Patterns of cell division, cell differentiation and cell elongation in epidermis and cortex of Arabidopsis pedicels in the wild type and in erecta.

Plant organ shape and size are established during growth by a predictable, controlled sequence of cell proliferation, differentiation, and elongation. To understand the regulation and coordination of these processes, we studied the temporal behavior of epidermal and cortex cells in Arabidopsis pedicels and used computational modeling to analyze cell behavior in tissues. Pedicels offer multiple advantages for such a study, as their growth is determinate, mostly one dimensional, and epidermis differentiation is uniform along the proximodistal axis. Three developmental stages were distinguished during pedicel growth: a proliferative stage, a stomata differentiation stage, and a cell elongation stage. Throughout the first two stages pedicel growth is exponential, while during the final stage growth becomes linear and depends on flower fertilization. During the first stage, the average cell cycle duration in the cortex and during symmetric divisions of epidermal cells was constant and cells divided at a fairly specific size. We also examined the mutant of ERECTA, a gene with strong influence on pedicel growth. We demonstrate that during the first two stages of pedicel development ERECTA is important for the rate of cell growth along the proximodistal axis and for cell cycle duration in epidermis and cortex. The second function of ERECTA is to prolong the proliferative phase and inhibit premature cell differentiation in the epidermis. Comparison of epidermis development in the wild type and erecta suggests that differentiation is a synchronized event in which the stomata differentiation and the transition of pavement cells from proliferation to expansion are intimately connected.

A theoretical description and experimental demonstration of diffraction in electron-stimulated desorption.

The role of diffraction in electron-stimulated desorption (DESD) is demonstrated experimentally and described theoretically. Specifically, initial state effects in DESD of Cl (+) from Si(111)-(1 × 1):Cl and Si(111)-(7 × 7):Cl are examined and a theoretical treatment that includes spherical-wave effects and multiple scattering of low-energy incident electrons is presented. Although contributions from complicated defect configurations such as SiCl2 and SiCl3 cannot be ruled out, comparison of the experimental data with theory indicates that Cl (+) desorption from Si(111)-(1 × 1):Cl and Si(111)-(7 × 7):Cl surfaces may be dominated by monochloride terminal sites. The initial states probably contain significant Si 3s and/or Si-Cl σ-bonding character. In the Si(111)-(7 × 7):Cl case, these excitations favor a propensity for Cl (+) desorption from the unfaulted, rather than faulted, zones of the 7 × 7 reconstructed rest atom area. This propensity may be related to increased screening and hole localization in the Si-Si backbonds within the faulted region. Finally, introducing Debye-Waller factors into each scattering path accounts for much of the experimentally observed DESD width broadening at room temperature.