Development of Female Gametophyte in Gladiolus italicus Miller (Iridaceae)

In this study gynoecium, megasporogenesis, megagametogenesis and female gametophyte of Gladiolus italicus Miller were examined cytologically and histologically by using light microscopy techniques. Ovules of G. italicus are of anatropous, bitegmic and crassinucellate type. Embryo sac development is of monosporic Polygonum type. Polar nuclei fuse before fertilization to form a secondary nucleus near the antipodals. The female gametophyte development of G. italicus was investigated for the first time with this study.

When disks collide: major and minor mergers in disk galaxies

We present the surface brightness profile fitting of a sample of double nucleus disk galaxies, minor merger candidates. We have decomposed these systems into two compact nuclear components and one or two extended galaxy disk components and estimated the luminosity of the primary and secondary nucleus and of the host galaxy and the separation between the two nuclei. Based on the ratio of nuclear luminosities we find that most of the sources qualify as major mergers despite their initial classification as minor merging systems. This is supported by the finding that 65% of the host galaxies are fitted only by one galaxy disk and that the luminosity of both the primary and the secondary nucleus decreases with decreasing nuclear separation, as expected from simulations of disk galaxy mergers. All these results indicate that these sources are most plausibly in the post-merger state of a major merger event. We also identify 19 candidates to binary active nucleus with nuclear separation ≤1 kpc.

Cytological and histological studies on female gametophyte of Leucojum aestivum (Amaryllidaceae)

In this study, gynoeceum, development of megasporangium, megasporogenesis, megagametogenesis and female gametophyte of Leucojum aestivum were examined cytologically and histologically. Ovules of L. aestivum are of anatropous, bitegmic and crassinucellate type. Inner integument forms the micropyle. Archesporial cell develops directly into a megasporocyte. Embryo sac development is of bisporic Allium type. Filiform apparatus is observed in synergids. Polar nuclei fuse before fertilization to form secondary nucleus near the antipodals.

Splitting of Periodic Comets

Among the comets that were observed to break in two or more fragments, only a few of them are periodic. So far the dynamic study of the relative motion of a secondary nucleus with respect to the primary has supposed that a cometary fragment is subject to a small and continuous radial nongravitational force after separation at rest. This force acts against the solar attraction and varies according to an inverse square law. A small impulse at break up may also be invoked in some case. A different approach is followed in this paper when dealing with a fragment of a periodic comet: after separation the motion of a secondary nucleus is characterized by nongravitational forces which vary according to the same g(r) law currently used for the primary.Results of the study of comets P/Biela and P/du Toit-Hartley show that the motion of their fragments after separation is characterized by nongravitational parameters which are larger than those of the parent bodies. Both fragments lasted for about 2 full revolutions and three returns.

Ovule maturity in relation to blossom age of a diploid clone of Solanum tuberosum

Numerous ovules from preanthesis and postanthesis gynoecia of a diploid clone of Solanum tuberosum L. were examined with the light microscope. In prefertilization female gametophytes, the formation of a secondary nucleus in the central cell in conjunction with the conspicuous vacuolation of the egg apparatus was identified as a consistent criterion for determining whether or not ovules were mature. There is a gradual increase in the percentage of mature, nonabortive ovules beginning on the day of anthesis and reaching a maximum 3 days later. In terms of pollination biology, mature ovules are available for pollen tube penetration over an extended period of several days.

Microkinetics of Lamellar Crystallization in a Long Chain Polymer

The growth kinetics of single crystal lamellae grown from poly(isoprene) melts have been determined by thin film electron microscopy which also reveals the relevant microstructure. The growth rate of a continuous lamellar crystal is independent of the length of the crystal and (in the absence of crosslinks) depends only upon temperature and the chemical composition of the polymer. The temperature dependence is closely similar to that observed for bulk crystallization and spherulite growth, with a maximum crystallization rate around —24° C. The growth rate at a given temperature is extremely sensitive to the chemical composition of the poly(isoprene), being for a polymer containing 10 per cent trans-poly(isoprene) one thousandth of that for pure cis-poly(isoprene). Between these extremes the logarithm of the growth rate decreases linearly with increasing trans content. A similar effect is produced by the introduction of chemical crosslinks instead of trans units. These effects are attributed to a reduction in the probability of formation of the ‘secondary nucleus’, whose size can be calculated from the experimental data. The secondary nucleus is found to correspond to three molecular folds and this agrees well with an independent estimate obtained, by using thermodynamical theory, from the dependence of lamellar thickness on temperature.

Microkinetics of lamellar crystallization in a long chain polymer

The growth kinetics of single crystal lamellae grown from polyisoprene melts have been determined by thin film electron microscopy which also reveals the relevant microstructure. The growth rate of a continuous lamellar crystal is independent of the length of the crystal and (in the absence of crosslinks) depends only upon temperature and the chemical composition of the polymer. The temperature dependence is closely similar to that observed for bulk crystallization and spherulite growth, with a maximum crystallization rate around — 24 °C. The growth rate at a given temperature is extremely sensitive to the chemical composition of the polyisoprene, being for a polymer containing 10% trans -polyisoprene one thousandth of that for pure cis -polyisoprene. Between these extremes the logarithm of the growth rate decreases linearly with increasing trans content. A similar effect is produced by the introduction of chemical crosslinks instead of trans units. These effects are attributed to a reduction in the probability of formation of the ‘secondary nucleus’, whose size can be calculated from the experimental data. The secondary nucleus is found to correspond to three molecular folds and this agrees well with an independent estimate obtained, by using thermodynamic theory, from the dependence of lamellar thickness on temperature.

The Anchoring of Nuclei by Cytoplasmic Microtubules in Acetabularia

The giant unicellular alga Acetabularia enters the reproductive phase of growth when the cap has completed its expansion. The primary nucleus then divides and the resulting secondary nuclei take up fixed equidistant positions in the coenocytic cap cytoplasm. These positions are visible as colourless areas in the otherwise green caps. Studies on the cytology and ultrastructure of the caps at this stage showed that each secondary nucleus was surrounded by a layer of cytoplasm which was free from chloroplasts and mitochondria, but which contained many microtubules, some of which were closely associated with the nuclear envelope, while others appeared to touch the plasmalemma adjacent to the cell wall. (Microtubules are not a regular feature of Acetabularia cells; they have not been reported in the stems, young caps or cysts, and vegetative growth is not inhibited by colchicine.) The function of the microtubules was investigated by treatment with colchicine or vinblastine sulphate, both of which depolymerize microtubules in many other systems. Administration of these drugs at 10-3 M and greater concentrations had the following effects: (1) the colourless areas were lost as chloroplasts and mitochondria invaded the cytoplasm around each secondary nucleus; (2) the nuclei began to migrate from their fixed positions; and (3) cyst formation (in which the cytoplasm cleaves into uniformly sized cysts, each containing a single secondary nucleus) either was inhibited, or proceeded abnormally. It is therefore proposed that a major function of these transitory microtubules is to anchor the secondary nuclei at fixed equidistant positions in the cytoplasm. This function is probably mediated by their association both with the nuclear envelope and cell membrane.