ULTRASTRUCTURE OF THE DEVELOPING MEGASPORE MOTHER CELL OF GINKGO BILOBA

The immature megaspore mother cell of Ginkgo biloba is essentially spherical and is surrounded by a thick, complex wall. A large nucleus occupies the central region of the cell, and the organelles appear to be randomly arranged in the cytoplasm. With approaching maturity and the onset of meiosis, the cell elongates in the direction of the ovular axis. An extensive system of ER develops at the micropylar pole of the cell during elongation, and the plastids and mitochondria migrate to the opposite or chalazal pole. The micropylar end of the mature megaspore mother cell is usually devoid of plastids and mitochondria, but these organelles are densely packed in the chalazal end of the cell below the nucleus. The dictyosomes and dense spherosome-like bodies do not show such polarity in their distribution. At meiosis I plastids and mitochondria are, as a rule, restricted to the chalazal dyad cell that is destined to produce the functional megaspore. The wall of the megaspore mother cell consists of a middle lamella which is irregularly thickened, an outer wall layer resembling the walls of the surrounding nutritive cells, and an inner layer resembling the middle lamella in appearance.     

水稻籼粳杂种F_1雌配子体败育的超微结构和酸性磷酸酶细胞化学研究(简报)

籼粳稻亚种间杂交是获得强优势种的有效途径之一。但主要存在如何克服杂种F_1结实率低的问题。影响F_1结实率的因素是很复杂的,部分胚囊败育而丧失授精能力是其中的主要原因。因此阐明籼粳杂种雌配子体败育的机理,成了当前籼粳亚种杂交优势利用研究的重要课题之一。目前对籼粳杂种雌配子体败育的细胞学研究特别是超微结构方面的研究不多。Oka和Doida观察到败育发生在大     

栽培甜菜大孢子发生的超微结构

栽培甜菜（Beta vulgaris）的大孢子发生为蓼型。减数分裂时,大孢子母细胞核中出现核液泡,形成联会复合体,细胞壁上有胼胝质加厚,并存在细胞质改组现象。大孢子母细胞减数第1次分裂形成二分体,2个细胞均被较厚的胼胝质壁包裹。合点端的二分体细胞中细胞器丰富,线粒体和质体的形态正常,表明完成了再分化。在大多数情况下,珠孔端的二分体细胞在减数第2次分裂前（或分裂的过程中）退化,合点端的细胞分裂产生大小不等的2个细胞,形成三分体。三分体合点端的大孢子体积较大,发育成单倍体的功能大孢子。     

栽培甜菜大孢子发生的超微结构

栽培甜菜(Beta vulgaris)的大孢子发生为蓼型。减数分裂时, 大孢子母细胞核中出现核液泡, 形成联会复合体, 细胞壁上有胼胝质加厚, 并存在细胞质改组现象。大孢子母细胞减数第1次分裂形成二分体, 2个细胞均被较厚的胼胝质壁包裹。合点端的二分体细胞中细胞器丰富, 线粒体和质体的形态正常, 表明完成了再分化。在大多数情况下, 珠孔端的二分体细胞在减数第2次分裂前(或分裂的过程中)退化, 合点端的细胞分裂产生大小不等的2个细胞, 形成三分体。三分体合点端的大孢子体积较大, 发育成单倍体的功能大孢子。     

竹节参雌配子体发育的研究

本文报道了竹节参(Panax japonicus C.A.Mey)雌配子体(胚囊)的发育过程。竹节参大孢子母细胞减数分裂产生线形排列的大孢子四分体。胚囊发育属蓼型,由合点端大孢子发育而成。游离核胚囊时期,胚囊珠孔端的细胞器种类和数量都较胚囊合点端多;胚囊合点端相邻的珠被细胞中有含淀粉粒的小质体,与胚囊珠孔端相邻的退化中的非功能大孢子中则有含淀粉粒的大质体和大类脂体。成熟胚囊中,反足细胞较早退化;极核融合成次生核;卵细胞高度液泡化,细胞器数量较少;助细胞则有丰富的细胞器和发达的丝状器。PAS反应表明,受精前的成熟胚囊中积累淀粉粒。次生核受精后,很快分裂产生胚乳游离核,到几十至数百个核时形成胚乳细胞。卵细胞受精后则要经过较长的休眠期。     

Ultrastructure of endopolyploid antipodals inAconitum vulparia Rchb.

Summary The ultrastructure of the antipodals ofAconitum vulparia Rchb. was studied in mature embryo sacs. Antipodal cell wall thickness varies in different parts of the cells. The antipodals resemble transfer cells with distinctly marked wall ingrowths which are particularly well developed in the chalazal part and between the antipodals. A few plasmodesmata occur in the cell wall between the antipodals and the central cell. The cytoplasm is rich in ribosomes which occur free or bound to the membranes of the well developed endoplasmic reticulum. Only in the micropylar region of the cells are some larger vacuoles found. The antipodals contain numerous mitochondria, plastids and apparently active dictyosomes. Vesicles with electron dense contents, microbodies, multivesicular bodies as well as lipid droplets and small multiple concentric cisternae are also present in the cytoplasm. The giant endopolyploid nuclei have lobed outlines, especially at the chalazal side of the nuclei.Ultrastructural features, especially the occurrence of numerous free ribosomes and the development of extensive rough endoplasmic reticulum, suggest high metabolic activity in the growing and differentiating antipodals of this species.     

Alisma embryogenesis: The development and ultrastructure of the suspensor

Summary The development of the suspensor (consisting of a basal cell and a few chalazal cells) inAlisma plantagoaquatica andA. lanceolatum was investigated using cytochemical methods, light and electron microscopy. The basal cell becomes differentiated during the first three days of embryo development. As a result of endopolyploidization the volume of the nucleus rapidly increases, as does the quantity of chromatin it contains and the size of the nucleolus. As basal cell grows, its cytoplasm increases in volume and the number of organelles increase, and wall ingrowths begin to form on the walls at the micropylar pole of the cell. The full development and functioning of the suspensor occurs during the next three days. The enormous basal cell then attains its maximum degree of differentiation: its nucleus reaches a ploidy of 256n or 512n, the micropylar transfer wall is fully developed, as is the cytoplasm, rich in proteins, ribonucleic acids (RNA) and organelles, particularly dictyosomes and long cisternae of the rough endoplasmic reticulum. The chalazal suspensor cells joining the embryo proper to the basal cell also become differentiated. In the seven-day embryo the suspensor begins to degenerate which coincides with the cellularization of the endosperm at the micropylar pole of the embryo sac. The senescence of the suspensor involves the degradation of the nucleus, increasing cytoplasmic vacuolization, and a distinct decrease in protein and RNA content, first in the basal cell, then in the chalazal suspensor cells. Analysis of the development and ultrastructure of the basal suspensor cell suggests that it plays the role of an active metabolic transfer cell, translocating nutrients from the maternal tissues via the chalazal suspensor cells to the growing embryo proper.     

Ultrastructure and development of the megagametophyte in Eleusine tristachya (Lam.) Lam. (Poaceae)

Eleusine tristachya (Lam.) Lam. is native from subtropical South American climates. Widespread in Argentina and Uruguay, it is frequently found in landscape prairies of the province of Buenos Aires. Megasporogenesis and megagametogenesis in this species were studied using light and transmission microscopy. The ovule is hemitropous, bitegmic and tenuinucellate. The megaspore mother cell enlarges and undergoes meiosis division resulting in a T-shaped tetrad of megaspores. The three micropylar megaspores degenerate, and the chalazal one develops into the Polygonum-type megagametophyte. The synergid cells have the cytoplasm very electron dense because it has got a rich complement of organelles. The synergid wall is strongly thickened at the micropylar pole, developing the filiform apparatus. At maturity, the antipodals originate a wall with large projections into the cytoplasm, acquiring transfer cells characteristics. The antipodals cytoplasm, enriched with organelles shows a high metabolic activity, and it is suggested that these cells perform as an efficient system for metabolites transport.     

Apomixis in plants: structural and functional aspects of diplospory inPoa nemoralis andP. palustris

Summary Data on structural and functional aspects of mitotic diplospory and later stages of apomictic seed formation are reported forPoa palustris andP. nemoralis. In this study, the plant material of two Russian populations ofP. nemoralis andP. palustris were used for transmission electron microscope observations. Seed formation was investigated by phase contrast microscopy in two populations ofP. nemoralis collected in The Netherlands. The processes of transformation of the megasporocytes to the megaspore mother cells of diplosporous embryo sacs, and thereafter to one- and two-nucleate diplosporous embryo sacs (Antennaria type) were characterized by an increase of cell size, structural and functional reorganization of the nucleus, nucleolus, and cytoplasm, and cell isolation as a result of thickening of the cell wall. These were accompanied by an increase in the cell metabolic activity inferred from visual evidence of the activation of nucleus, nucleolus, endoplasmic reticulum, dictyosomes, mitochondria, and from the appearance of a dense population of ribosomes and polysomes. The diplosporous embryo sac of the Antennaria type was characteristic for bothP. nemoralis andP. palustris. No signs of the presence of synaptonemal complexes were observed during the process of diplosporous-embryo-sac megaspore mother cell differentiation and division. About 90–95% of the diploid egg cells of diplosporous embryo sacs were able to produce apomictic embryos. These embryos developed before anthesis. However, many of them degenerated at the globular stage because of lack of endosperm. The ultrastructural events occurring during the process of diplospory of apomictic species, and meiosis and megagametogenesis of sexually reproduced plants are discussed.Abbreviations DMC megaspore mother cell of diplosporous embryo sac - TEM transmission electron microscopy - ER endoplasmic reticulum     

STRATIFICATION AND SUBSEQUENT BEHAVIOR OF PLANT CELL ORGANELLES

Living excised roots of pea were centrifuged at 20,000 g for 24 hours, and the behavior of organelles was followed by electron microscopy at various intervals after centrifugation. With these forces, organelles are not perceptibly or irreversibly damaged, nor is the viability of the whole root destroyed. Organelles stratify generally in the order of lipid (centripetal pole), vacuoles, smooth endoplasmic reticulum and dictyosomes, proplastids (without starch), mitochondria, rough endoplasmic reticulum, proplastids with starch. The nucleus distends from the vacuolar region to the extreme centrifugal pole of the cell, while the chromatin and nucleolus seek the centrifugal pole of the nucleus. During the redistribution of organelles the rough endoplasmic reticulum is among the first to reorient, and possible explanations for this are discussed. Mitochondria can be stretched elastically many times their original length, but proplastids seem fairly rigid. Small vacuoles, forced together during centrifugation, apparently may fuse to form a large unit. Lipid droplets, on the other hand, tend to remain separate. Dictyosomes and smooth endoplasmic reticulum layer in the same region of the centrifuged cell, indicating a density similarity between these two organelles.     

大车前胚乳发育的超微结构研究

采用透射电镜技术对大车前(Plantago major L.)胚乳发育的超微结构进行了研究。结果表明:(1)大车前为细胞型胚乳;初生胚乳核经一次横分裂产生1个珠孔室细胞和1个合点室细胞;珠孔室两次纵向分裂一次横向分裂形成2层8个细胞,位于上层的4个细胞发育为4个珠孔吸器,位于下层的4个细胞发育为胚乳本体;合点室细胞进行一次核分裂,发育为两核的合点吸器。(2)珠孔吸器呈管状插入珠被组织,珠孔端细胞壁加厚呈现少量分支并具有壁内突,壁内突周围细胞质里分布着大量线粒体、粗面内质网、高尔基体、质体等,细胞核与核仁明显,细胞质浓厚,代谢活动旺盛;球胚期,珠孔吸器的体积呈现最大值,珠孔吸器周围的珠被组织均被水解,形成明显的空腔。珠孔吸器从珠被组织吸收并转运营养物质至胚乳本体,参与胚乳的构建与营养物质的贮藏。球胚后期,珠孔吸器逐渐退化。(3)4个胚乳本体原始细胞具旺盛的分生能力,经不断的平周与垂周分裂增加胚乳细胞数目,使胚乳本体呈现圆球体状,并将胚包围其中;珠孔吸器、合点吸器以及珠被绒毡层吸收转运的营养物质贮存在胚乳本体;球胚后期,随着胚柄的退化,胚体周围的胚乳细胞被水解,为发育的胚所利用。(4)合点吸器的2个细胞核与核仁巨大,线粒体、质体、高尔基体、内质网主要绕核分布,液泡化明显;胚体与胚乳本体的体积增大,逐渐将合点吸器向胚珠合点部位挤压,合点吸器周围的合点组织逐渐被水解,形成巨大空腔。合点吸器自珠心组织吸收并转运营养物质至胚乳本体,参与胚乳的结构构建与营养物质的贮藏。球胚后期,合点吸器逐渐失去功能,呈现退化状态。     

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.     

CAPSELLA EMBRYOGENESIS: THE EGG,ZYGOTE, AND YOUNG EMBRYO

The ultrastructure and composition of the egg, zygote, and young embryo of Capsella bursa-pastoris were examined. The egg is a highly polarized cell; one-half to one-third of the micropylar end is filled with a large vacuole while the chalazal end contains the nucleus and much of the cytoplasm of the cell. The wall which surrounds the cell is incomplete at the chalazal end. Ribosomes fill the cytoplasm and show little or no aggregation into polysomes. The structure of the nucleolus suggests that ribosomes are not being produced. Following fertilization and the formation of the zygote, the cell decreases slightly in volume as the large central vacuole becomes smaller. The zygote soon increases in size as the small chalazal vacuoles present before fertilization begin to enlarge. The dictyosomes become active and a continuous wall forms around the zygote. Aggregation of the ribosomes begins and numerous polysomes are formed. Before division of the zygote all plasmodesmata between the zygote and the surrounding cells are lost. The first division of the zygote is unequal as a result of its marked polarity. A large basal cell and a small terminal cell are produced. The basal cell appears to contain more protein, RNA, carbohydrate, and cell organelles than the terminal cell. Ribosomal aggregation is even more pronounced at this stage. Starch accumulates in the plastids. Numerous plasmodesmata are present between the terminal and basal cells but there are no connections between the endosperm or other cells. The basal cell divides next to give rise to a three-celled linear embryo consisting of the basal cell, the suspensor cell, and the terminal cell. The terminal cell stains more intensely for protein and RNA as a result of increased numbers of ribosomes. Starch in all the cells is about equal and reaches a maximum in the embryo at this stage.     

Callose in cell walls during megasporogenesis in angiosperms

Summary Callose was detected by fluorescence microscopy in megasporogenesis in all investigated species with mono- and bisporic embryo-sac development. Callose occurs first in the meiotic prophase in the chalazal part of the megasporocyte wall and by the first meiotic metaphase the whole cell is enveloped in a callose-containing wall. Later, there is a marked decrease of callose fluorescence, usually at the chalazal end of the megasporocyte. In Oenothera, where the micropylar megaspore is active, decrease of fluorescence takes place at the micropylar pole of the megasporocyte. Callose appears centrifugally in the cell plates forming eventually the walls dividing the megaspores. It disappears from the walls of the megaspores during degeneration and differentiation.     

Ultrastructure of endothelium in ovules of Penstemon gentianoides Poir. (Scrophulariaceae) at mature embryo sac phase

In this study ultrastructural differences between endothelial cells of different location in Penstemon gentianoides have been examined with electron microscope at mature embryo sac phase. Embryo sac is of the Polygonum type and surrounded by endothelium except the micropylar region. The cuticle is located primarily around the chalazal three-fourths of the embryo sac. Endothelium cells around the chalaza and toward the micropylar region are rich in cytoplasmic organelles. The cytoplasm of endothelial cells near the central cell has large vacuoles and few organelles. There are also plasmodesmas on the anticlinal walls of endothelial cells. The endothelium and the micropylar integumentary cells play a role in transport of metabolites into the embryo sac.     

ULTRASTRUCTURE AND ACID PHOSPHATASE IN PEDICEL ABSCISSION OF HIBISCUS

Pedicel abscission in Hibiscus rosa-sinensis was investigated by light and electron microscopy. During the pre-abscission period endoplasmic reticulum declined somewhat, dictyosomes increased in number and apparent activity, and mitochondria maintained their numbers. The observations suggested that dictyosomal vesicles were migrating to and fusing with the plasma membrane. The enzyme acid phosphatase was associated with dictyosomes and dictyosomal saccules, with small vacuoles and invaginations of the plasma membrane, and in the paramural region between the plasma membrane and the cell wall. Our interpretation is that acid phosphatase, (and probably also the enzymes involved in cell wall dissolution) are transported via an endoplasmic reticulum-dictyosome-vesicle carrier system to the paramural regions of the cell. In more general terms, our observations support the view that the enzymes involved in the cell wall hydrolysis of abscission are synthesized within a compartmentalized, lysosomal system prior to their release and action.     

Studies of Ultrastructure and ATPase Localization of Mature Embryo Sac in Vicia faba L.

Studies of ultrastructure and ATPase localization of the mature embryo sac in Vicia faba L. show that the egg cell has no cell wall at thechalazal end, it has a chalazally located nucleus and a large micropylar vacuole. There are many nuclear pores in the nuclear membrane. The cytoplasm is restricted around the nucleus. Dictyosome and mitochondria are few. There are some starch grains and lipid grains in the egg cytoplasm. There are no obvious differences between two synergids. No cell wall is seen at the chalazal end either, but there are some vesicles which project to vacuole of the central cell and fuse with its vacuolar membrane. Plasmodesmata connections occur within the synergid wall where it is adjacent to the central cell. The synergid has a micropylarly located nucleus and a chalazal vacuole, the nucleus is irregularly shaped. The synergid cytoplasm is rich in organelles. The filiform aparatus is of relatively heterogeneous structure. The central cell is occupied by a large vacuole and its cytoplasm is confined to a thin layer along the empryo sac wall, but is rich in various organelles, starch grains and lipid bodies. Nucleolar vacuoles are often present two polar nuclei. The nuclear membranes of two polar nuclei have partly fused. ATPase reactive product was located obviously at the endoplasmic reticulum in cytoplasm of the egg cell and central cell. The embryo sac wall consists of different density of osmiophilic layer. There are some wall ingrowths in chalazal region of the embryo sac. The long-shaped and cuneate cells of chalazal region are peculiar. Special tracks of ATPase reactive products are visible at their intercellular space which may be related to transportation of nutrients.     

Histochemical and ultrastructural changes in the haustorium of date (Phoenix dactylifera L.)

Summary During imbibition ofPhoenix dactylifera embryos, all cotyledon cells show the same changes: protein and lipid bodies degrade, smooth endoplasmic reticulum (ER) increases in amount, and dictyosomes appear. At germination, the distal portion of the cotyledon expands to form the haustorium. At this time, epithelial cells have a dense cytoplasm with many extremely small vacuoles. Many ribosomes are present along with ER, dictyosomes, and mitochondria. The parenchyma cells have large vacuoles and a small amount of peripheral cytoplasm. Between 2 and 6 weeks after germination, epithelial cells still retain the dense cytoplasm and many organelles appear: glyoxysomes, large lipid bodies, amyloplasts, large osmiophilic bodies, and abundant rough and smooth ER which appear to merge into the plasmalemma. A thin electron-transparent inner wall layer with many small internal projections is added to the cell walls. Starch grains appear first in the subsurface and internal parenchyma and subsequently in the epithelium. Lipid bodies, glyoxysomes, protein, and osmiophilic bodies occur in the epithelial and subepithelial cell layers but not in the internal parenchyma. At 8 weeks after germination, the cytoplasm becomes electron transparent, vacuolation occurs, lipid bodies and osmiophilic bodies degrade, and the endomembranes disassemble. After 10 weeks, the cells are empty. These data support the hypothesis that the major functions of the haustorium are absorption and storage.     

Origin of Megagametophytes with Supernumerary Egg Cells Occurring in Oryza sativa

Cytoembryological observations were attempted to reveal the cytological origin of megagametophyte with supernumerary egg cells. It was shown that all ovules underwent a normal megasporogenesis. The meiosis of megasporocyte consisted of two successive divisions, which gave rise to four haploid megaspores. It was the chalazal spore that developed to form the megagametophyte while the three micropylar megaspores degenerated quickly. After first mitosis in the functional megaspore the two nuclei were separated to the micropylar and chalazal poles by a large central vacuole, meanwhile a differential enlargement of the two-nucleate embryo sac was visualized. The micropylar side enlarged quickly and in contrast, the chalazal side remains almost unchanged. Immediately afterward, the second mitosis took place forming four-nucleate embryo sac. During the second mitosis, nucleus located in the narrow area of chalazal side divided transversely, with its upper sister nucleus migrating to the central or micropylar part of the embryo sac, while the nucleus in the micropylar side divided at an angle of about 45° against the micropylar-chalazal axis. Through the third mitosis, two patterns of nuclear arrangement deviating from polygonum were observed. (i) One nuclear distribution pattern was two, two, four respectively in chalazal, central and micropylar parts. And during maturation the four micropylar nuclei differentiated as egg apparatus consisting of two egg cells and two synergids. The two central nuclei, which presumably suppressed the movement of nucleus toward centre part from both micropylar and chalazal sides developed into central cell with two polar nuclei. And the two chalazal nuclei organized into antipodal cells. Rarely indeed, one nucleus of either chalazal or micropyle side did migrate to join the formation of central cell. (ii) The other nuclei arrangement pattern was two and six respectively positioned in chalazal and micropylar sides. During maturation, five micropylar nuclei differentiated into egg apparatus consisted of three egg cells and two synergids. The sixth one migrated to form the upper polar nucleus. The lower nucleus of the chalazal side developed into antipodal cell which divided quickly, and the upper nucleus became the lower polar nucleus.     

甜菜无融合生殖单体附加系M14 成熟胚囊的超微结构特征

以甜菜无融合生殖单体附加系M14(Beta vulgaris, 2n=18+1)为实验材料, 利用电子显微镜技术对成熟胚囊及其超微结构进行研究。结果表明: M14成熟胚囊包括1个卵细胞、2个退化的助细胞、1个具有次生核的中央细胞和3-6个反足细胞。其卵细胞具有3种不同的形态: (1)极性正常的卵细胞, 细胞核位于合点端, 细胞质含有大量核糖体、线粒体、内质网等细胞器; (2)细胞核位于细胞中央; (3)细胞核位于珠孔端, 且后2种形态细胞器的种类与数量少。大多数胚囊中的2个助细胞在开花前已退化。中央细胞的次生核位于反足细胞附近; 未经受精自发分裂前的卵细胞与中央细胞的细胞核大、核仁明显, 细胞器的种类与数量多, 呈现旺盛代谢活动特征, 成为二倍体孢子无融合生殖过程中, 卵细胞与次生核自发分裂的细胞学标志。