The over 40 year old Hartman-Perdok (HP) [Hartman & Perdok (1955). Acta Cryst. 8, 49-52, 521-524, 525-529] theory for predicting crystal morphology is reconsidered. The new approach, which gives a physical foundation to the theory, is based on F faces having a roughening transition temperature higher than 0 K. The aim of this paper is to confront the field of crystal growth and in particular the classical HP theory with modern statistical thermodynamical treatments of models of surfaces of relatively simple crystal structures. It is shown that crystal faces (hkl) containing multiple connected nets with a relatively high energy content may have a very low roughening temperature and an unexpectedly high growth rate. In some cases, crystal faces become rough at OK because of multiple connected nets related by symmetry giving rise to symmetry roughening. The use of connected nets in the HP theory offers the possibility of extrapolating the results of statistical thermodynamical models of simple crystal surfaces to more complex crystals as encountered in practice. The role of the step free energy in understanding crystal morphology is emphasized.

Light spectrum plays a key role in the biology of symbiotic corals, with blue light resulting in higher coral growth, zooxanthellae density, chlorophyll a content and photosynthesis rates as compared to red light. However, it is still unclear whether these physiological processes are blue-enhanced or red-repressed. This study investigated the individual and combined effects of blue and red light on the health, zooxanthellae density, photophysiology and colouration of the scleractinian coral Stylophora pistillata over 6 weeks. Coral fragments were exposed to blue, red, and combined 50/50% blue red light, at two irradiance levels (128 and 256 μmol m−2 s−1). Light spectrum affected the health/survival, zooxanthellae density, and NDVI (a proxy for chlorophyll a content) of S. pistillata. Blue light resulted in highest survival rates, whereas red light resulted in low survival at 256 μmol m−2 s−1. Blue light also resulted in higher zooxanthellae densities compared to red light at 256 μmol m−2 s−1, and a higher NDVI compared to red and combined blue red light. Overall, our results suggest that red light negatively affects the health, survival, symbiont density and NDVI of S. pistillata, with a dominance of red over blue light for NDVI.

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The growth rate of NH 4NO3 phase III crystals is measured and interpreted using two models. The first is a standard crystal growth model based on a spiral growth mechanism, the second outlines the concept of kinetical roughening. As the crystal becomes rough a critical supersaturation can be determined and from this the step free energy. The step free energy versus temperature turns out to be well represented by a Kosterlitz-Thouless type model. Further a phenomenological treatment of some peculiar growth observations is given.