Early Inbreeding Depression and Pollen Competition inCalluna vulgaris(L.) Hull.

We investigated whether partial self-sterility inCalluna vulgarisresultsfrom abortion of selfed offspring owing to inbreeding depressionor a late-acting self-incompatibility mechanism, and whetherself-pollen interferes with normal functioning of cross-pollen.Self-pollination resulted in 75% less seed set than cross-pollination.Self-pollen tubes reached ovaries and penetrated ovules as oftenas those of cross-pollen. Following self-pollination, examinationof the size of undeveloped seeds showed that at least 70% resultedfrom ovule fertilization and arrest of development occurredat various stages. All self-pollinated plants produced seedsand self-fertility varied among plants. These results indicatethat the reduced seed set observed in self-pollination is morelikely the result of inbreeding depression rather than a late-actingself-incompatibility system. The fecundity component of inbreedingdepression was high (0.762). Seed set was reduced by an averageof 40% when self-pollen was mixed with cross-pollen, comparedto pure cross-pollination. Using genetic markers, we found about20% of seeds resulted from self-pollination in mixed-pollinatedfruits.C. vulgarisis likely to experience self-pollination innature and our data suggest this will reduce the number of ovulesthat might otherwise mature after cross-pollination.Copyright1999 Annals of Botany Company Calluna vulgaris(heather), self-pollination, pollen tube, ovule fertilization, early inbreeding depression, pollen interference.     

Megasporogenesis, megagametogenesis and ontogeny of the aril in Cytisus striatus and C. multiflorus (Leguminosae: Papilionoideae)

BACKGROUND AND AIMS: There are few embryological reports on wild legumes and even fewer on their seminal appendages. There are no existing studies on the complete ontogeny of these appendages in Cytiseae, a very important Papilionoideae tribe in Mediterranean ecosystems. In this work megasporogenesis, megagametogenesis and aril ontogeny were studied in Cytisus multiflorus and C. striatus, endemics from the western Mediterranean region. METHODS: Ovaries and ovules from flower buds, flowers at anthesis and hand cross-pollinated flowers were sectioned with a rotary microtome and studied under light and fluorescence microscopy. KEY RESULTS: A monosporic Polygonum-type of megagametogenesis is observed in both species but with megasporogenesis characterized by formation of a triad of cells after incomplete meiosis. The original cell wall of the megaspore mother cell and triad, including the transverse walls between the latter, are surrounded by a callose layer that isolates them from the surrounding diploid tissue; this callose layer gradually disappears during embryo sac formation. There are no antipodals in the mature embryo sac. Aril ontogeny starts in pre-anthesis with the formation of the aril primordium, and its normal development will occur only after fertilization, more specifically after endosperm initiation. After fertilization, a reactivation of meristem capacity takes place in the aril cells resulting in slow and sparse growth. Later, this type of development gradually decreases but the aril cells continue to grow by cell expansion, which in the last period of seed development is the only type of growth of the aril. In the mature seed, the seminal appendage acquires an irregular U-shape in transverse section, showing vacuolated cells with a large central vacuole that stores lipids and some proteins. CONCLUSIONS: Meiotic triad formation is due to a failure in meiosis II of the chalazal cell of the dyad. In Cytisus seeds the aril has a funicular origin with predominantly post-fertilization development, but a normal growth of the endosperm is needed for proper aril development.     

LIMITS TO REPRODUCTIVE SUCCESS IN A PARTIALLY SELF-INCOMPATIBLE HERB: FECUNDITY DEPRESSION AT SERIAL LIFE-CYCLE STAGES

Like many angiosperms, Crinum erubescens is partially self-compatible, producing fewer seeds upon selfing than after outcrossing. In this paper we test the relative magnitude of the prefertilization and postfertilization effects of self-incompatibility, inbreeding depression, or both in a natural population of this hermaphroditic tropical herb. We characterize prefertilization effects by examining pollen tube growth, while postfertilization effects are characterized by examination of embryo abortion and seed maturation. Statistical methods are developed to test the magnitude of these effects from one life-cycle stage to the next. We find that although pollen performance in selfed flowers is lower than that in outcrossed flowers, pollen performance is low overall. Postfertilization effects attributable to inbreeding depression account for a larger proportion of the reduction in fecundity in selfed compared to outcrossed flowers. Among naturally pollinated plants, despite ample pollen deposition, the numbers of fruits and seeds set are intermediate to selfed and outcrossed treatments.     

The embryology of <Emphasis Type="Italic">Xanthoceras</Emphasis> and its phylogenetic implications

Recent molecular analyses found the Chinese monotypic genus Xanthoceras to be either a sister genus to the rest of a broadly defined Sapindaceae sensu lato (s.l.) including Aceraceae and Hippocastanaceae or the only genus of the Xanthoceraceae. Here we investigate for the first time the embryology of Xanthoceras and compare the embryological data with related taxa to aid in understanding the phylogenetic relationships of this genus. The results indicate that Xanthoceras shares many embryological features with Sapindaceae sensu stricto (s.s.), Aceraceae, and Hippocastanaceae, including a glandular tapetum, simultaneous microsporogenesis, three-colporate pollen grains, a bitegmic and crassinucellate ovule, Polygonum type of embryo sac development, endosperm formation of the nuclear type, and exalbuminous mature seeds. The wide range of similarities in embryological features suggests that Xanthoceras is closely related to other members of Sapindaceae s.l. However, Xanthoceras differs from other Sapindaceae in several morphological characteristics, for instance, five to eight ovules in each locule in Xanthoceras versus one to two ovules in each locule in other Sapindaceae s.l., and five golden horn-like appendages present on both staminate and hermaphrodite flowers of Xanthoceras versus absent on the flowers of other Sapindaceae s.l. We propose that Xanthoceras merits recognition at the subfamily level as Xanthoceroideae, rather than at the family level as Xanthoceraceae. The present study also reveals that late-acting ovarian self-incompatibility (OSI) occurs in X. sorbifolium and that postzygotic OSI contributes to the self-sterility of the species.     

Self-sterility in Epilobium obcordatum (Onagraceae)

Greenhouse pollinations among 23 field-collected plants and their progeny revealed a high and variable degree of self-sterility in Epilobium obcordatum. Field-collected plants averaged 5% seed-set when selfed and 43% seed-set in crosses; greenhouse progeny 11% and 49%, respectively. In two of three pollen chase tests, previously or simultaneously applied self-pollen reduced seed-set to that resulting from pure self-pollination. Based on the relative survivorship of self- and outcross embryos, the mean number of lethal equivalents was determined to be 11.0 (range 2.9 to 17.6) in the field-collected plants, and 7.1 (range 0.62 to 21.8) in the greenhouse progeny. Pollinations among 91 individuals showed a significant negative correlation between seed-set and inbreeding coefficients of the resulting zygotes. Intrapopulation crosses yielded significantly fewer seeds than interpopulation crosses. Early embryonic mutational load, in combination with some level of biparental inbreeding, appears to be responsible for the level of self-sterility in natural populations of this species. A pollen dosage test for an interpopulation cross resulted in a maximum seed set of 47% at 16 tetrads. Additional unmeasured characteristics govern the probability of ovules becoming mature seeds.     

Reproductive phenology in three Genisteae (Fabaceae) shrub species of the W Mediterranean Region

We studied the flower duration, the phenology of flowering and fruiting, and flower, fruit, and seed production in three Genisteae shrub species of the W Mediterranean Region: Cytisus multiflorus, C. striatus , and Retama sphaerocarpa . Flower duration was negatively correlated with temperature, and in the case of C. striatus it was also influenced by pollination. In Cytisus multiflorus , which flowers during winter in the population studied, two floral morphs were recognized differentiated by flower size, phenological pattern, and production of the reproductive organs: morph LF (large flowers) and morph SF (small flowers). The former is earlier in phenology but its reproductive success is less than the second since it not only produces significantly fewer flowers/plant, but also a smaller crop of fruit and seeds. The flowering of the LF morph lasted some three months and of the SF morph 11 weeks. The winter-spring flowering C. striatus , with a four month duration of flowering time, is sympatric with C. multiflorus and their flowering periods overlap, but the former is more successful reproductively. Retama sphaerocarpa is clearly spring flowering, with an extremely short duration of flowering (c. 6.5 weeks), but, unlike the other two species which disperse their seeds in the same season in which they are produced, it staggers the dispersal of its diaspores over more than one year. Despite the great flower per plant production, especially in Retama sphaerocarpa , and of the number of fruit initiated, the final crop of fruit and seed/plant is low in the three species (C. multiflorus : fruit 8.40% and seeds 1.96%; C. striatus : fruit 13.09% and seeds 4.12%; Retama sphaerocarpa : fruit 5.65% and seeds 1.33%).     

Selfing and inbreeding depression in seeds and seedlings of Neobalanocarpus heimii (Dipterocarpaceae)

We evaluated the degree of selfing and inbreeding depression at the seed and seedling stages of a threatened tropical canopy tree, Neobalanocarpus heimii, using microsatellite markers. Selection resulted in an overall decrease in the level of surviving selfed progeny from seeds to established seedlings, indicating inbreeding depression during seedling establishment. Mean seed mass of selfed progeny was lower than that of outcrossed progeny. Since the smaller seeds suffered a fitness disadvantage at germination in N. heimii, the reduced seed mass of selfed progeny would be one of the determinants of the observed inbreeding depression during seedling establishment. High selfing rates in some mother trees could be attributed to low local densities of reproductive individuals, thus maintenance of a sufficiently high density of mature N. heimii should facilitate regeneration and conservation of the species.     

Genetic load,nutrient limitation,and seed production in Lupinus texensis (Fabaceae)

The interaction of mating system and nutrient limitation in determining seed production was investigated in the annual, self-compatible plant Lupinus texensis (Fabaceae). Abortion of developing seeds is a major factor limiting seed production in natural populations (17-28%). Selfing rates are generally low (0.02-0.21), suggesting that deleterious recessive genes may be maintained at significant levels in natural populations. The average inbreeding depression associated with seed development is δ = 0.24. Nutrient limitation reduced seed output across experimental treatments by a factor of 0.22 through decreased production of inflorescences, flowers, and ovules, and by a factor of 0.29 through increased abortion of fruits and of seeds within fruits. Competition for resources among fruits increased the frequency of seed abortion. Moreover, a greater proportion of selfed seeds were aborted as the overall abortion rate increased. Estimates of genetic load may therefore only be appropriate if undertaken in the field, and inbreeding depression may vary from year to year simply due to changes in environmental conditions rather than to underlying genetic changes in populations. The existence of inbreeding depression and the high frequency of abortions suggest that selective abortion favoring outcrossed progeny occurs in natural populations of L. texensis.     

Inbreeding depression in Lychnis flos-cuculi (Caryophyllaceae): effects of different levels of inbreeding

In a controlled crossing experiment on Lychnis flos-cuculi plants in the greenhouse, outbred and selfed maternal plants were each treated with pollen from unrelated plants, siblings and selves. The seeds thus obtained had expected inbreeding coefficients of 0, 0.25 and 0.5 for the outbred maternal plants, and 0, 0.5 and 0.75 for the selfed maternal plants. Seed abortion rate, seed weight and germination rate were estimated. Seedlings were transplanted to an outdoor garden, and monitored for survival, probability of flowering, number of capsules and area of capsules next spring. Inbred seeds germinated slower and in lower proportions than those less inbred, and seedlings had lower survival, flowering, fruit set and area of capsules if inbred. Combined fitness values were estimated from the survival and fecundity components, and severe inbreeding depression was detected for these estimates (0.51 and 0.56 for one generation of selfing). The fitness function decreased linearly with the increase in inbreeding coefficient, which is as expected if the inbreeding depression is additive among loci.     

Imprinting of the MEDEA polycomb gene in the Arabidopsis endosperm.

In flowering plants, two cells are fertilized in the haploid female gametophyte. Egg and sperm nuclei fuse to form the embryo. A second sperm nucleus fuses with the central cell nucleus that replicates to generate the endosperm, which is a tissue that supports embryo development. MEDEA (MEA) encodes an Arabidopsis SET domain Polycomb protein. Inheritance of a maternal loss-of-function mea allele results in embryo abortion and prolonged endosperm production, irrespective of the genotype of the paternal allele. Thus, only the maternal wild-type MEA allele is required for proper embryo and endosperm development. To understand the molecular mechanism responsible for the parent-of-origin effects of mea mutations on seed development, we compared the expression of maternal and paternal MEA alleles in the progeny of crosses between two Arabidopsis ecotypes. Only the maternal MEA mRNA was detected in the endosperm from seeds at the torpedo stage and later. By contrast, expression of both maternal and paternal MEA alleles was observed in the embryo from seeds at the torpedo stage and later, in seedling, leaf, stem, and root. Thus, MEA is an imprinted gene that displays parent-of-origin-dependent monoallelic expression specifically in the endosperm. These results suggest that the embryo abortion observed in mutant mea seeds is due, at least in part, to a defect in endosperm function. Silencing of the paternal MEA allele in the endosperm and the phenotype of mutant mea seeds supports the parental conflict theory for the evolution of imprinting in plants and mammals.     

SEED DEVELOPMENT IN CITRUS WITH SPECIAL REFERENCE TO 2X X 4X CROSSES

Comparative cytological and histological studies during embryogenesis of seeds from 2x X 2x and 2x x 4x crosses indicate that the ratio of ploidy level between embryo and endosperm is the most important factor affecting the course of seed development. The crosses produced seeds with the expected ploidy relationships between embryo, endosperm, and maternal tissue of 2:3:2 and 3:4:2 as well as the anomalous relationships 3:5:2, 4:6:2, and 6:10:2. All but 3:4:2 resulted in normal, germinable seeds. The ploidy level of the maternal tissue in relation to that of the embryo or endosperm did not appear to have any effect on seed development. About 92–99 % of seeds from 2x x 4x crosses containing triploid embryos with tetraploid endosperm aborted at different stages of embryogenesis. The abortion in all cases was preceded by abnormalities in the tetraploid endosperm. It is postulated that the unbalance of chromosome number between embryo and endosperm disturbs physiological relationships between these two tissues, leading first to the abortion of the endosperm and then of the embryo.     

Embryological Studies on Sagittaria guayanensis H. B. K. Subsp. lappula (D. Don) Bojin (Alismataceae)

This paper deals with the embryological characteristics of Sagittaria guayanensis H. B.K. subsp. lappula (D. Don) Bojin. The anther wall development follows the Monocotyledonous type. The cytokinesis of microspore mother cell in meiosis is of the Successive type. The tetrads of microspores show an isobilateral arrangement, and the mature pollen grains are 3-celled. The ovule is bitegminous, pseudo-crassinucellate and anatropous. The megaspore mother cell originates directly from a single archesporial cell. The mature embryo sac consists of 7 cells including 8 nuclei and conforms to the Allium type. The two polar nuclei do not fuse into a secondary nucleus before fertilization. Instead, one sperm fuses with the micropylar end polar nucleus first , and the fertilized polar nucleus then migrates to the chalazal end, where it fuses with the second polar nucleus, forming the primary endosperm nucleus. The embryo development conforms to the Caryophyllad type. The mature embryo is U-shaped and forms the embryonic shoot apex accompanied by two leaves. The endosperm development corresponds to the Helobial type. The primary endosperm nucleus (invariably lying in the chalazal part of the embryo sac) divides and forms two chambers:large micropylar one and small chalazal one. The chalazal endosperm chamber remains binucleate, while, in the micropylar chamber free nuclear divisions occur and then cellnlarization takes place. During the embryo formation the endosperm gradually degrades and can not be found in the mature seed. The subgenus Lophotocarpus is different from the subgenus Sagittaria in some embryological aspects, especially in the structure of mature embryo sac and the double fertilization process.     

冠果草的胚胎学研究

冠果草花药壁的发育为单子口十型,绒毡层为周原质团型。小孢子母细胞减数分裂为连续型,四分体呈左右对称式排列,成熟花粉为三细胞型。双珠被,假厚珠心,倒生胚珠。胚囊发育为葱型,成熟胚囊的特点是两个极核分别位于中央细胞两端,不融合成次生核。受精过程中,一个精于与卵核融合形成合子,另一精子先与珠孔端极核融合,之后受精极核再移动到合点端与另一极核融合,形成初生胚乳核。胚的发育为石竹型。成熟胚呈马蹄形,具有2片真叶。胚乳发育为沼生目型。随着胚的发育,胚乳细胞逐渐解体,成熟种子中无胚乳。     

Evidence for ovarian self-incompatibility as a cause of self-sterility in the relictual woody angiosperm,Pseudowintera axillaris (Winteraceae)

Species within the genus Pseudowintera exhibit high rates of self-sterility. Self-sterility in the genus has been previously posited-but not confirmed-to be the result of late-acting ovarian self-incompatibility (OSI) functioning within nucellar tissue of the ovule to prevent self pollen tubes from entering the embryo sac. Structural and functional aspects of pollen-carpel interactions and early seed development following cross- and self-pollination were investigated in P. axillaris to determine the site, timing and possible mechanisms of self-sterility. No significant differences were observed between pollen tube growth, ovule penetration and double fertilization following cross- and self-pollination. Pollen tubes exhibited phasic growth in an extracellular matrix composed of proteins and carbohydrates, as well as arabinogalactans/arabinogalactan proteins. A uniform failure in embryo sac development prior to division of the zygote was apparent within 15 d following double fertilization by self gametes. Results indicate that SI mechanisms in P. axillaris do not prevent double fertilization from occurring. Instead, mechanisms of self-sterility affect post-zygotic development of the embryo sac. Although self-sterility may be attributed to inbreeding depression, given the post-zygotic nature of failure in embryo sac development, the possibility of late-acting OSI is discussed.     

Fitness Traits and Dispersal Ability in the Herb Tragopogon pratensis (Asteraceae): Decoupling the Role of Inbreeding Depression and Maternal Effects

Abstract: Inbreeding depression can decrease several fitness traits and maternal effects can strongly influence the amount of inbreeding depression. Understanding the effects of inbreeding depression on plant fitness is especially important in the context of habitat fragmentation, where plant populations become smaller and more isolated, exhibiting increasing levels of inbreeding depression. We examined the joint influence of inbreeding depression and maternal effects on life cycle traits and dispersal ability in the herb Tragopogon pratensis that grows in fragmented populations in Europe. We conducted experimental crosses to obtain selfed and outcrossed progeny in two contrasted environments. In particular, we produced a first generation of seeds and plants that were self-pollinated again to produce a second generation of seeds. Individual seeds were weighed and their pappuses measured to estimate the dispersal potential. Pollination treatment only had a significant effect on seed mass and dispersal ability. Coefficients of inbreeding depression did not differ between selfed and outcrossed plants. Seed mass had a significant effect on germination date. Environment had a significant effect on mass of the second generation of seeds and the interaction between pollination treatment and family was significant for six traits, indicating the existence of strong maternal effects in T. pratensis. Results suggest population differentiation. Overall, T. pratensis populations exhibited a good performance under selfing, in terms of life cycle traits and dispersal ability, which would allow the species to cope with problems associated with fragmentation.     

Outcrossing rate and inbreeding depression in the perennial yellow bush lupine, Lupinus arboreus (Fabaceae)

Little is known about the breeding systems of perennial Lupinus species. We provide information about the breeding system of the perennial yellow bush lupine, Lupinus arboreus, specifically determining self-compatibility, outcrossing rate, and level of inbreeding depression. Flowers are self-compatible, but autonomous self-fertilization rarely occurs; thus selfed seed are a product of facilitated selfing. Based on four isozyme loci from 34 maternal progeny arrays of seeds we estimated an outcrossing rate of 0.78. However, when we accounted for differential maturation of selfed seeds, the outcrossing rate at fertilization was lower, ～0.64. Fitness and inbreeding depression of 11 selfed and outcrossed families were measured at four stages: seed maturation, seedling emergence, seedling survivorship, and growth at 12 wk. Cumulative inbreeding depression across all four life stages averaged 0.59, although variation existed between families for the magnitude of inbreeding depression. Inbreeding depression was not manifest uniformly across all four life stages. Outcrossed flowers produced twice as many seeds as selfed flowers, but the mean performance of selfed and outcrossed progeny was not different for emergence, seedling survivorship, and size at 12 wk. Counter to assumptions about this species, L. arboreus is both self-compatible and outcrosses ～78% of the time.     

The Role of Late-Acting Self-Incompatibility and Early-Acting Inbreeding Depression in Governing Female Fertility in Monkshood,Aconitum kusnezoffii

Reduced seed yields following self-pollination have repeatedly been observed, but the underlying mechanisms remain elusive when self-pollen tubes can readily grow into ovaries, because pre-, post-zygotic late-acting self-incompatibility (LSI), or early-acting inbreeding depression (ID) can induce self-sterility. The main objective of this study was to differentiate these processes in Aconitum kusnezoffii, a plant lacking stigmatic or stylar inhibition of self-pollination. We performed a hand-pollination experiment in a natural population of A. kusnezoffii, compared seed set among five pollination treatments, and evaluated the distribution of seed size and seed set. Embryonic development suggested fertilization following self-pollination. A partial pre-zygotic LSI was suggested to account for the reduced seed set by two lines of evidence. The seed set of chase-pollination treatment significantly exceeded that of self-pollination treatment, and the proportion of unfertilized ovules was the highest following self-pollination. Meanwhile, early-acting ID, rather than post-zygotic LSI, was suggested by the findings that the size of aborted selfed seeds varied continuously and widely; and the selfed seed set both exhibited a continuous distribution and positively correlated with the crossed seed set. These results indicated that the embryos were aborted at different stages due to the expression of many deleterious alleles throughout the genome during seed maturation. No signature of post-zygotic LSI was found. Both partial pre-zygotic LSI and early-acting ID contribute to the reduction in selfed seed set in A. kusnezoffii, with pre-zygotic LSI rejecting part of the self-pollen and early-acting ID aborting part of the self-fertilized seeds.     

Mixed pollen load and late‐acting self‐incompatibility flexibility in Adenocalymma peregrinum (Miers) L.G. Lohmann (Bignonieae: Bignoniaceae)

• Mixed cross and self‐pollen load on the stigma (mixed pollination) of species with late‐acting self‐incompatibility system (LSI) can lead to self‐fertilized seed production. This “cryptic self‐fertility” may allow selfed seedling development in species otherwise largely self‐sterile. Our aims were to check if mixed pollinations would lead to fruit set in LSI Adenocalymma peregrinum, and test for evidence of early‐acting inbreeding depression in putative selfed seeds from mixed pollinations.
• Experimental pollinations were carried out in a natural population. Fruit and seed set from self‐, cross and mixed pollinations were analysed. Further germination tests were carried out for the seeds obtained from treatments.
• Our results confirm self‐incompatibility, and fruit set from cross‐pollinations was three‐fold that from mixed pollinations. This low fruit set in mixed pollinations is most likely due to a greater number of self‐ than cross‐fertilized ovules, which promotes LSI action and pistil abortion. Likewise, higher percentage of empty seeds in surviving fruits from mixed pollinations compared with cross‐pollinations is probably due to ovule discounting caused by self‐fertilization. Moreover, germinability of seeds with developed embryos was lower in fruits from mixed than from cross‐pollinations, and the non‐viable seeds from mixed pollinations showed one‐third of the mass of those from cross‐pollinations.
• The great number of empty seeds, lower germinability, lower mass of non‐viable seeds, and higher variation in seed mass distribution in mixed pollinations, strongly suggests early‐acing inbreeding depression in putative selfed seeds. In this sense, LSI and inbreeding depression acting together probably constrain self‐fertilized seedling establishment in A. peregrinum.

     

Different levels of inbreeding depression between outcrossing and selfing Serapias species

We quantified inbreeding depression for fruit production, embryo vitality and seed germination in three deceptive orchids, Serapias vomeracea, S. cordigera and S. parviflora, which do not provide any reward to their pollinators, and are predicted to experience high outcrossing. Of the three species examined only S. parviflora was autonomously selfing. Both S. vomeracea and S. cordigera showed highly significant differences in fitness between selfed and outcrossed progenies, resulting in high levels of inbreeding depression, which increased in magnitude from seed set to seed germination. Inbreeding depression may promote outcrossing in Serapias by acting as a post-pollination barrier to selfing. Cumulative inbreeding depression across three stages in S. parviflora was lower that in both outcrossing species. The large difference in germination between selfed and outcrossed seeds is an important issue in conservation biology.     

Embryological Studies on Endangered Ophiopogon xylorrhizus (Liliaceae)

The embryological studies on an endangered species Ophiopogon xylorrhizus are reported in this paper. The anther was four-sporangiate, and its wall was composed of four layers: epidermis, fibrous endothelium, middle layer and glandular tapetum. Cytokinesis of microspore mother cell in meiosis was of the Successive type, and tetrads were tetrahedral or isobilateral. The mature pollen grains were 2-celled. Ovules were anatropous, bitegminous, and crassinucellate. Ovary wall ruptured 4～5 days after fertilization. The development of embryo sacs conformed to the Polygonum type. The two polar nuclei fused into a secondary nucleus before fertilization. The embryogeny was of the Asterad type, and endosperm development corresponded to the Nuclear type. About 55.8 % individuals in the population of O. xylorrhizus were found male sterile, and 43.3% of embryos aborted at early development stage. As a conclusion, we consider that (1)the failure of the xenogamy resulted from male sterility, autogamy and inefficient pollination agents, (2)embryo abortion and (3)exposure of young seeds during their early development are the weak links in the sexual reproductionof O. xylorrhizus, which would contribute to the endangerment of this species.