)

The paired flowers of all species of the Marantaceae studied, except Monotagma plurispicatum, are produced through the division of an apical meristem with a tunica-corpus structure. The solitary flowers of M. plurispicatum develop from a similar meristem which does not bifurcate. The paired flowers of Canna indica are produced in the axil of a florescence bract through the formation of a bract and an axillary flower on the side of the primordium which gives rise to the largest flower of the pair. The sequence of organ initiation for both families is: calyx, corolla and inner androecial whorl, outer androecial whorl, gynoecium. The sequence of sepal formation is opposite in the two families. In the Cannaceae it leads directly into the spiral created by the formation of the other organs, while in the Marantaceae the sequence of sepal formation follows a spiral opposite to that of the other floral organs. The members of the corolla and inner androecial whorl separate from common primordia. In general these common primordia separate into a petal and an inner androecial member through the initiation of two growth centers, at the same level, in the dorsal and ventral flanks of the primordium. In Ischnosiphon elegans and Pleiostachya pruinosa the stamen is initiated at a lower position than the petal in the ventral flank of the common primordium. A similar pattern of initiation is described for the callose staminode in Marantochloa purpurea and Canna indica. This pattern is interpreted as a variation on the more generalized pattern of inner androecial formation found in the other genera.     

Developmental study onHypolepis punctata (Thunb.) Mett.

The origins of the first and second petiolar buds ofHypolepis punctata were clarified in relation to the early development of the leaf primordium, which arises from a group of superficial cells of the shoot apical meristem. One of these superficial cells produces a two-sided leaf apical cell which subsequently cuts off segments to make a well-defined cell group, called here the leaf apical cell complex, on the distal part of the leaf primordium. Meanwhile, cells surrounding the leaf apical cell complex also divide frequently to form the basal part of the leaf primordium. Two groups of basal cells of the leaf primordium located on the abaxial and the adaxial sides initiate the first and the second petiolar buds, respectively. The initial cells are usually contiguous to the leaf apical cell complex, constructing the abaxial and adaxial flanks of the very young leaf primordium. However, the first petiolar bud sometimes develops from cells located farther from the leaf apical cell complex. These cells are derived from those originally situated in the peripheral region of the shoot apical meristem. This study was supported by a Grant-in-Aid for Encouragement of Young Scientists by the Ministry of Education, Science and Culture, of Japan No. 474322 in 1979.     

VARIATION IN THE DEGREE OF CLEISTOGAMY WITHIN AND AMONG SPECIES OF THE GRASS DANTHONIA

North American species of the grass Danthonia bear distinct chasmogamous and cleistogamous flowers. Four taxa occurring in North Carolina (D. compressa, D. epilis, D. sericea, and D. spicata) were examined for variation in the degree of cleistogamy. The flowering culms bear a single terminal panicle consisting of 4 to 12 spikelets of chasmogamous flowers. In each leaf axil, at the nodes of the flowering culm, is a single spikelet of cleistogamous flowers completely surrounded by the leaf sheath. The percentage of cleistogamous flowers produced on any culm depends on the number of chasmogamous and cleistogamous spikelets and the number of flowers in each type of spikelet. All four characters vary among the taxa examined. Danthonia compressa produces, on the average, 50% cleistogamous flowers; D. spicata, 25% cleistogamous flowers; D. sericea and D. epilis, 5% cleistogamous flowers. The degree of cleistogamy in D. spicata is associated with certain habitat features. Populations from mountain sites, disturbed sites, and non-woodland sites produced higher percentages of cleistogamous flowers than did populations from piedmont, undisturbed, and woodland sites, respectively. Grazing may favor increased cleistogamy because cleistogamous flowers are produced lower on the plant. In North Carolina, the most frequently grazed Danthonia taxa also produce the highest percentage of cleistogamous flowers.     

COMPARATIVE ANATOMY OF THE SPIKELET CALLUS OF ERIOCHLOA,BRACHIARIA, AND UROCHLOA (POACEAE: PANICEAE: SETARIINEAE)

Anatomical investigation of the spikelet base in Eriochloa reveals a nonvascularized cup-shaped structure composed of large parenchyma cells surrounding a column of tissue that is continuous with the pedicel. The cup and column are fused only at the base of the cup. No vestigial stelar node occurs in either the cup or the column. The stele branches above the cup at the rachillar node of the second glume. Therefore, the cup-shaped callus characteristic of Eriochloa appears to be formed entirely of parenchymatous tissue and is not derived from the first glume as previously interpreted. The bead of callus at the base of the spikelet in some species of Brachiaria also comprises an unbranched stele surrounded by parenchyma. The vascular tissue branches and enters the first glume at the apex of the bead. The spikelet base of Urochloa has a distinct first glume, nodal complex, and no callus parenchyma. Thus, the cup-shaped callus of Eriochloa and the callus bead of Brachiaria appear to be structurally similar.     

PHYLLODE THEORY IN RELATION TO LEAF ONTOGENY IN SANSEVIERIA TRIFASCIATA

Shoot apices of Sansevieria trifasciata have a three-layered mantle, a zone of subapical initials, a central meristem, and a peripheral meristem. Leaf initiation begins with periclinal divisions in L-3 and is followed by periclinal divisions in L-2 and anticlinal divisions in L-l. At first, the primordium is a mound of tissue at one point on the flank, but it soon takes the form of a low ridge encircling the apex. An ephemeral adaxial meristem differentiates in L-2 of the primordium when it is about 50 μ high and is active until the primordium is about 450 μ high. Then it ceases basipetally and is not observable after the primordium is about 600μ high. As the adaxial meristem ceases at the base of the radial tip, its two lateral regions become the submarginal meristems of the expanded portion. Marginal meristems differentiate from the protoderm, and oblique-anticlinal divisions of the marginal initials result in the formation of an abaxial and adaxial epidermis. These derivatives undergo a few anticlinal divisions, increasing marginal width, and then they divide periclinally, increasing marginal thickness. After the primordium is about 600-700 μ high it continues to grow in length by a diffuse basal intercalary meristem. When the leaf is 3 dm long, an adaxial rounding meristem differentiates in the region just above the sheath. Leaf vasculature consists of parallel bundles which anastomose acropetally. Vascular bundles are arranged in a semicircle in the expanded portion and in a circle in the radial tip. There is one centrally located bundle at the apex as a result of lateral anastomoses. Present evidence from leaf ontogeny and mature vasculature in S. trifasciata is interpreted as supporting the concept that the liliaceous leaf is homologous with the phyllodes of A corns and Acacia.