INFLORESCENCE AND FLORAL DEVELOPMENT IN HOUTTUYNIA CORDATA (SAURURACEAE)

The inflorescence of Houttuynia cordata produces 45–70 sessile bracteate flowers in acropetal succession. The inflorescence apical meristem has a mantle-core configuration and produces “common” or uncommitted primordia, each of which bifurcates to form a floral apex above, a bract primordium below. This pattern of organogenesis is similar to that in another saururaceous plant, Saururus cernuus. Exceptions to this unusual development, however, occur in H. cordata at the beginning of inflorescence activity when four to eight petaloid bract primordia are initiated before the initiation of floral apices in their axils. “Common” primordia also are lacking toward the cessation of inflorescence apical activity in H. cordata when primordia become bracts which may precede the initiation of an axillary floral apex. Many of these last-formed bracts are sterile. The inflorescence terminates with maturation of the meristem as an apical residuum. No terminal flowers or terminal gynoecia were found, although subterminal gynoecia or flowers in subterminal position may overtop the actual apex and obscure it. Individual flowers have a tricarpellate syncarpous gynoecium and three stamens adnate to the carpels; petals and sepals are lacking. The order of succession of organs is: two lateral stamens, median stamen, two lateral carpels, median carpel. The three carpel primordia almost immediately are elevated as part of a gynoecial ring by zonal growth of the receptacle below the attachment of the carpels. The same growth elevates the stamen bases so that they appear adnate to the carpels. The trimerous condition in Houttuynia is the result of paired or solitary initiations rather than trimerous whorls. Symmetry is bilateral and zygomorphic rather than radial. No evidence of spiral arrangement in the flower was found.     

ONTOGENY OF THE INFLORESCENCE OF SAURURUS CERNUUS (SAURURACEAE)

The spicate inflorescence of Saururus cernuus L. (Saururaceae) results from the activity of an inflorescence apical meristem which produces 200–300 primordia in acropetal succession. The inflorescence apex arises by conversion of the terminal vegetative apex. During transition the apical meristem increases greatly in height and width and changes its cellular configuration from one of tunica-corpus to one of mantle (with two tunica layers) and core. Primordia are initiated by periclinal divisions in the subsurface layer. These are “common” primordia, each of which subsequently divides to produce a floral apex above and a bract primordium below. The bract later elongates so that the flower appears borne on the bract. All common primordia are formed by the time the inflorescence is about 4.4 mm long; the apical meristem ceases activity at this stage. As cessation approaches, cell divisions become rare in the apical meristem, and height and width of the meristem above the primordia diminish, as primordia continue to be initiated on the flanks. Cell differentiation proceeds acropetally into the apical meristem and reaches the summital tunica layers last of all. Solitary bracts are initiated just before apical cessation, but no imperfect or ebracteate flowers are produced in Saururus. The final event of meristem activity is hair formation by individual cells of the tunica at the summit, a feature not previously reported for apical meristems.     

Homeosis, morphogenetic gradient and the determination of floral identity in the inflorescences ofPhilodendron solimoesense (Araceae)

A comparative developmental study of the inflorescence ofPhilodendron solimoesense was conducted using scanning electron microscopy. The spadix ofP. solimoesense is characterized by unisexual flowers. Staminate flowers are initiated on the upper portion of the spadix while pistillate flowers develop on the lower portion of the spadix. An intermediate zone located between the upper male and lower female portion of the inflorescence consists of sterile male flowers. Within this intermediate zone a row of flowers exhibit polarity with respect to the identity of sexual organs. Stamens are initiated on the flank of the floral meristem facing the upper male zone and carpels are initiated on the portion of the floral meristem facing the lower female zone. The resulting flowers therefore assume a bisexual identity. At the level of the inflorescence, all floral buds are initiated along a series of contact parastichies and the continuity of these parastichies is not disrupted at any level in the male, intermediate, and female zones on the spadix. Results from this study support the presence of a morphogenetic gradient acting at the level of the inflorescence and appears to be independent of the boundaries of floral primordia.     

INFLORESCENCE AND FLOWER DEVELOPMENT IN THE PIPERACEAE. II. INFLORESCENCE DEVELOPMENT OF PIPER

The inflorescence development of three species of Piper (P. aduncum, P. amalago, and P. marginatum), representing Sections Artanthe and Ottonia, was studied. The spicate inflorescences contain hundreds or even thousands of flowers, depending on the species. Each flower has a tricarpellate syncarpous gynoecium and 4 to 6 free stamens, in the species studied. No sepals or petals are present. In P. marginatum the apical meristem of the inflorescence is zonate in configuration and is unusually elongate: up to 1,170 μm high and up to 480 μm wide during the most active period of organogenesis. Toward the time of apical cessation both height and diameter gradually diminish, leaving an apical residuum which may become an attenuate spine or may be cut off by an abscission zone just below the meristem. The active apex produces bract primordia; when each is 40–55 μm high, a floral apex is initiated in its axil. Both bract and floral apex are initiated by periclinal divisions in cells of the subsurface layer. The bracts undergo differentiation rather early, while the floral apices are still developing. The last-produced bracts near the tip of the inflorescence tend to be sterile.     

HETEROMORPHIC FLOWER DEVELOPMENT IN NEPTUNIA PUBESCENS,A MIMOSOID LEGUME

The characteristic of heteromorphic inflorescences in some mimosoid legumes such as Neptunia is a puzzling one which can be approached developmentally. Each spicate inflorescence of Neptunia pubescens includes three types of flowers: perfect in the upper half, functionally male just below the middle, and sterile or neuter at the base. Developmental studies of the inflorescence show that order of initiation of bracts on the inflorescence is acropetal, but that order of subsequent development of flowers is both acropetal and basipetal on the axis. Bract growth and initiation of the axillary floral apices at the base are inhibited or retarded, while those in the middle and upper levels continue development without interruption. The three types of floral primordia are similar during initiatory stages of organ formation and through early development. At mid-development, differences arise in floral symmetry, petal form, stamen form, and size and shape of the carpel. The functionally male flowers become strongly dorsiventral and zygomorphic while the other two morphs remain actinomorphic or nearly so. Heteromorphy arises from a combination of early suppression of organogeny plus mid-stage innovations of zygomorphy and lateral expansion of stamen primordia. These divergent developmental pathways in one inflorescence can be interpreted in part using Gould's concept of heterochrony: changes in timing of developmental events to produce different structures. Other changes in Neptunia cannot be explained by this concept, however; such changes as omission of processes (i.e., meiosis) in some organs, or addition of processes not normally present (i.e., blade formation in stamen primordia which become staminodia). It is becoming evident from work on this and other legume flowers that actual loss of organs is rare, compared to initiation followed by suppression or modification.     

FLORAL DEVELOPMENT OF POTAMOGETON RICHARDSONII

The inception and development of the sterile floral appendages of Potamogeton richardsonii have been re-investigated with a refined dissection technique (Sattler, 1968) and improved microtechnical methods (Feder and O'Brien, 1968). The results obtained by Sattler (1965) are confirmed, i.e., the sterile appendages are initiated at the flanks of the floral apex before the stamen primordia are formed. Consequently, they may be homologized with tepals or perianth members, although in the mature flower they are inserted at the stamen connective, due to growth between and at the base of each developing tepal and stamen. Each carpel arises as a radial primordium which becomes peltate immediately after its inception. One ovule primordium is initiated at the cross-zone. The stigma becomes bilobed. A slight outgrowth develops at the abaxial side of the style. The floral apex has a two-layered tunica. The primordia of the tepals, carpels, and ovules arise by periclinal divisions in the second tunica layer, whereas the stamen primordia are initiated by periclinal divisions in the corpus and second tunica layer. Variation in floral pattern, especially with regard to the number of appendages, has been observed in flowers near the tip of the inflorescence axis.     

Is LEAFY a useful marker gene for the flower-inflorescence boundary in the Euphorbia cyathium?

The flower-like reproductive structure of Euphorbia s.l. (Euphorbiaceae) is widely believed to have evolved from an inflorescence, and is therefore interpreted as a special type of pseudanthium, termed a cyathium. However, fuzzy morphological boundaries between the inflorescence, individual flowers, and organs have fuelled the suggestion that the cyathium does not merely superficially resemble a flower but could actually share developmental genetic pathways with a conventional flower. To test this hypothesis, immunolocalizations of FLORICAULA/LEAFY (LFY), a protein associated with floral identity in many angiosperm species, were performed in developing cyathia of different species of Euphorbia. Expression of the LFY protein was found not only in individual floral primordia (as predicted from results in the model organisms Arabidopsis and Anthirrhinum), but also in the cyathium primordium and in the primordia of partial male inflorescences. These results provide further evidence that the evolution of floral traits in pseudanthial inflorescences often involves expression of floral development genes in the inflorescence apex. This finding blurs the conventional rigid distinction between flowers and inflorescences.     

FLOWER DEVELOPMENT IN DIOECIOUS SPINACIA OLERACEA (CHENOPODIACEAE)

Spinacia oleracea (Chenopodiaceae) is a potential model system for studies of mechanisms of sex expression and environmental influences on gender in dioecious species. Development of the male and female flowers and inflorescences of spinach were studied to determine when the two sex types can be distinguished. We found that female inflorescence apices are significantly larger than those of the male. Flower primordia are similar in size prior to perianth initiation, but the male primordia develop at a faster rate. Another distinguishing feature at this early stage is the larger bract subtending the female primordium. The two flower types become readily distinguishable when the perianth initiates. Male flowers produce four sepals and four stamens in a spiral pattern in close succession. Female flowers produce two alternate perianth parts that enlarge somewhat before the gynoecium becomes visible. There are no traces of gynoecia in male flowers or of stamens in female flowers. We propose that plant sex type is determined before inflorescence development, prior to or at evocation.     

FLORAL DEVELOPMENT IN MYRISTICA (MYRISTICACEAE)

Myristica fragrans and M. malabarica are dioecious. Both staminate and pistillate plants produce axillary flowering structures. Each pistillate flower is solitary, borne terminally on a short, second-order shoot that bears a pair of ephemeral bracts. Each staminate inflorescence similarly produces a terminal flower and, usually, a third-order, racemose axis in the axil of each pair of bracts. Each flower on these indeterminate axes is in the axil of a bract. On the abaxial side immediately below the perianth, each flower has a bracteole, which is produced by the floral apex. Three tepal primordia are initiated on the margins of the floral apex in an acyclic pattern. Subsequent intercalary growth produces a perianth tube. Alternate with the tepals, three anther primordia arise on the margins of a broadened floral apex in an acyclic or helical pattern. Usually two more anther primordia arise adjacent to each of the first three primordia, producing a total of nine primordia. At this stage the floral apex begins to lose its meristematic appearance, but the residuum persists. Intercalary growth below the floral apex produces a columnar receptacle. The anther primordia remain adnate to the receptacle and grow longitudinally as the receptacle elongates. Each primordium develops into an anther with two pairs of septate, elongate microsporangia. In pistillate flowers, a carpel primordium encircles the floral apex eventually producing an ascidiate carpel with a cleft on the oblique apex and upper adaxial wall. The floral ontogeny supports the morphological interpretation of myristicaceous flowers as trimerous with either four-sporangiate anthers or monocarpellate pistils.     

花叶芋(天南星科)的花器官发生

利用扫描电镜首次观察了天南星科花叶芋(Colocasia bicolor) 的花器官发生过程。花叶芋的肉穗花序由无花被的单性花构成, 雌花发生于花序基部, 雄花发生于花序上部, 中性花位于花序中间部位。雄花: 3 或4 个初生雄蕊原基轮状发生, 随后每个初生原基一分为二, 形成6或8个次生原基; 一部分次生原基在其后的发育过程中融合, 形成5 或7 枚雄蕊; 雄花发育过程中未见雌性结构的分化; 花药的分化先于花丝; 雄蕊合生成雄蕊柱。雌花: 合生心皮, 3或4个心皮原基轮状发生, 未见雄性结构的分化。中性花来源于雌雄花序过渡带上, 属于雄蕊原基的滞后发育以及发育成熟过程中的退化; 与彩叶芋属(Caladium)不同, 此过渡区未见畸形两性花。初生雄蕊原基二裂产生次生原基的次生现象在目前天南星科花器官发生中显得比较特殊, 同时初步探讨了次生原基的融合方式。     

RE-EVALUATION OF CERTAIN KEY RELATIONSHIPS IN THE ALISMATIDAE: FLORAL ORGANOGENESIS OF SCHEUCHZERIA PALUSTRIS (SCHEUCHZERIACEAE)

Transition to flowering in the North-temperate bog plant Scheuchzeria palustris occurs in early May and results in the formation of a simple raceme with six flowers. Five of the flowers are subtended by large foliar bracts, while the sixth and last-formed flower on the inflorescence remains ebracteate. The individual flowers develop along a clearly trimerous pattern. The three outer tepals develop first, arising almost simultaneously at the periphery of the triangular floral apex. They are followed closely by the development of the three anti-tepalous outer stamens. The three inner tepals are next in the developmental sequence, alternating with the outer whorl of tepal-stamen pairs but arising at a slightly higher level on the floral meristem. Three inner stamens are initiated opposite the inner tepal primordia. Finally, three gynoecial primordia are initiated on the remaining central portion of the floral apex and alternating with the inner whorl of tepal-stamen pairs. Each carpel develops at first as a horseshoe-shaped structure. Two ovules form in each carpel, initiating on the adaxial margin of the carpel wall. Histogenesis of all floral appendages involves initially periclinal divisions in the second tunica layer followed by corresponding anticlinal divisions in the first tunica layer and concurrent activity in the underlying corpus. Separate procambial strands differentiate acropetally from the inflorescence axis to each tepal-stamen pair and then bifurcate. The vascular connection to the gynoecium develops directly from the strands in the tepal-stamen pairs. The results of this developmental study of the flower of S. palustris have a significant bearing on the positioning of this and related taxa within the Alismatidae and on the speculation of the phylogeny of the monocotyledon flower.     

Study of homeosis in the flower of Philodendron (araceae): a qualitative and quantitative approach

This study deals specifically with floral organogenesis and the development of the inflorescence of Philodendron squamiferum and P. pedatum. Pistillate flowers are initiated on the lower portion of the inflorescence and staminate flowers are initiated on the distal portion. An intermediate zone consisting of sterile male flowers and atypical bisexual flowers with fused or free carpels and staminodes is also present. This zone is located between the sterile male and female floral zones. In general, the portion of bisexual flowers facing the male zone forms staminodes, and the portion facing the female zone develops an incomplete gynoecium with few carpels. The incomplete separation of some staminodes from the gynoecial portion of the whorl shows that they belong to the same whorl as the carpels. There are two levels of aberrant floral structures in Philodendron: The first one is represented by the presence of atypical bisexual flowers, which are intermediates between typical female flowers and typical sterile male flowers. The second one is the presence of intermediate structures between typical carpels and typical staminodes on a single atypical bisexual flower. The atypical bisexual flowers of P. squamiferum and P. pedatum are believed to be a case of homeosis where carpels have been replaced by sterile stamens on the same whorl. A quantitative analysis indicates that in both species, on average, one staminode replaces one carpel.     

MORPHOLOGY AND DEVELOPMENT OF THE FLOWERS OF BOOTTIA CORDATA,OTTELIA ALISMOIDES,AND THEIR SYNTHETIC HYBRID (HYDROCHARITACEAE)

The inferior ovary of Boottia cordata, Ottelia alismoides, and their hybrid is appendicular in nature, the carpels are congenitally only slightly connate, and they are unsealed. All floral organs except the sepals originate from common primordia in the female and bisexual flowers. A flat residual floral apex is pressnt. There is a vestigial superior ovary of three ontogenetically fused carpels in the male flower of Boottia cordata. The hybrid is intermediate in many characteristics and has partially fertile stamens and staminodia. The sequence of development in all flowers is acropetal. These plants appear to be related to the Butomaceae and they show evolutionary tendencies parallel to those in the Nymphaeaceae.     

Pistillate and staminate flower development in dioecious Pistacia vera (Anacardiaceae)

The development of staminate and pistillate flowers in the dioecious tree species Pistacia vera L. (Anacardiaceae) was studied by scanning electron microscopy with the objective of determining organogenetic patterns and phenology of floral differentiation. Flower primordia are initiated similarly in trees of both sexes. Stamen and carpel primordia are initiated in both male and female flowers, and the phenology of organ initiation is essentially identical for flowers of both sexes. Vestigial stamen primordia arise at the flanks of pistillate flower apices at the same time functional stamens are initiated in the staminate flowers. Similarly, a vestigial carpel is initiated in staminate flowers at the same time the primary, functional carpel is initiated in pistillate flower primordia. Differences between the two sexes become apparent early in development as, in both cases, development of organs of the opposite sex becomes arrested at the primordial stage. Male flowers produce between four and six mature functional stamens and female flowers produce a gynoecium with one functional and two sterile carpels.     

COMPARATIVE ONTOGENY OF THE INFLORESCENCE AND FLOWER OF HAMAMELIS VIRGINIANA AND LOROPETALUM CHINENSE (HAMAMELIDACEAE)

A comparative developmental study of the inflorescence and flower of Hamamelis L. (4-merous) and Loropetalum (R. Br.) Oliv. (4–5 merous) was conducted to determine how development differs in these genera and between these genera and others of the family. Emphasis was placed on determining the types of floral appendages from which the similarly positioned nectaries of Hamamelis and sterile phyllomes of Loropetalum have evolved. In Hamamelis virginiana L. and H. mollis Oliv. initiation of whorls of floral appendages occurred centripetally. Nectary primordia arose adaxial to the petals soon after the initiation of stamen primordia and before initiation of carpel primordia. In Loropetalum chinense (R. Br.) Oliv. floral appendages did not arise centripetally. Petals and stamens first arose on the adaxial portion, and then on the abaxial portion of the floral apex. The sterile floral appendages (sterile phyllomes of uncertain homology) were initiated adaxial to the petals after all other whorls of floral appendages had become well developed. In all three species, two crescent shaped carpel primordia arose opposite each other and became closely appressed at their margins. Postgenital fusion followed and a falsely bilocular, bicarpellate ovary was formed. Ovule position and development are described. The nectaries of Hamamelis and sterile phyllomes of Loropetalum rarely develop as staminodia, suggesting a staminodial origin. However, these whorls arise at markedly different times and are therefore probably not derived from the same whorl of organs in a common progenitor. This hypothesis seems probable when one considers that the seemingly least specialized genus of the tribe, Maingaya, bears whorls of both staminodia and sterile phyllomes inside its whorl of stamens.     

ORGAN INITIATION AND DEVELOPMENT OF INFLORESCENCES AND FLOWERS OF ACACIA BAILEYANA

Inflorescence and floral ontogeny are described in the mimosoid Acacia baileyana F. Muell., using scanning electron microscopy and light microscopy. The panicle includes first-order and second-order inflorescences. The first-order inflorescence meristem produces first-order bracts in acropetal order; these bracts each subtend a second-order inflorescence meristem, commonly called a head. Each second-order inflorescence meristem initiates an acropetally sequential series of second-order bracts. After all bracts are formed, their subtended floral meristems are initiated synchronously. The sepals and petals of the radially symmetrical flowers are arranged in alternating pentamerous whorls. There are 30–40 stamens and a unicarpellate gynoecium. In most flowers, the sepals are initiated helically, with the first-formed sepal varying in position. Petal primordia are initiated simultaneously, alternate to the sepals. Three to five individual stamen primordia are initiated in each of five altemipetalous sectorial clusters. Additional stamen primordia are initiated between adjacent clusters, followed by other stamens initiated basipetally as well as centripetally. The apical configuration shifts from a tunica-corpus cellular arrangement before organogenesis to a mantle-core arrangement at sepal initiation. All floral organs are initiated by periclinal divisions of the subsurface mantle cells. The receptacle expands radially by numerous anticlinal divisions in the mantle at the summit, concurrently with proliferation of stamen primordia. The carpel primordium develops in terminal position by conversion of the floral apex.     

Homeosis in Araceae Flowers: The Case of Philodendron melinonii

The early stages of development of the inflorescence of Philodendronmelinonii were examined using scanning electron microscopy.Pistillate flowers are initiated on the lower portion of theinflorescence and staminate flowers are initiated on the distalportion. The male flowers have four to five stamens. The femaleflowers have a multilocular ovary consisting of four to sixlocules. A transition zone consisting of sterile male flowersand bisexual flowers with fused or free carpels and staminodesis also present on the inflorescences. This zone is locatedbetween the male and female flower zones. Generally, the portionof the bisexual flower facing the male zone forms stamens, andthe portion facing the female zone develops an incomplete gynoeciumwith few carpels. In P. melinonii, the incomplete separationof staminodes from the gynoecial portion of the whorl showsthat the staminodes and carpels belong to the same whorl. Thebisexual flowers of P. melinonii are believed to be a case ofhomeosis where carpels have been replaced by sterile stamenson the same whorl. At the level of the inflorescence, pistillateand staminate flowers are inserted on the same contact parastichiesalong the inflorescence; there is no discontinuity between thefemale zone, the bisexual zone, and the male zone. The presenceof bisexual flowers is believed to correspond to a morphogeneticgradient at the level of the inflorescence as a whole. Copyright2000 Annals of Botany Company Flower, development, gradient, inflorescence     

THE DEVELOPMENTAL BASIS FOR SEXUAL EXPRESSION IN CERATONIA SILIQUA (LEGUMINOSAE: CAESALPINIOIDEAE: CASSIEAE)

The flowers of Ceratonia siliqua, an anomalous caesalpinioid legume in the tribe Cassieae, are unusual in being unisexual and in lacking petals. Inflorescence development, organogeny, and flower development are described for this species. All flowers are originally bisexual, but one sex is suppressed during late development of functionally male and female flowers. Ceratonia siliqua is highly plastic in sexuality of individuals, inflorescence branching pattern, racemose or cymose inflorescences, bracteole presence, terminal flower presence, organ number per whorl, missing floral organs, pollen grain form, and carpel cleft orientation. Order of initiation is: five sepals in helical order, then five stamens in helical order together with the carpel. Each stamen is initiated as two alternisepalous primordia that fuse to become a continuous antesepalous ridge; in some flowers, the last one or two stamens of the five may form as individual antesepalous mounds. Petal rudiments are occasional in mature flowers. Position of organs is atypical: the median sepal is on the adaxial side in Ceratonia, rather than abaxial as in most other caesalpinioids. This feature in Ceratonia may be viewed as a link to subfamily Mimosoideae, in which this character state is constant.     

NEW INTERPRETATION OF THE INFLORESCENCE OF FAGUS DRAWN FROM THE DEVELOPMENTAL STUDY OF FAGUS CRENATA,WITH DESCRIPTION OF AN EXTREMELY MONSTROUS CUPULE

The inflorescence of Fagus is generally considered to be a determinate one, i.e., an axillary dichasium, in contrast to those of most genera in the family, which are indeterminate, dichasial, or simple catkins. To understand the relationship between the two types, ontogenetic development of the inflorescence of Fagus crenata was investigated. The early developmental stages are similar in both the male and the female inflorescences. At first, the inflorescence is oval-shaped, then a swelling forms at the distal side of it. Subsequently, another swelling forms at the proximal side. The more or less conspicuous residual part of the primary inflorescence axis remains between the two swellings. The inflorescence becomes heart-shaped and the first flower forms at the summit of each swelling. Subsequently, higher-ordered flowers form dichasially in the male inflorescence, and the cupule valves differentiate in the female one. This organogenetic manner suggests that the inflorescence of Fagus is an indeterminate one, consisting of two dichasia arranged alternately on the primary axis. The scale leaves surrounding the inflorescence were also given a new interpretation. They were considered to be stipules of the bracts, because sometimes they constitute a continuous structure, together with an inconspicuous swelling between them. A proliferous-type monstrous cupule was interpreted as supporting evidence for the hypothesis.     

Floral development in bolting garlic

Garlic (Allium sativum L.) is a completely sterile plant, propagated only vegetatively. The aim of this research was to study the sequence of morphological processes occurring during floral initiation and development of a number of bolting garlic accessions from the Allium gene bank in Israel by using SEM. The garlic inflorescence is an umbel-like flower arrangement, the branches (flower clusters) of which arise from a common meristem. The numerous flowers have a distinct morphology typical of the genus Allium. Flower-stalk elongation precedes the swelling of the apical meristem and its subdivision into several centers of floral development. Within clusters, floral primordia develop unevenly. Differentiation of topsets begins after floral differentiation on the peripheral part of the apical surface, and their size, number and rate of development vary among genotypes. At least four morphological types differing in flower/topset ratio were distinguished among the 12 clones studied in this investigation. For further studies of flowering physiology and fertility restoration, only clones which can differentiate the greatest proportion of normal flowers and the least of topsets in the apical meristem should be selected. Received: 28 June 2000 / Revision accepted: 6 November 2000