Aspects of the sodium and potassium nutrition of the fynbos shrub Leucadendron salignum L. (Proteaceae)

The fynbos shrub Leucadendron salignum (Proteaceae) was used to study the effect of sodium and potassium supply in the nutrient medium on its growth and cation content. The plants were not highly sensitive to sodium, and luxury absorption of potassium was found with higher potassium supply, which was probably then used to maintain growth during a six-month period of potassium deficiency and sodium supply. Short-term absorption studies using excised roots showed active potassium and sodium absorption even when both ions were supplied together, especially from the low-concentration range. Potassium absorption was higher than that of sodium, and uptake by proteoid roots greater than that by ordinary roots

Surface instabilities of ferrofluids

We report on recent progress in understanding the formation of surface protuberances on a planar layer of ferrofluid in a magnetic field oriented normally to the surface. This normal field or Rosensweig instability can be tackled by a linear and a nonlinear description. In the linear regime of small amplitudes we focus on the wave number of maximal growth, its corresponding growth rate and the oscillatory decay of metastable pattern, accessible via a pulse technique. A quantitative comparison of measurements with predictions of the linear stability analysis is performed, whereby the viscosity and the finite depth of the liquid layer are taken into account. In the nonlinear regime the fully developed peak pattern can be predicted by a minimization of the free energy and by numerics employing the finite element method. For a comparison with the results of both methods, the three-dimensional surface profile is recorded by a radioscopic measurement technique. In the bistable regime of the flat and patterned state we generate localized states (ferrosolitons) which are recovered in analytical and numerical model descriptions. For higher fields an inverse hysteretic transition from hexagonal to square planforms is measured. % Via a horizontal field component the symmetry can be broken in the experiment, resulting in liquid ridges and distorted hexagons, as predicted by theory. Replacing ferrofluid by ferrogel also an elastic energy contribution has to be taken into account for a proper model description, yielding a linear shift of the threshold and an increased bistability range. Parametric excitation in combination with magnetic fields is widening the horizon of pattern formation even further. For the mono-spike oscillator harmonic and subharmonic response as well as deterministic chaos is observed and modeled. In a ring of spikes the formation of domains of different wavelengths and spatio-temporal intermittency is quantitatively studied. For an extended layer of ferrofluid we predict that a stabilizing horizontal field counteracted by vertical vibrations will result in oblique rolls with preselected orientation