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A wave-cut notch is formed at the base of vertical coastal cliffs made of Ryukyu Limestone in the Ryukyus, Japan. Many rectangular blocks, which appear to have originated in cliff failures, are observed on shore platforms in front of the cliffs. This fact suggests that the development of a notch increases the cliff instability and eventually results in a cliff failure. In order to determine the critical depth of a notch, the two limestone coasts were selected in Koroshima, Yaeyama Islands. We measured the horizontal depth of a notch, cliff height (h) and block length though field investigations. Notch depth, cliff height and block length range from 2.0-4.0 m. 1.5-2.4 m and 3.0-4.7 m, respectively. Assuming that the occurrence of cliff failure follows the principle of a cantilever, the critical depth of notch (l_c) should be given by: l_c=√^B where S_1 is the tensile strength in rock mass, g is the acceleration due to gravity, and B is a nondimensional constant. An analysis of the field data applying to this equation indicated that a boundary condition for delimiting the presence of notched cliffs and fallen blocks given by B = 1.5x 10^3.

陸生ラン色細菌のイシクラゲ(Nostoc commune)は南極や日本にも生育する汎存種である.イシクラゲの細胞の周りに存在する細胞外多糖(EPS)は,水分保持や細胞の保護に役立つと共に遮光性物質を蓄積しており,これが苛酷な環境への適応に重要な役割を果たしていると考えられている.南極産の貴重なイシクラゲを材料にしてこの仮説を検証するためには,EPSを除去した細胞を高収率で回収する必要がある.そこで,これまでに報告のあるEPS除去方法を改良し,Percollを用いた遠心分離で細胞の回収率向上を図った. その結果, イシクラゲ破砕懸濁液に10%濃度のPercollを混合して遠心することでEPSを効率良く除去できることが分かった.更に細胞画分を80% と50% のPercoll に上層して遠心することで他の不純物の除去が達成され,少量のイシクラゲを生理学実験に用いるのに適した細胞単離法が確立された., The terrestrial cyanobacterium Nostoc commune Vaucher ex Bornet et Flahault occurs worldwide, including in Japan and Antarctica. N. commune has a large amount of extracellular polysaccharides (EPS) that hold moisture and protect the cells and at the same time accumulate light-blocking substances which is believed to play an important part in adaptation to a severe environment. To evaluate the photoadaptation processes in N. commune and clarify the role(s) of EPS under ambient environmental condition at Antarctica, separation of cells from EPS is necessary. High yield is a prerequisite for the use of only small amount of natural N. commune from Antarctica. For this purpose, we developed a separation method by improving the Percoll density gradient centrifugation method using an EPS-coated field-grown Nostoc population. We established the most suitable condition to separate naked cells from EPS at high yield retaining high photosynthetic activity. The method is composed of centrifugation of cell homogenated N. commune in 10% (v/v) Percoll to separate cells efficiently from EPS followed by fractionating centrifugation to remove impurities using the gradient of Percoll (80% and 50%, v/v).