The Initiation, Development and Removal of Embryo Sac Wall Ingrowths in the Developing Seeds of Solanum nigrum L. An Ultrastructural Study

In developing seeds of Solanum nigrum L., wall ingrowths developedat the extreme micropylar and chalazal ends of the embryo sac.In the micropylar region, the wall ingrowths were initiatedat the three-celled endosperm stage starting at the base ofthe zygote then progressing for a short distance chalazalwards.They developed quickly with the most elaborate around the baseof the suspensor. The chalazal wall ingrowths developed alongthe surfaces of the chalazal cup, the antipodal cup and thehypostase. Those along the hypostase were initiated at the four-celled,those in the chalazal and antipodal cups at the 20-celled endospermstages. The most elaborate developed along the base of the antipodalcup; the most simple were along the base of the chalazal cup.Small electron-lucent invaginations of the plasmalemma whichlater became filled with fibrillar material, were the earliestindication of wall ingrowth formation. Removal of the wall ingrowthscommenced at the mid-globular stage of embryo development andwas completed by the mid-heart-shaped stage. In the micropylarregion, wall ingrowth removal was rapid, starting with the lossof the fibrillar component followed by the thinning of the cellwall. However, along the hypostase and antipodal cup, a heterogeneouslayer of varying electron densities and a thinner, more electrondense layer was laid down over the ingrowths. This was followedby the removal of the fibrillar component. The initiation, removaland location of the embryo sac wall ingrowths is discussed inconnection with understanding the nutritional relationshipsbetween maternal tissue, endosperm and embryo.Copyright 1995,1999 Academic Press Wall ingrowths, Solanum nigrum, transfer cells, zone of separation and secretion, hypostase     

Embryo and Endosperm Development in Isatis Tinctoria L. and the Histochemical Study of Its Storage Reserve

The ovule is anatropous and bitegmic. The nuceIlar cells have disorganized except the chalazal proliferating tissue. The curved embryo sac comprises an egg apparatus and a central cell with two palar nuclei and wall ingrowths on its micropylar lateral wall. The antipodal cells disappear. Embryo development is of the Onagrad type. The filament suspensor grows to a length of 785 μm and degenerats at tarpedo embryo stage. The basal cell produces wall ingrowths on the micropylar end wall and lateral wall. The cells of mature embryo contain many globular protein bodies, 2.5–7.5 μm in diameter, composed of high concentration of protein and phytin, insoluble polysaccharide and lipid. The cells, except procambium, also contain many small starch grains. Some secretory cavities scattered in the ground tissue have liquidlike granules composed of protein, ploysacchaide and lipid. Endosperm development follows the nuclear pattern. At the late heart embryo stage, the endosperm around the embryo and the upper suspensor and the peripheral endosperm of the basal region of the U-shaped embryo sac becomes cellular. The endosperm at micropylar and chalazal ends remains free nuclear phase until the late bended cotyledon stage. Wall ingrowths at both micropylar and chalazal end wall and lateral wall of the embryo sac become more massive during endosperm development. Wall ingrowths also occur on the outer walls of the outer layer endosperm cells at both ends and lateral region of the embryo sac. When the embryo matures, many layers of chalazal endosperm ceils including 2–4 layers of transfer cells, a few of micropylar endosperm cells and 1–5 layers of peripheral endosperm cells are present. The nutrients of the embryo and endosperm at different stages of development are also discussed.     

ADVENTIVE EMBRYOGENESIS IN CITRUS AND ITS RELATION TO POLLINATION AND FERTILIZATION

Cytological and histological studies of seeds from three facultative apomictic Citrus cultivars show that adventive embryos develop, as a rule, from the first few cell layers of the nucellus adjacent to the embryo sac in the micropylar half and occasionally from the chalazal end. The adventive embryos initiated in nucellar tissue away from the embryo sac and most of those initiated from the chalazal end of the nucellus do not develop beyond the one-celled stage. When two or more embryos are developing in the same seed, the successful development of a given embryo depends on its location in relation to access to nutrients from the endosperm. The presence of a zygote and triploid endosperm in seeds with adventive embryos, the abortion of seed when endosperm degenerates, and the lack of seed set without pollination indicate that pollination and fertilization are essential for in vivo adventive embryogenesis.     

ADVENTIVE EMBRYOGENESIS IN CITRUS (RUTACEAE) II. POSTFERTILIZATION DEVELOPMENT

Cytological and histological studies on postfertilization development of ovules were carried out in six facultatively apomictic Citrus cultivars. At the time of anthesis, adventive embryo initial cells (AEICs) were detected mainly in the cell layers of the nucellus around the chalazal half of the embryo sac. During the approximately 40 days rest period of the AEICs after fertilization, rapid cell division and enlargement in the endosperm and the chalazal half of the nucellus resulted in the split of AEICs into several separated areas forming the micropylar, lateral and chalazal islands surrounding the enlarging embryo sac. Both in diploid seeds with triploid endosperm and triploid seeds with pentaploid endosperm, the AEICs located in the micropylar half successfully developed into adventive embryos. In diploid seeds, almost all AEICs located in the chalazal half did not develop beyond the initial-celled stage, while in the triploid seeds, those located in the chalazal half occasionally developed into cotyledonary embryos. In seeds with aborted endosperm, the AEICs located in the chalazal half often developed into cotyledonary embryos. The chalazal expiants from normal seeds produced a large number of embryos in vitro. Four results can be summarized from these studies on adventive embryogenesis as follows: 1) All AEICs are initiated prior to anthesis. 2) Whether or not the AEICs successfully developed into adventive embryos is dependent upon their position in the seed. 3) The farther the AEICs are located from the micropylar end, the more adventive embryogenesis is suppressed by endosperm. 4) The degree of adventive embryogenesis in the chalazal half is affected by time and extent of malfunction of the endosperm. Under natural conditions, these regulatory systems of adventive embryogenesis contribute to high production of zygotic seedlings in apomictic Citrus species and cultivars.     

Accumulation of Metabolites during Seed Development in Trifolium repens L.

Metabolite deposition during seed development was examined histochemicallyin Trifolium repens by light- and fluorescence microscopy. Allendosperm haustorium at the chalazal pole of the embryo sacand wall protrusions in cell walls of the suspensor and theembryo sac suggest that transfer of metabolites from maternalto offspring tissue takes place primarily at these sites. Thisis further supported by prominent cutinization of the interpolarregion of the embryo sac wall, accumulation of starch in integumentaltissue at the embryo sac poles, and breakdown of interpolarendothelial cells. Decomposition of osteosclereid starch isfollowed by accumulation in the cellular endosperm and subsequentlyin the embryo parallel to endosperm degradation. The starchaccumulates gradually inward from the subepidermal cells ofthe embryo to the stele. Protein bodies are formed in the vacuolesalong the tonoplast, later to be cut off in vesicles releasedinto the cytoplasm. At maturity the embryo is packed with proteinand starch, but without lipid reserves. Phytin is observed inthe protein bodies. The mature embryo is surrounded by a proteinand starch containing aleurone layer which originates from theendosperm.Copyright 1994, 1999 Academic Press White clover, protein, starch, cuticle, embryo sac wall     

Embryo and Endosperm Function and Failure inSolanumSpecies and Hybrids

Although the importance of the endosperm as a food store inmany angiosperm seeds is well known, its significance duringearly embryogenesis has been neglected. In many interspecifichybrids, and in some other situations, embryos do not developfully and abort. It has often been stated that this is causedby the endosperm failing to conduct sufficient nutrients tothe embryo, but seldom has it been suggested that the endospermactively controls most of the early stages of morphogenesisof the embryo. Information gleaned from a broad survey of theliterature, combined with additional evidence presented here,obtained fromSolanum incanumand interspecific hybrids, indicatethat the endosperm is dynamic and very active in regulatingearly embryo development. This requires highly integrated geneticcontrol of rapidly changing metabolism in the endosperm. Ininterspecific hybrids, lack of coordination may cause unbalancedproduction of growth regulating substances by the endospermand hence abortion of the embryo, or even unregulated productionof nucleases and proteases resulting firstly in autolysis ofthe endosperm and then digestion of the embryo. The endospermmay thus serve to detect inappropriate hybridization of speciesor ploidy levels and so prevent waste of resources by producingseeds that would result in sterile hybrids or unthrifty subsequentgenerations. This discriminatory function of the endosperm hasdiminished during evolution and domestication of the crop plantSolanummelongenaL.Copyright 1998 Annals of Botany Company Solanum, embryo morphogenesis, endosperm, hybrid, seed development.     

Embryology of Early Abortion Due to Limited Maternal Resources in Pisum sativum L.

In most flowering plants, many embryos are aborted early intheir development due to limited maternal resources. The kin-conflictinterpretation of plant embryology predicts these abortionsshould be under maternal control. In a study of the abortionprocess in Pisum sativum, we found the first visible indicationof abortion was formation of a weak hypostase. Callose was locallydeposited around the chalazal endosperm haustorium, and ligninalong the outer cell walls of the remnant nucellar tissue. Thenucellus was compressed by proliferating adjacent inner integumentalcells. The endosperm haustorium's cytoplasm was forced backinto the embryo sac cavity. With suppression of haustorial activitythe endosperm nuclei gradually enlarged followed by enlargementof the embryo and suspensor nuclei. Finally, nuclei and cytoplasm throughout the endosperm and embryolost stainability and broke down. Four successive stages wererecognized in seed abortion. In seeds developing to maturity,no hypostase was developed and the haustorium continued to digestboth the remnant nucellus and the proliferated inner integumentalcells. These observations are consistent with the kin-conflicthypothesis. Pisum sativum, garden pea, ovule abortion, histology, hypostase, kin-conflict hypothesis     

The specialized chalazal endosperm inArabidopsis thaliana andLepidium virginicum (Brassicaceae)

Summary Endosperm of the nuclear type initially develops into a large multinucleate syncytium that lines the central cell. This seemingly simple wall-less cytoplasm can, however, be highly differentiated. In developing seeds of members of the family Brassicaceae the curved postfertilization embryo sac comprises three chambers or developmental domains. The syncytium fills the micropylar chamber around the embryo, spreads as a thin peripheral layer surrounding a large central vacuole in the central chamber, and is organized into individual nodules and a large multinucleate cyst in the chalazal tip. Later in development, after the endosperm has cellularized in the micropylar and central chambers, the chalazal endosperm cyst remains syncytial and shows considerable internal differentiation. The chalazal endosperm cyst consists of a domelike apical region that is separated from the cellularized endosperm by a remnant of the central vacuole and a basal haustorial portion which penetrates the chalazal proliferative tissue atop the vascular supply. In the shallow chalazal depression ofArabidopsis thaliana, the cyst is mushroom-shaped with short tentacle-like processes penetrating the maternal tissues. The long narrow chalazal channel ofLepidium irginicum is filled by an elongate stalklike portion of the cyst. In both, the dome contains a labyrinth of endoplasmic reticulum, dictyosomes with associated vesicles, nuclei, and plastids. The basal portions, which lack the larger organelles, exhibit extensive wall ingrowths and contain parallel arrays of microtubules. The highly specialized ultrastructure of the chalazal endosperm cyst and its intimate association with degrading chalazal proliferative cells suggest an important role in loading of maternal resources into the developing seed.     

Ultrastructure of the Developing and Fertilized Embryo Sac of Amaranthus hypochondriacus L.

Amaranthus hypochondriacus embryo sac development was investigatedbefore and after fertilization. During the early stages of development,the young embryo sac displays three antipodal cells at the chalazalpole that degenerate very early in the maturation process, beforethe synergids and egg cell are completely differentiated. Themature embryo sac is composed only of the female germ unit.The synergid cells organize a filiform apparatus accompaniedby the presence of mitochondria and dictyosomes with numerousvesicles. The involvement of the synergids in transport andsecretory functions related to pollen tube attraction and guidance,are discussed. The egg cell is located at the micropylar polenear the synergids and displays exposed plasma membranes atthe chalazal pole. The fertilized egg cell does not exhibitmarked changes after fertilization except for the closure ofthe cell wall. The central cell is the largest cell of thisvery long embryo sac. The fused nucleus is close to the eggapparatus before fertilization and displays a remarkable chalazalmigration after gamete delivery. The ultrastructure of the centralcell cytoplasm and the numerous wall ingrowths around this cellsuggest an important role in nutrient transportation. Aftergamete delivery, the embryo sac displays electron dense bodiesthat aggregate within the intercellular space between the synergids,egg cell and central cell. These bodies, that appear in theembryo sac of several plants, are probably involved in gametedelivery for double fertilization. The possibility of biparentalinheritance of mitochondria in this plant is also discussed.Copyright 1999 Annals of Botany Company Amaranthus hypochondriacus, grain amaranth, embryo sac, fertilization.     

A Reinvestigation of the Development of the Embryo Sac in Gastrodia elata Blume (Orchidaceae)

The embryo sac of Gastrodia elata was reinvestigated. The ovuleis anatropous, unitegmic and tenuinucellate. The chalazal megasporeof a triad develops into an embryo sac. Four nuclei are formedat the micropylar end but only two at the chalazal end. A typicalegg apparatus and a single polar nucleus are derived from themicropylar quartet, while the chalazal two disappear beforematuration of the embryo sac. Double fertilization takes placenormally.     

Ovule and Embryo Ontogenesis in Bombacopsis glabra (Pasq.) A. Robyns

Ontogeny of the ovule and development of the embryo in Bombacopsisglabra (Pasq.) A. Robyns were examined. The ovule is bitegmic,crassinucellate, and anatropous. The exostome is eccentric relativeto the endostome; stomata occur on the outer integument. Thesingle archesporial cell functions directly as the megasporemother cell. The embryo-sac is bisporic. The organization ofthe nuclei in the mature embryo-sac is normal. The antipodalcells disintegrate soon after formation. Double fertilization takes place; the zygote undergoes a longperiod of dormancy, but the primary endosperm nucleus dividesimmediately to produce first a nuclear-type, later a cellular-typeendosperm. The zygote is of the caryophyllad type. Adventive embryos arise from single cells of the nucellus inthe vicinity of the micropyle, and develop faster than the sexuallyproduced embryo; this leads to anomictic renroduction.     

黄顶菊的大孢子发生、雌配子体与胚胎发育

用常规石蜡制片对黄顶菊(Flaveria bidentis(L.) Kuntze)大孢子发生、雌配子体和胚胎的发育过程进行了观察.黄顶菊雌蕊柱头二裂,2心皮,1室,单胚珠,基生胎座,单珠被,薄珠心,倒生胚珠,具发达的珠被绒毡层.珠心表皮下分化出孢原细胞,孢原细胞直接发育为大孢子母细胞,大孢子母细胞减数分裂形成直列四分体...     

Development of Embryo and Endosperm and Nutrient Accumulation in Vigna sesquipedalis (L.) Fruwirth

The structure of embryo sac, fertilization and development of embryo and endosperm in Vigina sesquipedalis (L.) Fruwirth were investigated. Pollization occures 7–10h before anthesis, and fertilization is completed 10 h after anthesis. After fertilization, wall ingrowths are formed at the micropylar and chalazal ends of the embryo sac. Embryo development conforms to the Onagrad type, and passes through 2 or more celled proembryo, long stick-shaped, globular, heart shaped, torpedo, young embryo, growing and enlarging embryo and mature embryo. Wall ingrowths are formed on the walls of basal cells and outer walls of the cells at basal region of suspenser. The suspensor remains as the seed reaches maturity. The starch grains accumulate in the cells of cotyledons by 9–16 days after anthesis, and proteins accumulate by 12–18 days after. The endosperm development follows the nuclear type. The endosperm ceils form at the micropylar end, and remain free nuclear phase at chalazal end. The outer cells are transfer cells. Those cells at the micropylar end form folded cells with wall ingrowths. At heartembryo stage, the endosperm begins to degenerate and disintegrates before the embryo matures.     

延龄草(Trillium tschonoskii Maxim.)的胚胎学研究初报Ⅰ.——大孢子的发生及雌配子体的形成

本文描述延龄草(Trillium tschonoskii Maxim.)的大孢子发生,雌配子体的形成和雄配子体的形态。胚珠为倒生型,双珠被,厚珠心型。胎座为侧膜胎座向中轴胎座的过渡类型,胶囊发育为葱型的变异型。孢原细胞直接发生于幼胚珠的珠心表皮细胞之下,孢原细胞平周分裂,形成初生周缘细胞及初生造孢细胞。初生周缘细胞分裂先于初生造孢细胞,分裂结果与珠心表皮细胞共同形成了珠心组织。初生造孢细胞进一步发育,形成大孢子母细胞。大孢子母细胞经减数第一次分裂后,即出现壁,形成二分体。一般是珠孔端二分体细胞小于合点端二分体细胞,但偶尔也见到前者大于后者的情况。在二分体形成后珠孔端二分体细胞立即退化、或经减数第二次分裂后再退化(该次分裂多为斜向的)。合点端二分体细胞发育,经二核胚囊,四核胚囊,六核胚囊阶段至成熟胚囊。一般在珠孔端的周围淀粉粒丰富,并先于合点端的核进行分裂。珠孔端由二个助细胞,一个卵细胞构成卵器,助细胞具钩突,并具丝状器,两个极核。合点端常见多核仁的大核,成熟胚囊未见八核。成熟花粉粒为二细胞的,花药壁具变形绒毡层,花粉中充满淀粉粒。沼生目型胚乳。     

矮沙冬青雌配子体及胚胎发育研究

矮沙冬青子房单心皮1室,边缘胎座,弯生胚珠,胚珠具双珠被、厚珠心。大孢子孢原细胞发生于珠心表皮下,大孢子母细胞减数分裂形成直线排列的四分体,合点端大孢子具功能,并按蓼型胚囊发育,雌配子体成熟于4月中旬。双受精后,胚乳发育为核型。在矮沙冬青大孢子发生、雌配子体和胚胎发育过程中未发现异常现象,因此认为矮沙冬青濒危不存在雌性生殖结构与发育过程异常的内在因素。     

An Ultrastructural Study of The Embryo/Endosperm Interface in the Developing Seeds of Solanum nigrum L. Zygote to Mid Torpedo Stage

The early developmental sequences in the formation of the Zoneof Separation and Secretion in a hexaploid species of Solanumnigrum L. are described. Ultrastructural changes which occurredduring the development of the embryo/endosperm interface couldbe related to the different stages in the embryo's development.The first step was the completion of the cell wall around thechalazal end of the zygote; a thin wall was formed along theendosperm cell(s) abutting the zygote. From the mature zygotestage to the quadrant stage, minute plasmalemma invaginationsoccurred along the endosperm wall facing the zygote. These invaginationsenlarged, and from the mid-globular stage onwards became filledwith a fine fibrillar material; this material accumulated betweenthe endosperm cell wall and the plasmalemma before being releasedinto the developing periembryonic and intercellular spaces tobecome the extracellular matrix. Cell wall development in theendosperm cells abutting the embryo followed an unusual path.During the quadrant stage, whilst the outer embryo wall increasedin thickness due to vesicle fusion, the endosperm cell wallfacing the embryo showed a loosening of the wall fibrils aswell as partial separation of these same endosperm cells fromeach other. From the early-globular stage, the endosperm cellwalls opposite the embryo became electron-translucent, disappearinginto the extracellular matrix. Enzymic secretions by the embryomay account for the alteration in the abutting endosperm cellwalls. Enzymic activity may also explain the development ofa homogenous electron-opaque layer over the outer embryo wallas well as the differences in the width of the fibrillar layerwhich accumulated around the cotyledons as the embryo grew throughthe Zone of Separation and Secretion. The potential roles ofthe extracellular matrix are briefly discussed. Solanum nigrum L.; embryo/endosperm interface; Zone of Separation and Secretion; embryo development; cellular endosperm     

Origin of facultative heterochromatin in the endosperm ofGagea lutea (Liliaceae)

Summary Facultative heterochromatin occurs not only in certain animals in connection with sex determination but also in members of at least one plant genus,Gagea (Liliaceae s. str.), but here in the course of embryo sac development, fertilization, and endosperm formation. The present contribution intends to provide undebatable photographic and cytometric evidence, previously not available, for the events in the course of which three whole genomes in the pentaploid endosperm nuclei ofGagea lutea become heterochroma-tinized. In this plant, embryo sac formation usually follows the Fritillaria type, i.e., the embryo sac is tetrasporic, and a 1 + 3 position of the spore nuclei is followed by a mitosis in which the three chalazal spindles fuse and two triploid nuclei are formed. A triploid chalazal polar nucleus is derived from one of these, which contributes to the pentaploid endosperm. These nuclei in the chalazal part of the embryo sac show stronger condensation compared with the micropylar ones. The pycnosis of the triploid polar nucleus is maintained and even enhanced during endosperm proliferation, while the micropylar polar nucleus and the sperm nucleus maintain their euchromatic condition. The origin of the heterochromatic masses in the endosperm nuclei from the three chalazal genomes of the central cell is unambiguously evident from the distribution of heterochromatic chromosomes in the first endosperm mitosis and the following interphase. DNA content measurements confirm a 3 2 relationship of heterochromatic and euchromatic chromosome sets, which is usually maintained up to the cellularized endosperm. Pycnotic nuclei in the chalazal part of megagametophytes are characteristic of several embryo sac types, but only forGagea spp. it is documented that such nuclei can take part in fertilization and endosperm formation.Dedicated to Professor Walter Gustav Url on the occasion of his 70th birthday     

The Morphology and Anatomy of Ovule and Fruit Development in Arachis hypogaea L

The vascular system of the monocarpellary gynoecium with tenwell differentiated traces and a few cross links probably representsa precocious development of the post-fertilization vasculatureof the fruit wall. The restriction of the two integuments ofthe ovule to the micropylar half, and the endothecial natureof the chalazal cells adjoining the embryo sac appear to indicatea pathway of derivation of the unitegmic tenumucellate ovulefrom the bitegmic crassinucellate one. During double fertilization,a dark staining refractive body appears in the nucleolus ofthe egg as well as the fusion product of the polar nuclei. The peg that carries the ovary into the soil after fertilizationgrows by the activity of a rib meristem at the basal solid partof the gynoecium. During sub-soil fruit development, the ovarywall develops a prominent spongy inner zone which finally disappears,and a peripheral zone that forms the mature fruit wall. Theabinitio nuclear endosperm is much reduced and degenerates afterproducing a few cell layers in the chalazal half alone. Seeddevelopment is pachychalazal. The main vascular supply of theseed branches at the chalaza into eight to ten strands in theseed coat. All seeds that have a vascular ramification in theseed coat are probably pachychalazal. In the variety Valencia, diminutive fruits with viable seedmay develop aerially from pegs that fail to grow long enoughto reach the soil from the higher nodes. Arachis hypogaea L., groundnut, fruit development, seed development, carpel vasculature, seed vasculature, pachychalaza     

西瓜胚乳衰退过程中的超微结构变化及其对胚发育的作用

西瓜胚乳细胞衰退过程中 ,质膜、液泡膜突起、形成体积较大的囊泡 ,内质网断裂形成体积较小的囊泡 ;细胞质和细胞核降解形成电子致密的碎片沿细胞壁分布 ;细胞壁在衰退过程逐渐变薄 ,由于部分区域分解而使整个壁呈波浪型 ,细胞降解后的物质可直接穿越薄壁处或通过宽约 5 0 nm的胞间连丝向近胚端的胚乳细胞转移。胚乳与珠心组织分界壁 -胚囊壁上有发达的壁内突 ,有利于珠心组织内的物质向胚乳内转运 ;胚乳发育早期与胚共有的壁上内外两侧均有胼胝质沉积 ,壁上无外连丝型的胞间连丝存在 ,胚乳发育后期共有壁上的胼胝质消失 ,胚乳细胞降解物可穿越共有壁进入胚细胞内。实验结果表明西瓜胚乳在发育后期对胚的发育具有重要的作用。     

Light Microscope Study of Pollen Tube Growth, Fertilization and Early Embryo and Endosperm Development in the Avocado Varieties Fuerte and Hass

Pollen tube growth, fertilization and early embryo and endospermdevelopment were studied using light microscopy in the avocadovarieties Fuerte and Hass. The ovule was penetrated by a pollen tube by 24 h after pollination.On reaching the ovary, the pollen tube grew along the surfaceof the inner ovary wall. It then grew around the funicle, throughthe micropyle in the inner integument and between the papillatecells at the apex of the nucellus. It entered the embryo sacvia a synergid. Sperm nuclei were present in the embryo sacat 48 h after pollination and fusion of the polar and spermnuclei took place before fusion of the egg and sperm. The endospermnucleus was the first to divide and cell wall formation occurredfollowing division. The first division of the zygote occurredat 5 or 6 days after pollination. In the variety Fuerte less than 20 per cent of the 1- and 2-day-oldembryo sacs had been penetrated by a pollen tube although tubeswere often observed in the integument or nucellus. In the varietyHass over 60 per cent of the embryo sacs were penetrated. Inwas concluded that low yields of the variety Fuerte may be partlyattributable to the failure of the pollen tube to penetratethe embryo sac. Persea americana Mill, avocado, pollen tube, fertilization, embryo, endosperm