swell perennial streams. Mountain snow does not lie in beds of uniform thickness and some parts are more exposed to the sun and warm air than others. Beds of snow commonly alternate with exposed slopes covered with brilliant vegetation without an obvious boundary of perpetual snow. A seemingly clear boundary seems visible when a high mountain chain is viewed from a distance: Similar conditions are repeated at many different points, so that the level at which large snow beds show themselves can be seen as approximately horizontal. But this is true only as far as the conditions are similar. On the opposite sides of the same chain the exposure to the sun or to warm winds may cause a wide difference in the level of permanent snow, though the increased fall of snow on the side exposed to moist winds may more than compensate for the sun's rays. Still, the "line of perpetual snow" is not fixed. The occurrence of favorable meteorological conditions during several successive seasons may and does increase the extent of the snowfields and lower the limit of seemingly permanent snow, while the opposite may cause the limit to rise higher on the flanks of the mountains. Attempts to fix accurately the level of perpetual snow in the Alps are difficult. Local accuracy might be established for a particular district. In some parts of the Alps, the limit is about elevation, while in others it cannot be placed much below . As very little snow remains on rocks angled more than 60°, this is soon removed by the wind, some steep masses of rock remain bare even near the summits of the highest peaks, but as almost every spot offering the least hold for vegetation is covered with snow, few flowering plants are seen above . Given the extremely harsh climatic conditions of this zone, few plant species can find a favorable environment to thrive. A few high altitude plants, however, manage to grow in certain crevices of rocks sometimes at very high altitudes. A specimen of glacier buttercup was found at the top of the Finsteraarhorn at an altitude of more than In the glacial zone, the species distribution is very different between calcareous and siliceous rocks. On limestone rock, we can find Androsace helvetica, saxifrages, Athamanta cretensis, and Draba tomentosa. On siliceous rock, one will come across Androsace vandellii, two characteristic species of houseleek (Sempervivum