On the Megasporogenesis inChamaecyparis LawsonianaParl.

Abstract

SUMMARYIn Chamaecyparis lawsoniana Parl, the megaspore mother cell (MMC) differentiates in the middle and innermost region of the nucellus as is generally the case in Gymnosperms. During its development the MMC attains a size much larger than that of the surrounding somatic cells and accumulates a considerable amount of food-storage substances. These consist of starch and droplets which could be, at least partly, of lipidic nature. During prophase I the MMC becomes a fully polarized cell since all or almost all the mitochondria are restricted to the chalazal cytoplasm. In the authors' opinion this is determined by the trophically privileged position of the chalazal cytoplasm. The polarization of the chondriome determines the course of the subsequent events in such a way that the meiotic divisions give rise to a linear triad, in which the two micropylar megaspores soon degenerate while the chalazal one inherits all of the mithochondria from the megasporocyte and remains functional. On the other hand the food-storage substances appear more or less uniformly distributed at all megasporogenesis stages.The preparation for the female gametophyte development should therefore be accomplished via an accumulation of the cell organelles necessary for the consumption of the food-storage substances, rater than via an accumulation of the food-storage substances themselves. The latter may in fact be supplied later by the nearby vascular system. This behaviour of the megasporocyte of C. lawsoniana is discussed in connection with that previously observed in Encepbalartos poggei and in Ginkgo biloba.In the EM preparations the primary wall of the megasporocyte appears thicker than that of the adjacent cells. After the first meiotic division the wall separating the dyad from the tapetum shows different thickenings on the two sides. Finally, new wall material is laid down by the cytoplasm of the chalazal functional megaspore. These thickenings seem to occlude, at least partially, the plasmodesmata connecting at early stages the fertile cytoplasm to the somatic.The ultrastructural features of the nucellus are briefly described. We observe a gradual morphological change from the innermost nucellar cells, which form the tapetum and are less differentiated, to the external ones showing thicker walls, greater vacuolation and more starch.Some nucellar cells divide, while some others, mainly in the tapetum region, degenerate.The walls of the apical nucellar cells show very electron-transparent inclusions. These are considered to be the sites of enzymatic activities which should initially produce the pollination drop, and then the separation and degeneration of the above cells.Ovules with megasporocytes or whole triads degenerating have been frequently observed.