THE STIGMATIC PAPILLAE OF AMYEMA (LORANTHACEAE): DEVELOPMENTAL RESPONSES TO PROTANDRY AND SURFACE ADAPTATIONS FOR BIRD POLLINATION

The flowers of Amyema miquelii and A. miraculosum are protandrous and pollinated by birds. Their dry-type stigmatic surface is composed of unicellular papillae. At the male phase, these papillae are constricted with rugulose surfaces. During the transition to the female (pollen receptive) phase these cells expand, almost doubling in width while their surface becomes much smoother. Beneath the thin proteinaceous pellicle, the papillar wall consists of an extraordinarily thick bi-layered cuticle overlying the primary wall. The two layers of the cuticle are stained by lipid dyes, but are distinguished by their different responses to other cytochemical tests. The reaction product for the enzyme esterase is present within crenulations on the papillar surface in small amounts, and in dense deposits in the cuticular clefts at the base between papillae. Not surprisingly, pollen tubes are unable to penetrate the thick papillar cap and enter the style through these clefts. The unusual thickness of the cuticle is interpreted as an adaptive response to pollination by perching birds (passerines) probing for nectar.     

Papillar integrity as an indicator of stigmatic receptivity in kiwifruit (Actinidia deliciosa)

Stigmatic receptivity is a major factor limiting fruit set inkiwifruit (Actinidia deliciosa). The aim of this work was toknow what determines the cessation of stigmatic receptivityin this species. Stigmatic receptivity has been analysed inkiwifruit through the capacity of the stigma to sustain pollengermination and has been related to stigmatic development anddegeneration.The stigma of kiwifruit flowers has a papillatesurface which from anthesis is covered with an abundant exudate.Papillae are unicellular and contain a number of phenolic deposits.During the lifetime of the flower these papillae gradually loseturgidity, while stigmatic secretion increases until 5 d afteranthesis when papillae start rupturing and the papillar contentis liberated into the germination medium. Stigmatic receptivityis high at anthesis and lasts for the next 4 d. However, itdrastically decreases 5 d after anthesis and it is nil 2 d later.The pattern of stigmatic receptivity closely fits that of papillarintegrity, indicating that stigmatic receptivity relies on thisintegrity. Since papillar integrity can easily be evaluatedin stigmatic arms, this criterion can be used as a quick methodto estimate stigmatic receptivity. Key words: Actinidia deliciosa, kiwifruit, stigma, receptivity, pollen     

STIGMA OF SOLANUM TUBEROSUM CV SHEPODY: MORPHOLOGY,ULTRASTRUCTURE,AND SECRETION

The stigma of Solarium tuberosum L. cv Shepody has a bilobed papillate surface covered with a viscous secretion at anthesis. The secretion originates as osmiophilic droplets in the cytoplasm, accumulates in the intercellular space, and fills the base of the papillae, after lifting and rupturing the cuticular layer covering the stigma surface. Cytochemical evidence shows that the stigmatic secretion is lipidic in nature; it did not stain with the periodic acid-Schiffs reaction for carbohydrate or Coomassie Brilliant Blue R-250 for proteins, but did stain with Sudan black or Nile red, a fluorescence probe for lipids. Sodium dodecylsulfate-polyacrylamide gel electrophoresis has revealed that the secretion contained three polypeptides that appeared to be 'indigenous' to the stigmatic secretion, and not contaminants. Comparative analyses of this and other proteins found in stigma secretions may provide clues to their possible roles in pollen grain adhesion and germination.     

Structural analysis of stigma development in relation with pollen–stigma interaction in sunflower

Structural analysis of stigma development in sunflower highlights the secretory role of papillae due to its semi-dry nature. Production of lipid-rich secretions is initiated at the staminate stage of the flowers in stigma development and increases at the receptive stage, coinciding with an extensive development of elaioplasts and endoplasmic reticulum network in the basal region of the papillae. Transfer cells, earlier identified only in the wet type of stigma, are also present in the transmitting tissue of the sunflower stigma. Attainment of physiological maturity by the stigmatic tissue, accompanying development from bud to pistillate stage, appears to affect the initial steps of pollen–stigma interaction. The nature of self-incompatibility in Helianthus has also been investigated in relation with pollen adhesion, hydration and germination. Pollen adhesion to the stigma is a rapid process in sunflower and stigma papillae exhibit greater affinity for pollen during cross pollination as compared to self-pollination. Components of the pollen coat and the pellicle on the surface of stigmatic papillae are critical for the initial phase of pollen–stigma interaction (adhesion and hydration). The lipidic components of pollen coat and the proteinaceous and lipidic components from the surface of the papillae coalesce during adhesion, leading to the movement of water from stigma to the pollen, thereby causing pollen hydration and its subsequent germination. Pollen germination (both in self-and cross-pollen) on the stigma surface and the growth of the pollen tube characterize the flexibility of self-incompatibility in sunflower. Compatible pollen grains germinate and the pollen tube penetrates the stigma surface to enter the nutrient-rich transmitting tissue. The pollen tube from incompatible pollen germination, however, fails to penetrate the stigmatic tissue and it grows parallel to the papillae. Present findings provide new insights into structural and functional relationships during stigma development and pollen–stigma interaction.     

Characterization of C₃--C₄ intermediate species in the genus Heliotropium L. (Boraginaceae): anatomy, ultrastructure and enzyme activity

Photosynthetic pathway characteristics were studied in nine species of Heliotropium (sensu lato, including Euploca), using assessments of leaf anatomy and ultrastructure, activities of PEP carboxylase and C₄ acid decarboxylases, and immunolocalization of ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) and the P‐subunit of glycine decarboxylase (GDC). Heliotropium europaeum, Heliotropium calcicola and Heliotropium tenellum are C₃ plants, while Heliotropium texanum and Heliotropium polyphyllum are C₄ species. Heliotropium procumbens and Heliotropium karwinskyi are functionally C₃, but exhibit ‘proto‐Kranz’ anatomy where bundle sheath (BS) cells are enlarged and mitochondria primarily occur along the centripetal (inner) wall of the BS cells; GDC is present throughout the leaf. Heliotropium convolvulaceum and Heliotropium greggii are C₃–C₄ intermediates, with Kranz‐like enlargement of the BS cells, localization of mitochondria along the inner BS wall and a loss of GDC in the mesophyll (M) tissue. These C₃–C₄ species of Heliotropium probably shuttle photorespiratory glycine from the M to the BS tissue for decarboxylation. Heliotropium represents an important new model for studying C₄ evolution. Where existing models such as Flaveria emphasize diversification of C₃–C₄ intermediates, Heliotropium has numerous C₃ species expressing proto‐Kranz traits that could represent a critical initial phase in the evolutionary origin of C₄ photosynthesis.     

ROLE OF CALCIUM IN THE CALLOSE RESPONSE OF SELF-POLLINATED BRASSICA STIGMAS

In Brassica oleracea, sporophytic self-incompatibility prevents germination of self pollen, or normal growth of self pollen tubes. After self-pollination, the papillae of stigmas synthesize callose. The role of Ca⁺⁺ in the formation of stigmatic callose was tested by adding compounds that interact with Ca⁺⁺ to suspensions of pollen that were known to induce callose formation in self stigmas. The calcium channel antagonist, lanthanum, and the calcium chelating agent, EGTA, reduced or abolished the callose response to self-pollen suspensions. In the presence of Ca⁺⁺, the calcium ionophore, A23187, induced callose in stigmatic papillae when added to pollen suspensions, or alone. Therefore, callose deposition in response to incompatible pollinations appears to be a calcium-dependent process. Pretreatment of pistils with 100 μm 2-deoxy-D-glucose abolished the callose response to self-pollination, while self pollen remained inhibited and cross pollen grew normally in treated pistils. Thus, callose formation in the stigma is not an essential part of the self-incompatibility mechanism preventing the growth of self pollen in Brassica.     

POLLEN-PISTIL INTERACTIONS IN TRISTYLOUS PONTEDERIA SAGITTATA (PONTEDERIACEAE). I. FLORAL HETEROMORPHISM AND STRUCTURAL FEATURES OF THE POLLEN TUBE PATHWAY

Heteromorphic characters and structural features of the pollen tube pathway are described in tristylous Pontederia sagittata to assess their influence on the pollination process and in mediating self-incompatibility behavior. Heteromorphic characters that distinguish the floral morphs include style length, stigma depth, stigmatic papillae length, stylar parenchyma cell length, area of the stylar canal, stamen height, anther size, and pollen size. Unlike several distylous species that have been investigated, the exine of pollen in P. sagittata was not strongly differentiated among the pollen types, and no differences in stigma cytochemistry were apparent. Features common to the floral morphs were a wet stigma, a hollow trilobed stylar canal separating into two sterile and one fertile canal, and a single anatropous ovule with a highly elaborated integumentary obturator. The similarity in structural features of the pollen tube pathway of P. sagittata to those found in monocotyledonous taxa with gametophytic self-incompatibility suggests that phylogenetic constraints may have influenced the evolution of trimorphic incompatibility in the Pontederiaceae.     

Comparative floral structure and systematics in Crossosomatales (Crossosomataceae, Stachyuraceae, Staphyleaceae, Aphloiaceae, Geissolomataceae, Ixerbaceae, Strasburgeriaceae)

Floral structure of all putative families of Crossosomatales as suggested by molecular studies was comparatively studied. The seven comprise Crossosomataceae, Stachyuraceae, Staphyleaceae, Aphloiaceae, Geissolomataceae, Ixerbaceae, and Strasburgeriaceae. The entire clade (1) is highly supported by floral structure, also the clades (in sequence of diminishing structural support): Ixerbaceae/Strasburgeriaceae (2), Geissolomataceae/Ixerbaceae/Strasburgeriaceae (3), Aphloiaceae/Geissolomataceae/Ixerbaceae/Strasburgeriaceae (4), and Crossosomataceae/Stachyuraceae/Staphyleaceae (5). Among the prominent floral features of Crossosomatales (1) are solitary flowers, presence of a floral cup, imbricate sepals with outermost smaller than inner, pollen grains with horizontally extended endoapertures, shortly stalked gynoecium, postgenitally united carpel tips forming a compitum, stigmatic papillae two‐ or more‐cellular, ovary locules tapering upwards, long integuments forming zigzag micropyles, cell clusters with bundles of long yellow crystals, mucilage cells, seeds with smooth, sclerified testa and without a differentiated tegmen. Clade (2) is characterized by large flowers, petals forming a tight, pointed cone in bud, stamens with long, stout filaments and sagittate anthers, streamlined, conical gynoecium, antitropous ovules, rudimentary aril, lignified, unicellular, T‐shaped hairs and idioblasts with striate mucilaginous cell walls. Clade (3) is characterized by alternisepalous carpels, punctiform stigma formed by postgenitally united and twisted carpel tips, synascidiate ovary, only one or two pendant ovules per carpel, nectary recesses between androecium and gynoecium. Clade (4) is characterized by pronounced ‘pollen buds’. Clade (5) is characterized by polygamous or functionally unisexual flowers, x‐shaped anthers, free and follicular carpels (not in Stachyuraceae). Crossosomataceae and Aphloiaceae, although not retrieved as a clade in molecular studies, share several special floral features: polystemonous androecium; basifixed anthers without a connective protrusion; stigma with two more or less decurrent crests; camplyotropous ovules and reniform seeds; simple, disc‐shaped nectaries and absence of hairs. © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society, 2005, 147 , 1–46.     

Unravelling the stylar polymorphism in Melochia (Malvaceae): reciprocity and ancillary characters

Heterostyly is a genetic polymorphism in which plant populations possess two (distyly) or three (tristyly) morphs with flowers differing reciprocally in stigma and anther height. Sex organ deployment has been described as being highly variable among and within species of several distylous taxa belonging to different taxonomic groups. However, the number of studies considering within‐species disparities is still limited. For a better overview of the existing amount of variation that can occur within and between heterostylous species, we sampled 46 populations of six Melochia spp., a style‐polymorphic genus in Cuba. We characterized the floral morphology in all populations and described a set of ancillary characters per species. All of these Melochia spp. are distylous, except for the monomorphic M. nodiflora. The S‐morph produces fewer, larger pollen than L‐morphs, and has verrucose ornamentation. The L‐morph produces reticulate pollen and has larger stigmatic papillae than the S‐morph. The monomorphic M. nodiflora shows ancillary characteristics that are similar to the L‐morph individuals in the related species. As expected, there are differences in ancillary characters among species and also dissimilarities in reciprocity among and within species of Melochia. Our results highlight the importance of considering intraspecific variation in the morphometric characterization of heterostylous taxa. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 176 , 147–158.     

Localization of adenylate cyclase activity in Populus: its relation to pollen-pistil recognition and incompatibility

Summary Adenylate cyclase has been localized cytochemically in female and male parents as well as during the pollen-stigma interaction with an original technique employing strontium as the capture ion and adenyl imidodiphosphate as the specific substrate. The specificity of the reaction was checked by using several controls. No final specific reaction product was detected in unpollinated P. deltoides stigmas or in the P. deltoides or P. alba pollen grains used for compatible and incompatible pollinations. In the compatible cross between P. deltoides × P. deltoides, fine dense precipitates were observed in the dictyosomes and the plasma membrane and exterior to the exine of hydrated pollen grains adhering to the stigma surface. Labeling of the stigmatic pellicle was also observed after pollen adhesion and hydration. This was accompanied by a strong reactivity of the cell wall and plasmalemma of the stigma papillae at the sites of pollen tube germination on the stigma surface and at the sites of penetration of pollen tubes between adjacent papillae. In the incompatible cross between P. deltoides x P. alba, adenylate cyclase activity was still present but reduced at the stigma surface following adhesion, hydration, and germination of P. alba pollen. This activity was completely abolished after the penetration of pollen tubes between stigma papillae. These findings suggest that in Populus, adenylate cyclase activity is correlated to pollen adhesion, hydration, and germination at the stigma surface, and that the abolition of this enzyme activity could be one of the cellular events governing the gametophytic phenotype of incompatibility in the cross between P. deltoides and P. alba.     

A comparative analysis of the distribution and composition of lipidic constituents and associated enzymes in pollen and stigma of sunflower

Spatial distribution and compositional analyses of the lipidic constituents in pollen and stigma of sunflower (Helianthus annuus L. cv. Morden) were conducted using ultrastructural, histochemical, and biochemical analysis. Detection of secretions at the base of stigmatic papillae and neutral lipid accumulations on the surface of stigmatic papillae and between adjacent pseudopapillae demonstrates the semidry nature of stigma surface in sunflower. Pollen coat is richer in lipids (8%) than stigma (2.2%) on fresh weight basis. Nile Red-fluorescing neutral lipids are preferentially localized in the pollen coat. Neutral esters and triacylglycerols (TAGs) are the major lipidic constituents in pollen grains and stigma, respectively. Lignoceric acid (24:0) and cis-11-eicosenoic acid (20:1) are specifically expressed only in the pollen coat. Similar long-chain fatty acids have earlier been demonstrated to play a significant role during the initial signaling mechanism leading to hydration of pollen grains on the stigma surface. Lipase (EC 3.1.1.3) activity is expressed both in pollen grains and stigma. Stigma exhibits a better expression of acyl-ester hydrolase (EC 3.1.1.1) activity than that of observed in both the pollen fractions. Expression of two acyl-ester hydrolases (41 and 38 kDa) has been found to be specific to pollen coat. Specific expression of lignoceric acid (24:0) in pollen coat and localization of lipase in pollen and stigma have been discussed to assign possible roles that they might play during pollen–stigma interaction.     

Isolation and characterization of a polymorphic stigma-specific class III peroxidase gene from <Emphasis Type="Italic">Senecio squalidus</Emphasis> L. (Asteraceae)

A novel stigma-specific class III peroxidase gene, SSP (Stigma-Specific Peroxidase), has been isolated from the self-incompatible daisy Senecio squalidus L. (Asteraceae). Expression of SSP in flower buds is developmentally regulated, with maximal levels of expression coinciding with anthesis, when stigmas are most receptive to pollen and when self-incompatibility is fully developed. In situ hybridization revealed SSP expression to be localized exclusively to the specialized secretory epidermal cells (papillae) of the stigma, which receive and discriminate pollen. SSP is therefore the first tissue-specific and cell-specific peroxidase gene identified in a plant. SSP belongs to a distinct clade of class III plant peroxidases that possess two introns, instead of the more normal situation of three conserved introns. The deduced amino acid sequence of SSP revealed a 27 amino acid signal peptide, suggesting that the SSP protein is secreted to the cell wall of the stigmatic papillae. In-gel peroxidase activity assays showed that SSP has relatively low peroxidase activity compared to other, as yet uncharacterized, peroxidases present in stigmatic extracts. Six SSP alleles have been cloned from different lines of S. squalidus carrying a range of self-incompatibility (S)-alleles but there was no consistent association between the presence of a particular SSP allele and S-genotype indicating that SSP is not the female determinant of SSI in S. squalidus. Nevertheless, the precise expression of SSP in stigmatic papillae suggests that it may have a more general function in pollen–stigma interactions, or alternatively in protection of stigmas from pathogen attack. Extensive database screens have identified homologues of SSP in other plant species, but available expression data for these genes indicates that none are flower-specific, suggesting that SSP represents a new functional type of class III peroxidase specific to the stigma. We discuss the possible function(s) of S. squalidus SSP in pollen–stigma interactions and in protection of stigmas from pathogen attack.     

Reproductive biology of Nymphaea capensis Thunb. var. zanxibariensis (Casp.) Verdc. (Nymphaeaceae)

Anthesis in Nymphaea capensis var. zanzibariensis is diurnal with flowers opening and closing for three consecutive days. On the first day of anthesis, the stigmatic papillae secrete fluid and the outermost anthers are dehiscent. On the second day of anthesis the stamens form a cone above the dry stigmatic cup. The middle stamens open and turn outward. On the third day of flowering, all the stamens open and the dry stigmatic cup is exposed. The flowers are homogamous and not protogynous as the other Nymphaea. The gynoecium of the self-compatible N. capensis var. zanzibariensis , is characterized by a wet papillate stigma, a short hollow style, and secretory cells on the ventral surface of the ovary. The pollen is released on the receptive stigma. Following initial growth in intercellular spaces in the transmitting tract of the stigma, pollen tubes travel through the stylar canal and into the ovary.     

Quantitative evaluation of stigma polymorphism in a tristylous weed,Lythrum salicaria (Lythraceae)

Tristyly involves three different forms of flowers that differ reciprocally in the heights of stigmas and anthers within flowers. Apart from the style and stamen lengths, heterostylous species also demonstrate pollen and stigma polymorphisms. We quantified stigma polymorphism in tristylous Lythrum salicaria by measuring the stigma diameters, structure of papillae, and density and distribution of papillae on the stigma from flower samples of 201 individuals belonging to three morphs. The diameter of the stigma and the distribution of papillae were quantified using a scanning electron microscope, and the structure of papillae was determined using a light microscope. The stigma diameter in the long morph was significantly greater than in the mid and short morphs. While the density of stigmatic papillae was significantly greater in the mid and short morphs than in the long morph, the total number of papillae per stigma did not differ across morphs. The length and diameter of papillae at the apex, neck, and base were significantly greater in the long morph followed by the mid and short morphs. A discriminant function analysis separated the long morph from the mid and short morphs based on the canonical scores of measurements of papillae structure. The stigma polymorphism coupled with those of pollen may play a functional role in self-incompatibility mechanisms.     

3种生殖方式蒲公英花粉活力和柱头形态及可授性对比

利用离体萌发法、电镜扫描和联苯胺-过氧化氢法对有性生殖的东北蒲公英、专性无融合生殖的丹东蒲公英和兼性无融合生殖的蒙古蒲公英花粉活力、柱头表面形态及可授性进行观察和测定。结果显示:(1)3种蒲公英柱头均为干柱头,单细胞乳突,属于原始进化类型,都存在雌雄异熟和雌雄异位花部特征;东北蒲公英乳突饱满、紧密,蒙古蒲公英次之,丹东蒲公英干瘪、稀少。(2)东北蒲公英和丹东蒲公英柱头呈Y状时期花粉萌发率最高,分别为67.3%和19.8%,蒙古蒲公英柱头呈棒状时期萌发率最高,为23.3%。(3)3种蒲公英随着开花进行,柱头可授性增加,并且柱头在大Y状时期3种蒲公英为最佳授粉期,可授性柱头占样本柱头的比率分别是98.7%(东北蒲公英)、84.0%(蒙古蒲公英)、21.3%(丹东蒲公英)。研究表明,有性生殖蒲公英与无融合生殖蒲公英在花粉萌发力、柱头可授性和乳突上产生明显差别,为蒲公英在长期进化中性器官能量再分配提供证据。     

The effect of temperature on stigmatic receptivity in sweet cherry (Prunus avium L.)

Plant reproduction is highly vulnerable to environmental conditions such as temperature and, consequently, planet warming may have significant consequences on the reproductive phase with serious implication in agricultural crops. Although pollen tube growth is clearly affected by temperature, little information is available on its effect on the female side and on flower receptivity. In this work, the effect of temperature has been evaluated on stigmatic receptivity of sweet cherry in vivo, in the laboratory, and in planta, in the field. Results herein show that temperature has a clear effect on the duration of stigmatic receptivity. Thus, whereas high temperature reduced stigmatic receptivity, low temperature enlarged it. The stigma lost the capacity to offer support first for pollen penetration, second for pollen germination and, finally, for pollen adhesion. The effect of temperature was more pronounced on pollen germination and penetration than on pollen adhesion. High temperature reduced the germination capacity of the pollen as early as the first day after anthesis, a time when no apparent signs of stigma degeneration are apparent. This clear effect of temperature on stigmatic receptivity and pollen performance may have clear implication in crop performance and in establishing screening criteria of best‐adapted genotypes.     

Self-incompatibility and the pollen-stigma interaction inTradescantia ohiensis rafin

Summary Self-incompatibility (SI) is reported for an accession ofTradescantia ohiensis. Pollen tube inhibition is stigmatic. The stigma is wet and papillate, the papillr surface bearing conspicuous blebs particularly in the mid- and basal regions of the cell. A proteinaceous pellicle is present on the surface of the papillar cuticle. The penetration of the stains calcofluor white and alcian blue into the cell wall of fresh stigma papillae strongly indicates that the permeability of the papillae is greatest at the mid-region of the cell and not at the tip. When single pollen grains are attached to the tip of a papilla there is either no response at all or the pollen adheres to the papilla. When attached in the mid-region pollen adheres, and often germinates. It is concluded that the sites of pollen receptivity are the mid- and basal regions of the papular cell.     

STIGMA OF NICOTIANA: ULTRASTRUCTURAL AND BIOCHEMICAL STUDIES

The aim of the present study has been to elucidate further the cytology of the glandular stigma of Nicotiana tabacum, the mechanism of the secretory process during stigma development and the biochemical composition of the exudate. The stigma consists of two distinct zones: a glandular zone formed by the papillae and 2–3 layers of cells (basal cells) immediately below them, and a non-glandular region formed by vacuolated cells which are in continuity with the transmitting tissue. The stigmatic exudate is a complex mixture of different chemical compounds such as proteins, saccharides, fatty acids and phenols. The role of stigma secretion is discussed in relation to pollen activation, recognition and pollen tube growth.     

Female flowers and inflorescences of Didymelaceae

 In molecular analyses Didymelaceae together with Buxaceae form a fairly well-supported clade among families near the base of eudicots. Only little is known, however, about the flowers and inflorescences of Didymelaceae. In this study, the structure of the female flowers and inflorescences of Didymeles integrifolia was studied. Flowers are unicarpellate and orientation of the carpel is slightly deflected abaxially as in Proteaceae. Otherwise, Didymelaceae share many features of the gynoecium with Buxaceae and some other basal eudicots: the carpels are ascidiate in the lower half; anthetic carpels are completely closed by postgenital fusion; stigma is double-crested and widely decurrent; stigmatic papillae are unicellular and pear-shaped; the pollen tube transmitting tract is extensive and prominently differentiated; fruits are fleshy drupes with persistent stigma and style. However, the exceedingly elongate base of the integuments of Didymelaceae is an unusual feature among basal eudicots and even angiosperms. Received October 31, 2002; accepted December 17, 2002 Published online: March 31, 2003     

POLLINATION IN LUPINUS NANUS SUBSP. LATIFOLIUS (LEGUMINOSAE)

Floral morphology and phenology and the timing of stigmatic receptivity and pollen viability were studied to elucidate the mechanisms by which self-pollination in Lupinus nanus subsp. latifolius is minimized under natural conditions. Pollen germination and pollen tube growth suggest that a physiological self-incompatibility system does not exist. Instead, self-pollination is minimized by protandry and by a collar of peristigmatic hairs, which initially inhibit access of autologous pollen to the stigma. These hairs subsequently wilt, permitting self-pollination of unvisited flowers. Pollen germinates in vivo one day before substantial metabolic enzyme activity can be detected. Citric acid cycle enzymes are not detectable in pollen, but those of anaerobic metabolism are. Beginning on the second day postanthesis, stigmatic secretions exude from weak areas on lateral walls of the elongate epidermal papillae, welling up in the interstices between these cells. This contrasts with the stigmas of other papilionoid legumes, in which secretions accumulate beneath the cuticle covering the stigmatic cells, and the often-thin cuticle must be ruptured before pollen and exudate can come into contact.