No abstract

We use atomic force microscopy (AFM) in conjunction with fluorescence relaxation dynamics to establish the relationship between the physical morphology and optical response of zirconium−phosphonate (ZP) monolayers containing oligothiophene chromophores. For ZP monolayers formed on SiO x , island structures are seen with AFM in the size range 40−100 Å with the nearest neighbor island spacing varying from 80 to 200 Å. For these same surfaces, the fluorescence population relaxation dynamics we measure are nonexponential, chromophore concentration-independent, and identical for two different chromophores. This finding is fully consistent with the AFM data, and the results can be understood using an excitation hopping model. The spectroscopic data are consistent with aggregated islands ∼50−100 Å in diameter. We demonstrate that the spectroscopic and physical domain sizes present on ZP/SiO x monolayers are the same. Studies using several initial priming schemes point to the silanol group distribution on the surface determining structural heterogeneity with intermolecular interactions between chromophores playing a secondary role.

The structure and distribution of cytoplasmic membranes during mitosis and cytokinesis in maize root tip meristematic cells was investigated by low and high voltage electron microscopy. The electron opacity of the nuclear envelope and endoplasmic reticulum (ER) was enhanced by staining the tissue in a mixture of zinc iodide and osmium tetroxide. Thin sections show the nuclear envelope to disassemble at prophase and become indistinguishable from the surrounding ER and polar aggregations of ER. In thick sections under the high voltage electron microscope the spindle is seen to be surrounded by a mass of tubular (TER) and cisternal (CER) endoplasmic reticulum derived from both the nuclear envelope and ER, which persists through metaphase and anaphase. At anaphase strands of TER traverse the spindle between the arms of the chromosomes. The octagonal nuclear pore complexes disappear by metaphase, but irregular-shaped pores persist in the membranes during mitosis. It is suggested that these form a template for pore-complex reformation during telophase. Phragmoplast formation is preceded by an aggregation of TER across the spindle at anaphase. Evidence is presented to suggest that the formation of the desmotubule of a plasmodesma is by the squeezing of a strand of endoplasmic reticulum between the vesicles of the cell plate.