SUMMARY-It is known that in Taxus (Taxaceae) megasporogenesis is carried out, for the most part, in a single megasporocyte. Upon maturity, this cell presents a grouping of the greater part of both the mitochondria and the amylipherous reserves in its chalazal cytoplasm. The chalazal megaspore, which inherits a larger supply of the said organelles, gives rise to the female gametophyte. These aspects essentially agree with what has been generally observed in gymnosperms. Nevertheless, research carried out under the LM shows that in T axus there is -even if sporadically -differentiation of several megasporocytes. Moreover, the behaviour of the kinoplasmic mass is variable and any of the megaspores arising from the same meiotic act can be the functional one. Finally, more than one megaspore, even up to four, can begin to develop. The aim of this investigation was to identify in the megasporocyte of Taxus baccata L. the possible ultrastructural bases for the afore-mentioned uncommon aspects. The most significant new result of this study shows that frequent plasmodesmata, previously observed only in the walls of the other nucellus cells, are present in the megasporocyte wall, as well. The peculiar fine structure and the delayed occlusion of these cytoplasmic connections suggest that the additional multi-directional simplastic circulation of nutrients before and during meiosis takes place. This condition may allow for the maturation of some meiocytes and may even allow for at least the initial development of all the megaspores which arise from a single meiotic process. However, the megasporocyte chalazal cytoplasm is usually the most privileged in relation to the development of the female gametophyte because of its nearness to the conducting tissues of the ovules.
There is, however, a particular aspect in Tillandsia worth underlining, that is the mechanism by which the occlusion of the nectariferous slits is attained once the epithelium has become inactive. This mechanism does not only involve the approaching of the epithelium tangential cell walls, which at this stage are considerably thick, but it is due mainly to the production of a cementing substance made of acid polysaccharides.
SUMMARY -The mature megasporocyte of various gymnosperms appears as a clearly polarized cell. With reference to the genus Larix Miller (Pinaceae), previous LM observations showed the presence of starch accumulations in the chalaza! cytoplasm in L. occidentalis. A peculiar polarization, characterized by the presence of «reticular plasma» in the micropylar cytoplasm, and by a noteworthy abundance of starch in the chalaza! region, has been observed in L. sibirica. The present TEM study examines the differentiation of the megasporocyte and the behaviour of the organelles during meiosis in L. leptolepis (Sieb. et Zucc.) Gordon. The most important results are the presence of abundant perinuclear RER complexes in the micropylar cytoplasm of the mature megasporocyte -such a finding can clarify the «reticular plasma» observed in L. sibirica -and the localization of the majority of the mitochondria and amyliferous reserves in the chalaza! cytoplasm. It follows that the four megaspores produced by meiosis have different cytoplasmic contents and are of different sizes. The chalaza! megaspore, which will give rise to the female gametophyte, is clearly privileged. This agrees with observations reported by other authors as regards other gymnosperms. We have found in L. leptolepis the segregation of the majority of the megasporocyte RER into the micropylar megaspores. These latter remain alive for a long period of time. Increases in RER and ribosomes and the localization of numerous RER elements around autophagic-type vacuoles, degenerating plastids and mitochondria suggest a transformation of micropylar megaspore contents into nutrients which may be used for the gametophyte development. In agreement with what has generally been observed in gymnosperms, the meiotic process involves a certain isolation of the megasporocyte cytoplasm from the surrounding cells. The megasporocyte wall thickening in L. leptolepis seems to be determined by an abundant deposition of components similar to those also present in the walls of the surrounding cells. Notwithstanding this, these components are thought to have a peculiar arrangement which is responsible for ultrastructural layering.
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