. The British fern gazette. Ferns. 316 BRITISH FERN GAZETTE: VOLUME 10 PART 6 (1973) andesitic, and trachytic lavas but nearly the whole surface is covered by volcanic loose material — tuffs and ashes with lapilli and pumice (Zbyszewski et al. 1958, 1959). This soft material can rapidly be colonised by plants. Besides the small hills, characteristic landscape elements are the volcanic cones, craters (calderas), often containing circular lakes, hot springs, shore cliffs and deep ravines with boulders in the river-beds running down from the high mountains. Some data concerning the climate (Walte
. The British fern gazette. Ferns. 316 BRITISH FERN GAZETTE: VOLUME 10 PART 6 (1973) andesitic, and trachytic lavas but nearly the whole surface is covered by volcanic loose material — tuffs and ashes with lapilli and pumice (Zbyszewski et al. 1958, 1959). This soft material can rapidly be colonised by plants. Besides the small hills, characteristic landscape elements are the volcanic cones, craters (calderas), often containing circular lakes, hot springs, shore cliffs and deep ravines with boulders in the river-beds running down from the high mountains. Some data concerning the climate (Walter & Lieth 1960) can be given here: Ponta Delgada (22 m) has a mean annual rainfall of 697 mm which increases in the islands to the west; Angra do Heroismo has 953 mm; Flores (according to Ward, 1970) 1350 — 1800 mm. All stations quoted are in the lower altitudes of the islands. The mean annual temperature in Ponta Delgada is ° C with small differences between summer and winter; the absolute minimum in 30 years has been +° C. Walter & Lieth demonstrate a slight period of dryness in July and August. It has to be remembered, however, that the mountain vegetation gets additional water by frequent fog and clouds as even on clear days one may see "cloud caps" on the mountain peaks. The precipitation map (Fig. 2) is a construction by S. de Brito (1955) according to the records of 16 observation points and extrapolated according to the relief. Below 600 m there is no frost to be expected although on higher mountains the temperatures may occasionally fall below 0° C (Tutin 1953). In spite of the well known azorean anticyclone it is a typical oceanic climate with a number of rainy days and high relative FIGURE 2: Average rainfall. 1 = below 1000 mm; 2 = 1000-1250 mm; 3 = 1250-1500 mm; 4= 1500-1750 mm; 5 = 1750-2000 mm; 6 = 2000-2250 mm; 7 = above 2250 mm. Nearly all the vegetation is anthropogenous or at least influenced by man. T
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectferns, bookyear1909
