The Structural Changes of the Basal Region of Wheat Proembryo in Relation to Nurture Absorption

There are some cellular fail and degeneration in the parietal area of the basal region of developing wheat proembryo. Electron microscopic studies reveal that the envelopment of peripheral wall to the proembryo is partly ruptured in this area and the disassembled protoplasm of the degenerated cells mixes with the disintegrated constituents of adjacent endosperm cells. Hence, in the limited area a direct communication between the inner surviving proembryo cells and the surrounding medium is established. A number of ectodesma-like plasmodesmata and open channels appear at the boundary wall, various nutrients may enter the proembryo via symplastic pathway or by endocytosis. The surrounding macromolecules (disassembled nuclei, mitochondria, cytoplasmic granules and vesicles packed with fibrils) appear to traverse across the wall continually, and it seems that this is'an important mode of nurture translocation. Also, within the proembryo some of the densely distributed plasmodesmata undergo modification and become fully opened for macromolect, les traversing, which is in favor of re-distribution of cell contents amongst proembryo cells. Presumably, the structural changes occurred in the basal region is a special kind of differentiation which results in function of this local area as apparatus of nurture absorption. Evidently, it would enhance the incorporation of external materials into the proembryo, and then the normal proliferation, development and differentiation of proembryo cells would be ensured.     

Transport of Disassembled Protoplasm from Degenerated Nucellus into Embryo Sac and Its Role in Feeding the Proliferating Antipodals in Wheat

The wheat embryo sac is pear-shaped and deeply imbedded in fleshy nucellus of uneven thickness, which, in mm, is enclosed partially by two layers of integument and is in intimate connection with the procambium around the chalazal region (Text fig. 1 ). This connection seems to be the main inlet passage of nutrient in the ovule. Accordingly the nutrient has to pass from cell to cell and to be incorporated in the nucellus before it is fed to the enlarging embryo sac. Though as a whole the nucellus is transitory in existence, establishment of new peripheral layers and decline of inner layers occur at the same time. While cells in the outer layers multiply by mitosis; cells in the intermediate layers begin to exhibit “nuclear extrusion” (an indication of transcellular movement of protoplasm) which becomes more frequent in inner layers (Text Fig. 2); and cells in the innermost layer, embracing the embryo sac, actively undergo disintegration, showing walls in rupture and cellular contents in disassembly and in retreat (Fig. 7,8). A distinguished feature of high activity of ATPase located in extruding nucleus has been observed in chalazal region (Fig. 4) and in degenerating nucellag cells (Fig. 5). The embryo sac is delimited from the nucellus by an incomplete envelop at the mycopylar end, and the envelop is reinforced by successive deposition of wall debris of the diminished nucellar layers (Fig. 9); whereas at the chalazal end the envelop is lacking and the anfipodals can communicate directly with the disintegrating layer of the thickened portion of nucellus. The antipodals grow steadily as the embryo approaches maturity and the number of cells can be increased 7–8 times(from 3 to 20 or more). Conceivably, proliferation of the antipodals is sustained at the expense of the disintegration of nucellar tissue. The present investigation has confirmed our previous statement that transport of disassembled protoplasm is involved in the feeding of antipodals by nucellus. Some electron micrographs are chosen to reveal details of this particular process. Some findings of special interest are listed below: 1. Cells that make up the nutritional pathway at the chalazal end are small, closely packed, and rich in mitochondria. Its wall is thickened irregularly by heavy deposition of el ectron-translucent material and is interspersed with prominent bundles of plasmodesmata (Fig. 2). It seems likely that the inlet passage is predominantly symplastic in nature. 2. Wi thdrawal of cell contents from the nucellar tissue at the early stage is carried out by efflux of nuclear substance (karyoplasm) through enlarged openings on the nuclear envelop, and by exokaryosis of vesicles packed with ribosome-like granules (Fig. 6). These vesicles can then be trapped in the ER cavity. Breakdown of the endomembrane system follows next. A multitude of small vacuoles, vesicles (coated or not) impregnated with sap, fibrils and granules respectively, deformed mitocondria, chromatic aggregates, etc. can be found in suspension within the deteriorating cell (Fig. 7, 10). In addition, degradation of polysaccharides can also take place. Withdrawal of the cell content from shrinking nucellar layer and its flow into the antipodal section is through the ruptured wall where the cellulose skeleton is turned loose and fluffy at the opening. The protoplasmic fragments in transport are those structures of definite submicroscopic constitution, resulted from disassembly and disinteg ration of the protoplasm and from reassorment of protoplasmic constituents. 3. The antipo dal cells are separated from each other by partial walls riddled with cytoplasmic canals. The naked portion of the ceU can be in direct access to the invading fragments, which can be utilized and incorporated by the antipodal cell and participate in the building of new cell possibly by reorgnization (Fig. 12a) which may be a special mode of cell proliferation in antipodals. Amitosis of antipodal nucleus also has been observed (Text Fig. 3). Discussion is made in regards to the physiological significance of the nutritional supply in form of protoplasmic fragment and of the self-propelling mobility of the fragment. Alth ough the antipodals still proliferate to some extent even after fertilization, they meet the same fate as its predecessor, the nucellus, and soon vanish during the establishment of en dosperm. In food transport, the interconversion of polymer-monomer of saccarides, etc. is fre quently involved. In the present case of material transport, interconversion at higher levels plays a dominant role as shown in the assembly and disassembly of protoplasmic constituents and in the orgnization and disorgnization of ephemeral tissue.     

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。     

Intercellular Transport of Macro-Molecular Substances as a Means of Redistribution of Cellular Contents in Detatched Garlic Scape

A detached garlic scape in long storage will eventually give rise to a whorl of freshy aerial cloves at its apex (Text fig. 2). This can only be brought about at the expense of the stalk proper, where withering starts from the lower end and extends gradually upward until the whole stalk is completely exhausted. The material transfer involved must be mainly concerned with the redistribution and reultilization of cellular contents from the senescing stalk to the growing cloves. The present systematic investigation on the whole process is primarily based upon serial microscopic and electronmicroscopic examination on conducting channels and withering parenchyma. Our previous investigations on garlic have shown that the exhaustive withdrawl of cellular contents from the senescing tissue is finally accomIished by intercellular movement of the partially disassembled protoplasm itself. The present result are essentially in agreement with such a general scheme. Light and electron-micrographs that show nuclear material and other macro-molecular substances tranversing through the plasmodesmata are rather common. The high resolving electronmicrographs have enabled us to detect the finer details in intercellular transport as given below: 1. Filamentous and fluffy material, somewhat similar in structure to P-protein in sieve tube, can be found in abundance in senescing parenchyma cells in which the demar- kations between protoplasmic components gradually become indistinct. The filamentous material is in transit through plasmodesmata between parenchyma cells and also between parenchyma and sieve tube (Plate Ⅱ, 16, 18). 2. Withdrawl of cellular contents from the deteriorating parenchyma may assume the form of vesicular transport through plasmodesmata (Plate Ⅰ 9, 10, 11). Some of the vesicles are simply filled with vacuolar sap; some fully packed with prefabricated material of maeromolecolar structure; and some actually loaded with disassembled protoplasmic fragments. 3. Fully packed vesicles as well as disassembled protoplasmie components (including disintegrated nucleus, degenerated mitoehondrion, etc.) may extrude into the intercellular spaces and may invade the vessel eavity (Plate Ⅱ, 12, 13, 20; Plate Ⅲ, 21, 22, 23, 24). The fine structure of the moving protoplasm in the vessel is quite distinct from that of the residual deposits which may cause plugging in the same cavity (Plate Ⅲ, 25, 26).     

The globby1-1 (glo1-1) mutation disrupts nuclear and cell division in the developing maize seed causing alterations in endosperm cell fate and tissue differentiation

Cereal endosperm tissues account for most of the world's calorific intake, yet the regulation of monocot seed development remains poorly understood. The maize endosperm originates with a series of free-nuclear divisions, followed by cellularisation and subsequent formation of a range of functional cellular domains. We describe the isolation and characterisation of a mutation that induces aberrant globular embryo and endosperm morphology, globby1-1 (glo1-1). Our data indicate that glo1-1 plays a role in nuclear division and cytokinesis in the developing seed. Pattern formation in the embryo is severely impaired with development arresting at premature stages, while in the endosperm, the effects of the glo1-1 mutation are manifest at the free-nuclear or syncytial stage. During cellularisation, and at later stages of development, aberrant cell division and localised domains of cell proliferation are apparent in glo1-1 endosperms. As a consequence, cell fate acquisition and subsequent differentiation of endosperm tissues are affected to varying degrees of severity. To date, it has been hypothesised that BETL cell fate is specified in the syncytium and that cell files subsequently develop in response to a gradient of signal(s) derived from the maternal pedicel region. Based on our findings, however, we propose that specification of BETL cells is an irreversible event that occurs within a narrow window of syncytial development, and that BETL cell identity is subsequently inherited in a lineage-dependent manner. Additionally, our data suggest that acquisition of aleurone cell fate does not solely rely upon signalling from the maternal surrounding tissue to the periphery of the endosperm, as previously thought, but that other factor(s) present within the endosperm are involved.     

Ultrastructure of endopolyploid antipodals inAconitum vulparia Rchb.

Summary The ultrastructure of antipodals ofAconitum vulparia Rchb. was studied at two stages of development: at the earlier stage the endosperm has several nuclei, at the later one the endosperm is multinucleate. Over the investigated period the antipodal size enlarges distinctly. The wall ingrowths increase in size and number. Finally, they occur throughout the antipodal walls except for a small area in the extreme chalazal part, sunk deep into the nucellar podium. There are no plasmodesmata in the antipodal cell walls. The cytoplasm is dense and rich in ribosomes; it shows weak vacuolation. The rough endoplasmic reticulum is well developed. At the later stage dilated cisternae of endoplasmic reticulum are formed. Mitochondria, plastids and active dictyosomes are abundant. At the later stage some giant mitochondria are present; their matrix contains a large clear area with fine fibrils and an aggregation of fibrillar material. At this stage of development plastids have two types of inclusions: electron-transparent vacuoles and aggregations of electron-dense granules. The giant endopolyploid nuclei are considerably larger than those at the mature embryo sac stage; they are lobed on all sides.During the studied periodA. vulparia antipodals seem to be at their most active state.     

ULTRACYTOCHEMICAL LOCALIZATION OF PLASMA MEMBRANE-ASSOCIATED PHOSPHATASE ACTIVITY IN DEVELOPING TOBACCO SEEDS

Plasma membrane-associated phosphatase activity was found in integumentary cells of developing tobacco ovules from the megaspore tetrad stage to seed maturity. Enzyme activity is greatest in the innermost layers of the integument from the mature megagametophyte stage on. The egg, zygote, and synergids almost totally lack plasma membrane-associated reaction product, while the antipodals show some activity at their chalazal ends. The endosperm has much plasma membrane-associated phosphatase activity in most of its cells during development, but it is primarily the outermost plasma membranes of the surface cells of the embryo that have associated reaction product. It is concluded that the plasma membrane-associated phosphatase activity is related to active transport of assimilates and that the integument is the most important site of active transport in the young ovule. After fertilization, in addition to the innermost layers of the integument, the endosperm and the outermost cells of the embryo become involved in active transport, which continues to seed maturity.     

Embryological features of Tofieldia glutinosa and their bearing on the early diversification of monocotyledonous plants

Background and Aims

Although much is known about the vegetative traits associated with early monocot evolution, less is known about the reproductive features of early monocotyledonous lineages. A study was made of the embryology of Tofieldia glutinosa, a member of an early divergent monocot clade (Tofieldiaceae), and aspects of its development were compared with the development of other early divergent monocots in order to gain insight into defining reproductive features of early monocots.

Methods

Field-collected developing gynoecial tissues of Tofieldia glutinosa were prepared for histological examination. Over 600 ovules were sectioned and studied using brightfield, differential interference contrast, and fluorescence microscopy. High-resolution digital imaging was used to document important stages of megasporogenesis, megagametogenesis and early endosperm development.

Key Results

Development of the female gametophyte in T. glutinosa is of a modified Polygonum-type. At maturity the female gametophyte is seven-celled and 11-nucleate with a standard three-celled egg apparatus, a binucleate central cell (where ultimately, the two polar nuclei will fuse into a diploid secondary nucleus) and three binucleate antipodal cells. The antipodal nuclei persist past fertilization, and the process of double fertilization appears to yield a diploid zygote and triploid primary endosperm cell, as is characteristic of plants with Polygonum-type female gametophytes. Endosperm development is helobial, and free-nuclear growth initially proceeds at equal rates in both the micropylar and chalazal endosperm chambers.

Conclusions

The analysis suggests that the shared common ancestor of monocots possessed persistent and proliferating antipodals similar to those found in T. glutinosa and other early-divergent monocots (e.g. Acorus and members of the Araceae). Helobial endosperm among monocots evolved once in the common ancestor of all monocots excluding Acorus. Thus, the analysis further suggests that helobial endosperm in monocots is homoplasious with those helobial endosperms that are present in water lilies and eudicot angiosperms.Key words: Tofieldia, Tofieldiaceae, Alismatales, monocots, embryology, female gametophyte, antipodals, development, endosperm     

Embryology and seed development of Syngonanthus caulescens (Poir.) Ruhland (Eriocaulaceae-Poales)

The embryology and the seed development of Syngonanthus caulescens are presented. This species possesses: a bithecous and tetrasporangiate anther, with a four-layered wall, a conspicuous endothecium of the baseplate type, a secretory tapetum formed by uninucleate cells, successive microsporogenesis resulting in isobilateral microspore tetrads, spiraperturate and binucleate pollen grains, an orthotropous, pendulous, bitegmic and tenuinucellate ovule, with a micropyle formed only by the inner integument, a megagametophyte of the Polygonum type, with formation of an antipodal cyst, free-nuclear and starchy endosperm, a broad and bell-shaped embryo, operculate and endotestal seeds, a seed coat derived from the inner layers of both integuments, and tanniniferous endotegmen. These embryological aspects are characteristic not only for Syngonanthus, but for the whole family, with few differences between genera. Furthermore, the pollen grain of the spiraperturate type and the cystic arrangement of the antipodals in the megagametophyte are peculiar and very distinctive features of Eriocaulaceae within the other Poales (commelinids).     

DEVELOPMENT OF NORMAL AND SEEDLESS ACHENES IN CICHORIUM INTYBUS (COMPOSITAE)

Cross- and partially cross-pollinated capitula of Cichorium intybus (Compositae, Lactuceae) were examined for a study of normal and seedless fruit development respectively. Embryos develop according to the Asterad pattern, and the free-nuclear endosperm becomes cellular 15–17 hrs after pollination. A zone of disorganized cellular material surrounds the embryo sac at anthesis, and, in normal achenes, this zone expands as the seed develops. Initially the developing seed elongates and comes into contact with the top of the ovary by 48 hrs. In contrast to this pattern, the ovule in developing seedless achenes degenerates within 72 hrs. Irregularities, such as an abnormally proliferating endothelium, embryo formation without endosperm, and endosperm formation without an embryo often accompany this degeneration. Differentiation of the pericarp in seeded achenes begins between 48 and 72 hrs, starting at the apex and proceeding basipetally; in seedless fruits the process is similar though initiated somewhat later. The normal pericarp at maturity exhibits a pigmented exocarp, a broad mesocarp of thick-walled lignified cells, and a tenuous endocarp. In seedless achenes the fruit coat is similar except that the exocarp is colorless and the cells of the mesocarp are relatively small.     

Studies on nuclear degeneration during programmed cell death of synergid and antipodal cells in<Emphasis Type="Italic"> Triticum aestivum</Emphasis>

Morphological changes in the nuclear degeneration of the synergid (mainly the synergid that receives the pollen tube) and antipodal cells in Triticum aestivum were studied. Although located in the same embryo sac, and derived from the same megaspore, nuclear degeneration of the synergid and antipodal cells differs greatly. Nuclear degeneration in the synergid is characterized by pycnosis, i.e., total chromatin condensation, nuclear deformation and distinct shrinkage in volume, followed by the formation of an irregular and densely stained mass—the degenerated nucleus—while the nucleolus disappears prior to the degradation of chromatin. In contrast, in the nuclear degeneration of antipodal cells, chromatin is only partly condensed and the nuclear volume changes only slightly after the distinct chromatin condensation. Chromatolysis then occurs, i.e., stainable contents disappear while the nuclear envelope is retained. The nucleoli persist after the disappearance of the chromatin. The possible functions of nuclear degeneration of synergid and antipodal cells are discussed, especially with respect to the guidance of pollen tube growth and the proliferation of free-nuclear endosperm. The degeneration of synergids and antipodal cells in T. aestivum are distinct forms of programmed cell death, regarded as cytoplasmic cell death and nuclear degradation in advance of cell death, respectively.     

水稻淀粉胚乳细胞编程性死亡中细胞核变化特征

应用透射电子显微镜技术 ,观察了水稻 (OryzasativaL .)淀粉胚乳细胞编程性死亡过程中核的变化特征。伴随胚乳的发育进程 ,淀粉胚乳细胞核表现出衰退特征 :核变形、染色质凝缩、核膜多处被降解破坏、核基质外泄等。DNALadder显示核内大片段DNA呈严重的弥散状拖尾现象 ,而核内和胞质中在 14 0～ 180bp处有明显的条带。在核衰退的同时 ,其胞质中的粗面内质网、淀粉质体和线粒体等细胞器具有正常的代谢功能 ,细胞仍在合成并积累营养物质 ,淀粉胚乳细胞一边衰退一边行使其功能 ,直至死亡。这些结果表明 ,水稻淀粉胚乳在核衰退的同时 ,细胞仍在积极合成与积累贮藏产物 ,表现为一种特殊形式的植物细胞编程性死亡现象。此外 ,对淀粉胚乳细胞特有的核质关系、植物细胞编程性死亡过程中细胞核的变化等问题进行了讨论。     

EMBRYOLOGY AND DEVELOPMENT OF THE ENDOSPERM HAUSTORIUM OF ARCEUTHOBIUM DOUGLASII

Arceuthobium douglasii develops a dome-like structure, the ovarian papilla, in which 2 megasporocytes are formed. The papilla is not a true ovule, for no integuments are formed, and it is forced aside by the developing endosperm. Megasporocytes are differentiated in the spring, but meiosis does not occur until the following spring. A tetrasporic embryo sac is developed which is 8-nucleate at maturity. Pollination and fertilization occur approximately 13–14 months after initiation of the inflorescence. Only 1 of the 2 embryos develops after fertilization. After fertilization, the embryo sac segregates into 2 parts, one containing the zygote and the disintegrating synergids, the other the primary endosperm nucleus and the degenerating antipodals. This primary endosperm cell elongates toward the base of the ovarian papilla. Cytokinesis then forms an endosperm cell, adjacent to the zygote, and a haustorial cell. The haustorial cell forms several tiers of cells which persist during the development of the embryo and endosperm. The zygote, while still contained within the ovarian papilla, divides, forming a 2-celled sphere. It remains unchanged until after it is conveyed out of the ovarian papilla by the developing endosperm. The development of the embryo and endosperm is arrested in the autumn approximately 3 months after their initiation. They complete their development the following spring and summer.     

The dynamics of protein body formation in developing wheat grain

Wheat is a major source of protein in the diets of humans and livestock but we know little about the mechanisms that determine the patterns of protein synthesis in the developing endosperm. We have used a combination of enrichment with ¹⁵N glutamine and NanoSIMS imaging to establish that the substrate required for protein synthesis is transported radially from its point of entrance in the endosperm cavity across the starchy endosperm tissues, before becoming concentrated in the cells immediately below the aleurone layer. This transport occurs continuously during grain development but may be slower in the later stages. Although older starchy endosperm cells tend to contain larger protein deposits formed by the fusion of small protein bodies, small highly enriched protein bodies may also be present in the same cells. This shows a continuous process of protein body initiation, in both older and younger starchy endosperm cells and in all regions of the tissue. Immunolabeling with specific antibodies shows that the patterns of enrichment are not related to the contents of gluten proteins in the protein bodies. In addition to providing new information on the dynamics of protein deposition, the study demonstrates the wider utility of NanoSIMS and isotope labelling for studying complex developmental processes in plant tissues.     

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.     

Roles of isoamylase and ADP-glucose pyrophosphorylase in starch granule synthesis in rice endosperm

Amyloplast-targeted green fluorescent protein (GFP) was used to monitor amyloplast division and starch granule synthesis in the developing endosperm of transgenic rice. Two classical starch mutants, sugary and shrunken, contain reduced activities of isoamylase1 (ISA1) and cytosolic ADP-glucose pyrophosphorylase, respectively. Dividing amyloplasts in the wild-type and shrunken endosperms contained starch granules, whereas those in sugary endosperm did not contain detectable granules, suggesting that ISA1 plays a role in granule synthesis at the initiation step. The transition from phytoglycogen to sugary-amylopectin was gradual in the boundary region between the inner and outer endosperms of sugary. These results suggest that the synthesis of sugary-amylopectin and phytoglycogen involved a stochastic process and that ISA1 activity plays a critical role in the stochastic process in starch synthesis in rice endosperm. The reduction of cytosolic ADP-glucose pyrophosphorylase activity in shrunken endosperm did not inhibit granule initiation but severely restrained the subsequent enlargement of granules. The shrunken endosperm often developed pleomorphic amyloplasts containing a large number of underdeveloped granules or a large cluster of small grains of amyloplasts, each containing a simple-type starch granule. Although constriction-type divisions of amyloplasts were much more frequent, budding-type divisions were also found in the shrunken endosperm. We show that monitoring GFP in developing amyloplasts was an effective means of evaluating the roles of enzymes involved in starch granule synthesis in the rice endosperm.     

Observations on the Rate of Early Embryogenesis and Cytomixis in Spring Wheat

The rate of early embryogenesis and cytomixis of spring wheat has been observed. The results obtained are summarized as follows: 2 h. after pollination, the two sperms entered into the egg nucleus and the polar nucleus respectively; after 6 h. triple fusion has been completed, a male nucleolus is discernible in egg nucleus. Twenty-four h. after pollination, 2-celled proembryo can be detectable, after 6 d the differentiation in some embryo has initiated. Three d after pollination, the formation of endosperm cells are proceeding, upto 6 d, the embryo sac are full of endosperm cells. After pollination through full development, the degeneration of the antipodals appears, 4 d later the structure of cell and nucleus disappeared. Only naked nucleoli and chromatin mass are remained. After 8 d the degeneration of antipodals almost has completed. Cytomixis has been seen in the cell of nucellus, endosperms, ovary, proembryo and differentiated embryo and it frequently appeared near the proembryo.     

An electron microscopic study of the mature megagametophyte in Zea mays

With light microscopy maize megagametophytes stained with Alcian blue-periodic acid-Schiff (AB-PAS) reveal acid or neutral polysaccharides in various cell walls. Comparative fine structural studies were made of permanganate- or OsO₄-fixed material. Organelle distribution is random in the vacuolate and multinucleate antipodal cells; organelles are abundant; starch is scarce. Antipodal cell walls have large openings forming several syncytia. Some walls are papillate. In the central cell (primary endosperm cell) a thin peripheral layer of cytoplasm surrounds the large vacuole; organelle number is moderate; starch is abundant. The central cell wall is also papillate adjacent to the antipodals and around the egg apparatus. In the synergids organelle distribution is non-random; nuclei and numerous organelles occupy the micropylar cytoplasm of each synergid; vacuoles dominate the chalazal cytoplasm of these cells. The filiform apparatus stains with AB-PAS and is composed of both lightly and darkly stained amorphous material. In the egg, organelle distribution is perinuclear with vacuoles proximal to the micropyle; mitochondria are large, abundant and polymorphic; starch is abundant. Nucleolar diameter is five times greater in the central cell and egg than in the antipodal cells and ten times greater than in the synergids. Plasmodesmata occur in all cell walls within the gametophyte, but none appear in the gametophyte wall itself. It is suggested that the antipodals and synergids might be secretory, the latter probably being involved in pollen tube attraction, and that stored metabolites in the central cell and egg cytoplasm support rapid increase in metabolism following fertilization.     

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.     

Cytological Studies on the Embryogenesis of Hybrids between Oryza sativa L.(Pennisetum sp.

(1) The pollen grains of Pennisetum can germinate normally on the stigma of rice and the pollen tubes can grow into the style and enter the embryo sacs. However, the process of double fertilization is slow and more or less abnormal and phenomenon of simple fertilization often occurs. (2) It has been found that in the majority of cases the development of the embryos is slow and stays long in the stage of globular embryos, thus, the differentiation of the embryos is very difficult and degeneration of the embryos appears many times. Simple differentiation was observed only in some embryos during 16–24 days after pollination. Normal differenting and developing embryos were not observed. The cause of the degeneration of the embryos is related to the state of endosperm development and also to the non-coordination of the genomes of both parents. (3) The development of the endosperm is abnormal. The change from the free nuclei into the cells in the endosperm is delayed as late as the 8th day after pollination. The whole endosperm tissue is composed of the cell masses which are quite different both in shape and function, a part of these endospemn cells lacks the ability to synthesize starch. The disintegration of the endosperm could be frequently observed during their development. (4) A lots of starch are accumulated in the nucellar cells near the antipodals, It is shown that there was some metabolic confusion resulted from the crossing in the embryo sacs. Based on the above mentioued results the authers consider that the failure of producing seeds by crossing is at least related to the nutrient condition which are essential for the development of embryos. If embryo culture technique is employed at the early stage of the embryo development the hybrid seeds could be obtained.