Judul: Keanekaragaman Selaginella di Jawa Tengah
Abstrak: Selaginella is a single genus belong to family Selaginelaceae which include approximately 750 species spread all over the world. The diversity of Selaginella can be found in lowland to mountain rainforest. This study aimed to describe the diversity of morphological characters and the distribution of Selaginella in Central Java, then to provide descriptions and identification key of Selaginella species in Central Java. The method used to assess the diversity Selaginella in Central Java included observation and identification of morphological characters. In Central Java, there were found 10 species of Selaginella, namely S. plana, S. remotifolia, S. involvens, S. ornata, S. ciliaris, S. opaca, S. modica, S. aristata, S. subalpina, and Selaginella sp. Selaginella varied on some morphological characteristics such as branching pattern, rhizofor, lateral leaves, median leaves, axillary leaves, and Strobilus shape. The distribution of Selaginella is fairly even in Central Java ranging from the altitude of 200 to 2771 m asl. Selaginella species found in the lowest altitude was Selaginella sp. (200 m asl), while at the highest altitude was S. plana (2771 m asl).
Keyword: Selaginella, Selaginellaceae, Taxonomy, Central Java.

Judul: Keanekaragaman Selaginella di Jawa Barat.
Abstrak: Selaginella is a genus of ferns in the family Selaginellaceae. It is characterized by having small leaves and the spongarium on the tip of each branch. The society of West Java have been a long time used Selaginella as vegetable and dill, but the taxonomic information of Selaginella in West Java is still poorly known. The aim of this research was to describe the diversity of Selaginella in West Java. This research used 316 herbarium specimens of Selaginella collected by Plant Taxonomy Laboratory, Department Biology of Bogor Agricultural University and Herbarium Bogoriense Botanical, Research and Development Cibinong. From each specimen was observed the mophological characters, recorded the spread site, identified, then arranged the identification key. This study showed that 18 species found in West Java, namely S. aristata, S. biformis, S. ciliaris, S. frondosa, S. intermedia, S. involvens, S. mayeri, S. opaca, S. ornata, S. plana, S. remotifolia, S. rothertii, S. subalpina, S. subspinulosa, S. uncinata, S. willdenovii, S. zollingerian and Selaginella sp 1. Selaginella is able to live at altitudes ranging from 20 m above sea level (asl) to > 2500 m asl, with a variety of habitats including primary forest, in the rice fields, near the waterways, and in the open area. An identification key of Selaginella in West Java was provided in this paper.
Keyword: identification key, Selaginella, taxonomic, West Java.

Judul: Hamiltonian formulation for linear waves motion in two layers fluid
Abstrak: The surface wave could be considered as a wave that separates two fluids, namely water and air. Based on this assumption, it is introduced the interfacial wave, a wave between two layers of fluid with different density. The formulation of interfacial waves motion begins with deriving the base equation of irrotational ideal fluid. Furthermore, according to irrotational fluid assumption, the base equation can be stated in velocity potential. In this derivation, the fluids domain is assumed to be restricted by rigid lid boundary conditions, both at the upper and lower limit. Therefore, the interfacial waves motion can be explained in a hamiltonian formulation. In the hamiltonian formulation, total energy is defined as the sum of kinetic and potential energy. The hamiltonian system is obtained from reduction of kinetic energy by using the Dirichlet Neumann Operator. The resulted kinetic energy equation is nonlinear. Therefore, this form is linearized by first part of the Taylor expansion. This linearization gives a dispersion relation of linear wave. Based on this dispersion relation, the phase speed of the linear wave depends on the density ratio of the two layers fluid.
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