THE INTRAFLORAL PHENOLOGY OF STREPTANTHUS TORTUOSUS (BRASSICACEAE)

The sequence of floral events during anthesis was examined in Streptanthus tortuosus to determine the relationship between the male and female floral phases. The flowers are strongly protandrous. In the staminate phase, the anthers mature sequentially over a 3–4-day period. Because pollinators quickly remove pollen from the anthers, sequential anther maturation prolongs the male phase relative to what it would be if anthers did not mature sequentially. Pollen applied to the stigma during the staminate phase does not adhere readily and does not germinate. The length of the pistillate phase depends on pollinator activity, as pollination accelerates the abscission of floral parts. Unpollinated flowers remain pistillate for 3–4 days, during which time stigmatic receptivity declines gradually. In the field, 72% to 80% of flowers are staminate at any time, indicating that the staminate phase is three times longer than the pistillate phase when pollinators have access to the flowers. The consequences of the relative length of the floral phases and the schedule of stigmatic receptivity are discussed in terms of outcrossing mechanism, floral longevity, and sexual selection models.     

INVESTIGATIONS OF ANGIOSPERMS FROM THE EOCENE OF NORTH AMERICA: A CATKIN WITH JUGLANDACEOUS AFFINITIES

Three specimens of one type of fossil catkin from the Middle Eocene of Tennessee are excellently preserved and have been investigated morphologically. The flowers on these catkins are subtended by elongate, three-lobed bracts, are exclusively staminate, and have three conspicuous, obovate, perianth parts that bear large peltate scales. The stamens are well preserved and contain triporate pollen grains that are equivalent to the dispersed pollen genus Momipites. Floral morphology, cuticular features, and pollen indicate close affinities with the extant genera Engelhardia, Oreomunnea, and Alfaroa of the Juglandaceae; but because the fossil catkins are distinct and are a dispersed plant organ, they are placed in a new form genus: Eokachyra. These fossil flowers represent a rare opportunity to correlate the micro- and macrofossil record and to compare the relative rates of evolution of these features. The fossil catkins also demonstrate that much structural information may be gained from the study of fossil angiosperm flowers. The similarities between the staminate flowers of the fossil catkins and the staminate flowers of Engelhardia, Oreomunnea, and Alfaroa confirm the idea that this complex has had a long evolutionary history and suggest that the pollination system of certain extant genera was well developed during Middle Eocene times.     

CLONAL STRUCTURE OF ARCTIC DWARF BIRCH (BETULA GLANDULOSA) AT ITS NORTHERN LIMIT

Betula glandulosa Michx. (Betulaceae) at Tarr Inlet, Baffin Island, Northwest Territories reproduces primarily by vegetative layering with little, if any, viable seed production or seedling recruitment. Information from polymorphic enzyme loci revealed 15 three-locus genotypes with a single genotype usually dominating each of the nine sampled sites. Clonal reproduction combined with self-incompatibility may be one factor contributing to a decreased seed set by reducing the probability of xenogamy. However, multiple genotypes were found at some sites providing an opportunity for cross compatible pollen to effect fertilization. The lack of seed set in these areas appears to be due to additional factors inhibiting sexual reproduction. Based on pollen records, the present limited distribution of Betula glandulosa on Baffin Island represents the remnants of once more widely distributed populations, and the failure of sexual reproduction at the northern edge of its range may be responsible for the decline in abundance. Genotypic diversity observed in the Tarr Inlet area appears to be residual from a once sexually reproducing population. No evidence was found for differential selection of genotypes indicating that random processes are probably eroding the genotypic diversity during the decrease of population size in this species.     

SEX EXPRESSION,BREEDING SYSTEM,AND POLLEN BIOLOGY OF RICINOCARPOS PINIFOLIUS: A CASE OF ANDRODIOECY?

A population of 54 Ricinocarpos pinifolius (Euphorbiaceae) plants contained male plants, which produced only staminate flowers, and hermaphrodites, which produced staminate and pistillate flowers. The fraction of pistillate flowers ranged continuously from 0 to 0.68. Insect pollination was effective and fruit set virtually complete except for losses to herbivores. Self pollen, outcross pollen from male plants, and outcross pollen from hermaphrodites were all equivalent in viability, germination, tube growth, ovule penetration, and fruit setting ability. Inbreeding depression was manifested as late abortion of some selfed seeds. Geitonogamous selfing is largely prevented by temporal separation of male and female functions within plants. This temporal separation, combined with population-wide synchrony of flowering, may create unusual conditions allowing male plants at low frequency to match hermaphrodites in reproductive success.     

Reproductive Biology of the Monoecious Understory Palm Prestoea schultzeana in Amazonian Ecuador1

Prestoea schultzeana is a monoecious, protandrous palm in the forest understory of Amazonian Ecuador. We studied its leaf production, population density, sexual expression, phenology, pollination, and the specificity of the floral visitors. On average, 1.4 leaves and 0.9 inflorescences are produced per individual per year. The number of staminate flowers per inflorescence is relatively constant compared with the number of pistillate flowers which varies greatly. Flowering occurs in staminate and pistillate phases of approximately 19 and 0–7 days duration, respectively. Flowers open in the morning, and staminate flowers abscise in the afternoon of the same day whereas pistillate flowers last for two days. Flowers are whitish-yellow with a sweet odor and produce nectar. They were visited by Coleoptera (Chrysomelidae, Curculionidae, Nitidulidae, Ptiliidae, Staphylinidae), Hemiptera, Diptera (Drosophilidae, Syrphidae, Ceratopogonidae), Lepidoptera (Nymphalidae), and Hymenoptera (Formicidae, Halictidae). All examined individuals of the syrphid fly Copestylum sp. visiting pistillate flowers carried 100–500 grains of P. schultzeana pollen. Pollen occurred on all body parts, but especially on the legs, and this makes Copestylum sp. the most important pollinator. Most floral visitors were also frequent on the flowers of co-occurring plant species; notably the palm Hyospathe elegans shared most visitor species with P. schultzeana.     

The dynamics of pollen removal and deposition,and its effects on sexual phases in a protandrous plant: Glechoma longituba (Lamiaceae)

The duration of sexual phases in dichogamous plants are affected by many factors. Using both experimental and observational studies, we investigated natural patterns of pollen removal and deposition, visiting frequency of pollinators, patterns of nectar secretion, and effects of pollen removal and stigmatic pollen deposition on the duration of sexual phases in a protandrous plant, Glechoma longituba. We found that visiting frequency of pollinators correlated with the nectar secretion pattern. The nectar volume during the male phase was higher than during the female phase. In the morning, the main pollinator, the bee Anthophora plumipes, mainly foraged for nectar and showed no preference for flowers in male or female phase, despite male phase flowers producing higher amounts of nectar. However, in the afternoon, they changed their behavior and foraged mainly for pollen, and then showed a preference for flowers in male phase. Furthermore, the rates of pollen removal and stigmatic pollen deposition can affect the starting time and the duration of the female phase. When pollen removal and pollination rates are low due to scarcity of pollinator services, the sexual phase can be prolonged, leading to an overlap, and thereby enhance the chance for sexual reproduction through pollinator‐facilitated self‐pollination. We consider the variation of sexual phases in Glechoma longituba an adaptive mechanism prepared for both cross‐pollination enhancement and reproductive assurance depending on the available pollination services.     

FLOWERING,SEX RATIOS,POLLEN-OVULE RATIOS,FRUIT SET,AND REPRODUCTIVE EFFORT OF A DIOECIOUS TREE,MYRISTICA INSIPIDA (MYRISTICACEAE), IN TWO DIFFERENT RAIN FOREST COMMUNITIES

The flowering of Myristica insipida R. Br. was studied in two rain forest communities in northern Queensland. This dioecious, subcanopy tree had a male-biased sex ratio at both study sites. In the lowland population the male-bias could be attributed to males (trees producing staminate flowers) starting to flower at a smaller average size than females (trees producing pistillate flowers). There were no intersexual differences in spacing or distribution within the study sites. Males trees flowered earlier, flowered longer, and produced over twice as many flowers as females during the study season. Although the onset of flowering was rather variable, 18–22 days following heavy rains, most trees had a synchronous period of maximum flowering. Pollination manipulations determined that there was no fruit development without pollination, and that increasing pollen loads resulted in increased fruit set with diminishing effect. Taking into account the sex-ratios and intersexual differences in flower production, the pollen-ovule ratio was calculated to be 16,000–19,000. Male trees were found to expend more energy on flowering than female trees. Open-pollination resulted in 1.0% of female flowers setting fruit. The much greater cost of fruit production resulted in females expending 421% more energy on reproduction than males. Fruit and seed production were judged to be pollination-limited. Nonetheless, this species exhibited several characteristics that are predicted if dioecy evolved by means of sexual selection.     

POLLEN REMOVAL AND POLLEN DEPOSITION AFFECT THE DURATION OF THE STAMINATE AND PISTILLATE PHASES IN CAMPANULA RAPUNCULOIDES

We examined factors affecting the duration of the staminate and pistillate phases in the protandrous flowers of Campanula rapunculoides L. (Campanulaceae). Under conditions of natural pollinator visitation, flowers experiencing low rates of pollen removal lasted significantly longer than flowers that had faster rates of pollen removal. Experimental manipulations showed that low levels of pollen removal resulted in extension of the staminate phase. Hand-pollinations in which we varied the amount and source of pollen showed that when the number of fertilized ovules within an ovary is low, senescence of the flower is delayed, resulting in extension of the pistillate phase. We also report on pollinator foraging patterns within the vertical inflorescences of C. rapunculoides and the limiting factor for seed set in this population. The results are relevent to recent suggestions that floral characters often serve to reduce interference between the sexual functions in cosexual plants.     

SEX DIFFERENTIAL FLORAL LONGEVITY,NECTAR SECRETION,AND POLLINATOR FORAGING IN A PROTANDROUS SPECIES

Sex differential nectar production, floral longevity and pollinator foraging were examined in Lobelia cardinalis, a self-compatible, protandrous species that is hummingbird pollinated. The staminate phase of the flowers lasts significantly longer and produces significantly more nectar (total sugar) per day than the pistillate phase of the flowers. Additional pollen is presented throughout the staminate phase. Because inflorescences of L. cardinalis mature acropetally, the nectar reward on any given day is greatest at the top of the inflorescence (where staminate phase flowers are located). Hummingbirds appear to be sensitive to this pattern of nectar presentation as they most commonly began foraging in the middle of an inflorescence and proceeded upward. This foraging pattern tends to promote outcrossing and suggests that staminate phase flowers are visited more often than pistillate phase flowers. We conclude that L. cardinalis emphasizes the male function at anthesis. Others have hypothesized that the features of this species are a logical consequence of intrasexual selection, but further research is needed before we place great confidence in a sexual selection interpretation of our data.     

Subdioecy in Consolea spinosissima (Cactaceae): breeding system and embryological studies

The breeding system and the embryology of Consolea spinosissima, a tree-like opuntioid endemic to Jamaica, were investigated. Morphological and embryological studies revealed that the species is subdioecious, with three sexual morphs present in the 150 × 120 m plot studied at Hellshire Hills, Jamaica. The female morph has pistillate flowers with open stigma lobes, no pollen grains, and sets fruit. The male morph has cryptic staminate flowers with closed stigma lobes, viable pollen grains, and a nonfunctional gynoecium that does not set seed. The weak hermaphrodite morph has low fruit set and "perfect" flowers that superficially resemble the functionally staminate flowers of the male morph. These perfect flowers reach anthesis with viable pollen grains, with no or only a few functional ovules, and with the style supporting pollen tube growth. Embryological studies showed that the critical stage for sex determination occurs earlier in pistillate than in staminate and perfect flowers. Anthers of pistillate flowers abort prior to microspore tetrad formation, whereas ovules of the staminate and perfect flowers degenerate after the complete maturation of the embryo sac. Based on flower structure and embryological data, we hypothesize that the ancestor of C. spinosissima is/was hermaphroditic.     

Pollination in Geonoma macrostachys and Three Congeners,G. acaulis,G. gracilis,and G. interrupta

A continuous 15 month study of the floral ecology of four syntopic understorey palm species of Genoma was conducted in Amazonian Peru lowland rainforest. The spicate inflorescences of G. macrostachys, G. acaulis and G. gracilis are strictly protandrous and the plants are functionally dioecious. Data suggest that in G. macrostachys and G. acaulis pollination is based on a mimicry system, the pistillate flowers mimicking the staminate ones in colour, shape and scent. Pollen-collecting meliponine bees (Hymenoptera, Apidae, Meliponinae) and pollen-feeding syrphid flies (Diptera, Syrphidae) which visit inflorescences during both sexual stages are the pollinators of G. macrostachys. Geonoma acaulis is pollinated by small pollen-feeding weevils (Coleoptera, Curculionidae, Derelomini) that visit male and female spikes. Additionally, in G. macrostachys another pollinator type, viz. euglossine bees (Hymenoptera, Apidae, Euglossinae), which are attracted and rewarded by both types of flowers may account for long-distance pollination. The palm G. gracilis shows a very distinct pollination system. Although opportunistic insect visitors are attracted to the inflorescences of this species it seems to be mainly anemophilous because pollen becomes powdery during an thesis. The branched inflorescences of G. interrupta are also protandrous, but unlike the other species of Geonoma observed, staminate and pistillate anthesis of individual flowers are, for the most, overlapping. A broad spectrum of visitors is attracted (bees, wasps, flies, and beetles), which all may act as pollinators. Outcrossing is especially encouraged during the purely female phase at the end of the flowering cycle when there are no more staminate flowers in the inflorescence. Effects on the reproductive biology and population structure of different pollination systems and breeding system are discussed.     

Pollen movement to flowering canopies of pistillate individuals of three rain forest tree species in tropical Australia

This study measured the quantities of effective pollen vectors and their pollen loads arriving at the canopies of dioecious tropical rain forest trees in north-east Queensland. Population flowering synchrony, effective pollinator populations and pollen loads transferred between staminate and pistillate trees were compared among three insect-pollinated tree species. All three were visited by a wide range of insects, 75% of which (mostly 3–6 mm long) carried conspecific pollen. Fewer than 8% of individual insects were found to be carrying single-species pollen exclusively and none could be described as specialist pollen foragers. The introduced honeybee carried greater quantities of pollen than any native species but was not necessarily a reliable pollinator. The brief flowering periods in Neolitsea dealbata (3–4 weeks) and Litsea leefeana (4–5 weeks) populations were synchronized among individuals. Flowering in the Diospyros pentamera population extended over 15 weeks and most individuals were in flower for most of this period. Staminate trees began flowering earlier, produced more flowers and attracted relatively more insects than did pistillate trees, suggesting a density-dependent response of pollinators to flowering performance. Pollen was trapped in greater quantities on insects at staminate trees than at pistillate trees. Insect numbers increased at peak flowering periods and Diptera were the most abundant flower visitors. Anthophilous Coleoptera were more numerous at staminate than at pistillate trees in all three tree species populations. Larger quantities of pollen were mobilized during peak flowering times although the greatest quantities were transferred to pistillate canopies towards the end of the population flowering periods. Diptera carried pollen more often to pistillate N. dealbata and L. leefeana trees than did other groups whereas Coleoptera carried pollen more often to pistillate D. pentamera trees. The two contrasting flowering performances in the three tree species are discussed with reference to mechanisms that facilitate pollen transfer between staminate and pistillate trees.     

Morphological variation in the flowers of Jacaratia mexicana A. DC. (Caricaceae), a subdioecious tree

The Caricaceae is a small family of tropical trees and herbs in which most species are dioecious. In the present study, we extend our previous work on dioecy in the Caricaceae, characterising the morphological variation in sexual expression in flowers of the dioecious tree Jacaratia mexicana . We found that, in J. mexicana , female plants produce only pistillate flowers, while male plants are sexually variable and can bear three different types of flowers: staminate, pistillate and perfect. To characterise the distinct types of flowers, we measured 26 morphological variables. Our results indicate that: (i) pistillate flowers from male trees carry healthy-looking ovules and are morphologically similar, although smaller than, pistillate flowers on female plants; (ii) staminate flowers have a rudimentary, non-functional pistil and are the only flowers capable of producing nectar; and (iii) perfect flowers produce healthy-looking ovules and pollen, but have smaller ovaries than pistillate flowers and fewer anthers than staminate flowers, and do not produce nectar. The restriction of sexual variation to male trees is consistent with the evolutionary path of dioecy from hermaphrodite ancestors through the initial invasion of male-sterile plants and a subsequent gradual reduction in female fertility in cosexual individuals (gynodioecy pathway), but further work is needed to confirm this hypothesis.     

Resource partitioning to male and female flowers of Spinacia oleracea L. in relation to whole-plant monocarpic senescence

Male plants of spinach (Spinacea oleracea L.) senesce following flowering. It has been suggested that nutrient drain by male flowers is insufficient to trigger senescence. The partitioning of radiolabelled photosynthate between vegetative and reproductive tissue was compared in male (staminate) versus female (pistillate) plants. After the start of flowering staminate plants senesce 3 weeks earlier than pistillate plants. Soon after the start of flowering, staminate plants allocated several times as much photosynthate to flowering structures as did pistillate plants. The buds of staminate flowers with developing pollen had the greatest draw of photosynthate. When the staminate plants begin to show senescence 68% of fixed C was allocated to the staminate reproductive structures. In the pistillate plants, export to the developing fruits and young flowers remained near 10% until mid-reproductive development, when it increased to 40%, declining to 27% as the plants started to senesce. These differences were also present on a sink-mass corrected basis. Flowers on staminate spinach plants develop faster than pistillate flowers and have a greater draw of photosynthate than do pistillate flowers and fruits, although for a shorter period. Pistillate plants also produce more leaf area within the inflorescence to sustain the developing fruits. The (14)C in the staminate flowers declined due to respiration, especially during pollen maturation; no such loss occurred in pistillate reproductive structures. The partitioning to the reproductive structures correlates with the greater production of floral versus vegetative tissue in staminate plants and their more rapid senescence. As at senescence the leaves still had adequate carbohydrate, the resources are clearly phloem-transported compounds other than carbohydrates. The extent of the resource redistribution to reproductive structures and away from the development of new vegetative sinks, starting very early in the reproductive phase, is sufficient to account for the triggering of senescence in the rest of the plant.     

Thrips pollination of androdioecious Castilla elastica (Moraceae) in a seasonal tropical forest

Androdioecy is a rare sexual system in nature, as predicted theoretically. Among the androecious species reported so far, Castilla elastica (Moraceae) is unique in that flowers are unisexual and staminate and pistillate flowers on cosexual plants are produced on different inflorescences. In addition, inflorescence structure of staminate inflorescences on males and staminate and pistillate inflorescences on cosexes is markedly different. Staminate inflorescences on males are bivalvate, while staminate inflorescences on cosexes are "fig-like" and urceolate. Pistillate inflorescences are discoidal. The difference may reflect different roles and requirements of the three inflorescences in pollination and protection from herbivores. This study reports thrips pollination of C. elastica, demonstrated by a pollinator introduction experiment. Thrips pollination of the species may be an example of mutualism originating from plant-herbivore interactions. In the Moraceae, shifts from simple herbivores on flowers to pollinators might have occurred many times, evolving into diverse pollination systems including the fig-fig wasp mutualism. The family, of which little is known about pollination systems, provides interesting and unique opportunities to study evolution of pollination systems and roles of nonpollinating associates of inflorescences.     

Specialised protagonists in a plant–pollinator interaction: the pollination of Blumenbachia insignis (Loasaceae)

• Analyses of resource presentation, floral morphology and pollinator behaviour are essential for understanding specialised plant‐pollinator systems. We investigated whether foraging by individual bee pollinators fits the floral morphology and functioning of Blumenbachia insignis, whose flowers are characterised by a nectar scale‐staminode complex and pollen release by thigmonastic stamen movements.
• We described pollen and nectar presentation, analysed the breeding system and the foraging strategy of bee pollinators. We determined the nectar production pattern and documented variations in the longevity of floral phases and stigmatic pollen loads of pollinator‐visited and unvisited flowers.
• Bicolletes indigoticus (Colletidae) was the sole pollinator with females revisiting flowers in staminate and pistillate phases at short intervals, guaranteeing cross‐pollen flow. Nectar stored in the nectar scale‐staminode complex had a high sugar concentration and was produced continuously in minute amounts (~0.09 μl·h^?1). Pushing the scales outward, bees took up nectar, triggering stamen movements and accelerating pollen presentation. Experimental simulation of this nectar uptake increased the number of moved stamens per hour by a factor of four. Flowers visited by pollinators received six‐fold more pollen on the stigma than unvisited flowers, had shortened staminate and pistillate phases and increased fruit and seed set.
• Flower handling and foraging by Bicolletes indigoticus were consonant with the complex flower morphology and functioning of Blumenbachia insignis. Continuous nectar production in minute quantities but at high sugar concentration influences the pollen foraging of the bees. Partitioning of resources lead to absolute flower fidelity and stereotyped foraging behaviour by the sole effective oligolectic bee pollinator.

     

Neither insects nor wind: ambophily in dioecious Chamaedorea palms (Arecaceae)

Pollination of Neotropical dioecious trees is commonly related to generalist insects. Similar data for non‐tree species with separated genders are inconclusive. Recent studies on pollination of dioecious Chamaedorea palms (Arecaceae) suggest that species are either insect‐ or wind‐pollinated. However, the wide variety of inflorescence and floral attributes within the genus suggests mixed pollination mode involving entomophily and anemophily. To evaluate this hypothesis, we studied the pollination of Chamaedorea costaricana, C. macrospadix, C. pinnatifrons and C. tepejilote in two montane forests in Costa Rica. A complementary morphological analysis of floral traits was carried out to distinguish species groups within the genus according to their most probable pollination mechanism. We conducted pollinator exclusion experiments, field observations on visitors to pistillate and staminate inflorescences, and trapped airborne pollen. A cluster analysis using 18 floral traits selected for their association with wind and insect pollination syndromes was carried out using 52 Chamaedorea species. Exclusion experiments showed that both wind and insects, mostly thrips (Thysanoptera), pollinated the studied species. Thrips used staminate inflorescences as brood sites and pollinated pistillate flowers by deception. Insects caught on pistillate inflorescences transported pollen, while traps proved that pollen is wind‐borne. Our empirical findings clearly suggest that pollination of dioecious Chamaedorea palms is likely to involve both insects and wind. A cluster analysis showed that the majority of studied species have a combination of floral traits that allow for both pollination modes. Our pollination experiments and morphological analysis both suggest that while some species may be completely entomophilous or anemophilous, ambophily might be a common condition within Chamaedorea. Our results propose a higher diversity of pollination mechanisms of Neotropical dioecious species than previously suggested.     

The role of growth regulators in the differentiation of flowers and inflorescences

Growth regulators participate in the differentiation of floral parts, determining the developmental path of the respective type of inflorescence. The effect depends on the expression of the peculiarities of floral part differentiation, the recognition of the character of endogenous substances in certain stages and the choice of the suitable regulator for application. In the primitive flower ofPapaver petals and stamens are formed from the peripheral meristem with a lower content of auxins and a higher level of gibberellic substances. The pistil arises later from central tissues with a higher level of auxins and inhibitory substances. The stamens are more sensitive to the higher level of auxin substances, and by a suitable application of GA₃ and BAP they can be transformed into petals; in this way double flower forms arise. In the differentiation of floral parts ofCampanula, Rosa andMelandrium similar regularities assert themselves in time successions, but in another spatial arrangement. Sex differentiation of diclinous flowers ofMelandrium is based on differences in heterochromosomes XY and XX. The rise of the zygomorphic flower ofVeronica is accompanied by a different distribution of endogenous substances which affect the development of petals, stamens and the pistil. The differentiation of flowers in the racemose inflorescence occurs in the acropetal succession, and lateral primordia inCampanula develop into actinomorphic regular flowers, whereas inDigitalis they are zygomorphic and only the terminal flower is peloric. In the initial phases the staminate tassel and the pistillate ear in maize are identical. Earlier differentiation of the terminal pistillate tassel is connected with a higher level of gibberellins and the later development of the lateral pistillate ear is accompanied by the increase in auxin-like substances and inhibitions. Similar correlations were found in the development of staminate catkins and the differentiation of pistillate flowers in terminal buds ofJuglans regia. By the application of auxin-like substances it is possible to achieve the transformation of primordia of the staminate tassel into the pistillate ear in maize or to regulate the number of staminate catkins and pistillate flowers on twigs of the walnut tree. In the capitulum of the sunflower differences arise between peripheral pistillate ray flowers and hermaphrodite tubular ones. By applying GA₃ and BAP the number of ray flowers is increased. If the normal course of inflorescence differentiation is affected with a suitable type of regulator, a range of floral abnormalities appears which permit to assess the intervention in different developmental stages and the reaction of the primordium to the applied type of regulator. Abnormalities also suggest some phylogenetic correlations.     

MORPHOLOGY,VASCULAR ANATOMY AND EMBRYOLOGY OF PISTILLATE AND STAMINATE FLOWERS OF ASPARAGUS OFFICINALIS

Flowers of three pistillate (female), two heterogametic staminate (male) and two homogametic male genotypes of Asparagus officinalis L. were compared for morphology and vascular anatomy of the flower and for embryological development to the stage of mature ovules and pollen. Flowers are liliaceous, the staminate with rudimentary pistils and the pistillate with collapsed anthers. The uncomplicated vascular pattern differs between staminate and pistillate flowers only in the size and degree of maturation of bundles to stamens and carpels. Longer styles appear to be correlated with a greater extent of ovule development in ovaries of staminate flowers. Microsporogenesis in males is normal with wall development corresponding to the Monocotyledonous type. The tapetum is glandular and binucleate, cytokinesis successive, the tetrads isobilateral or occasionally decussate, and the mature pollen grain two-celled. A pair of heteromorphic, possibly sex, chromosomes was observed in heterogametic male plants. Anther development is initially the same in pistillate flowers, but the tapetum degenerates precociously followed by collapse of microspore mother cells. In pistillate flowers the ovules are hemitropous, bitegmic, and slightly crassinucellate. Megasporogenesis-megagametogenesis conforms to the Polygonum type. In staminate flowers ovule development is like that in pistillate flowers until degeneration starts in nucellar and integumentary cells at the chalazal end. Ovules in both homogametic male genotypes rarely complete meiosis, while in the heterogametic males it is normally completed with about one ovule in 20 flowers forming a mature megagametophyte. Since manipulation of sex expression in Asparagus could be important in developing inbred male and female lines for breeding purposes, those aspects of the morphological and embryological observations presented which might be useful in planning experiments to induce sex changes are discussed briefly.