886 C. SKOTTSBERG 



(no. 20, 1930), for the foliose Hepaticae Th. Herzog (no. 24, 1942), and for the 

 Lichens A. Zaiilbruckner (no. ii, 1924, with a few additions in no. 13, 1926). 

 Later, Hchenologists engaged in critical studies have, in a number of instances, 

 diverged from Zahlbruckner's opinion; the genus Pseudocyphellaria, not recognized 

 by him, has been taken up again, and a number of specimens of this and other 

 genera have been revised: see G. Degehus in Acta Phytogeogr. Suec. 7 (1935), 

 p. 52, I. Mackenzie Lamb in Lilloa 13 (1947), A. H. Magnusson in Meddel. Goteb. 

 Bot. Tradg. 14 (1940), pp. 18, 34 and in K. Sv. Vetenskaps.-akad. Handl. Ser. 3, 

 vol. 7:4 (1929), p. 86, G. A. Malme in Arkiv for botanik 26 A: 13 (1935), p. 5, 

 J. Motyka, Lich. gener. Usnea Studium Monogr., Lwow 1936 — 38, and R. Santesson 

 in Arkiv for Botanik 30 A: 10 (1942), p. 16 and ibid. 31 A: 7 (1944), pp. 14,15. 

 I owe this information to Dr. Santesson. Prof. G. E. Du Rietz kindly supplied 

 some corrections. 



Stratification. Four layers were distinguished: IV, Tree layer, above 6 m, 

 subdivided in an upper, IV: 2, and a lower, IV: i, with the limit between them at 

 12 m; III, Shrub layer, 6 — 1.8 m; II, Field layer, composed of 3 strata, upper (II: 3), 

 1.8 — 0.6 m, middle (II: 2), 0.6 — 0.2 m, and lower (II: i), under 0.2 m; Bottom or 

 Ground layer (I), mostly formed by Cryptogams. 



Degree of covering. The relative role played by each species in a de- 

 fined stratum is indicated with the Hult-Sernander scale from i to 5; with regard 

 to the forest communities, I now would have preferred to simplify this scale; the 

 figures given are, of course, not a result of measuring, but of estimation. 



The Plant Communities of Masatierra. 



Remarks on the Topography. 



A geographical sketch will be found in vol. I, where also the geology has 

 been described (no. 3, by P. D. Ouensel). Here I shall confine myself to a few 

 introductory remarks. 



Masatierra is a deeply eroded, volcanic. Tertiary mountain range, extending 

 E — W and built up by basalt beds of varying thickness; agglomerate banks and 

 tuff layers also occur, but no recent volcanic products. In the eastern half the 

 strata dip gently N, the crest, about 500 to 800 m high, follows the south coast- 

 line and falls abruptly into the sea, forming a magnificent escarpment, cut by 

 shallow, almost vertical hanging gulches, separated by narrow buttresses and more 

 or less inaccessible. With the prevailing wind directions, the clouds discharge 

 their moisture on the high ridges and feed the series of parallel wooded valleys 

 trending north. West of the bold summit, El Yunque, the main ridge traverses the 

 broadest part of the island, with peaks rising to 600 — 700 m; here erosion has cut 

 deep back from both sides, leaving a narrow, precipitous rock wall standing, crossed 

 by the Portezuelo pass and separating the two principal valleys of Masatierra, 

 the Cumberland valley system and the Villagra valley. The main ridge or back- 

 bone now turns northwest, at first cut by rather deep valleys going north and 

 then following close to the north shore, which forms a high unbroken, inaccessible 



