Manual de Plantas de Costa Rica 103 
of vascular plants from the island, in 60 families and 132 genera. Ferns are represented 
by 91 species, 43% of the total flora. This is quite high by continental standards, with 
ferns accounting for only about 12% of the total Costa Rican flora. However, high per- 
centages of pteridophytes are common for tropical oceanic islands, presumably due in 
part to the small, widely dispersible spores and bisexual gametophytes of most ferns 
(see Smith, 1972). It is worth noting that several taxa known both from this island and 
the Central American mainland have a Caribbean distribution on the mainland, e.g., 
Bertiera angustifolia, Calophyllum inophyllum, Campnosperma panamense, Hypo- 
lytrum amplum, Ilex yurumanguinis, Maieta poeppigii, Sacoglottis, and Selaginella 
porelloides. This phenomenon was noted by Fournier O. (1966) and Gémez P. (1986), 
and for non-vascular plants by Dauphin (1999), who found approximately 10% of the 
island’s total bryophyte flora (153 spp.) to have such a distribution. Gdmez P. (1986) 
mentioned that Pacific island species of eastern Amazonian origin might have dispersed 
before the closure of the Central American isthmus in Panama. For more information 
about the vegetation of Cocos Island, see Fosberg & Klawe (1966), Fournier O. (1966), 
Gomez P. (1975a, 1975b, 1986), Dauphin (1999), and Bernecker-Liicking (2000). 
The cordilleras 
Costa Rica is divided into two almost equal parts by way of a central mountain chain 
that forms the two slopes (Caribbean and Pacific) already mentioned. This mountain 
system trends from northwest to southeast, and is made up of four distinct ranges: the 
Cordillera de Guanacaste, the Cordillera de Tilaran, the Cordillera Central, and the 
Cordillera de Talamanca. The Cordillera de Guanacaste and the Cordillera Central are 
volcanic in origin (see the chapter on The physical environment). Below we will briefly 
present relevant details of the floristic composition of each of these cordilleras and the 
floristic relationships among them. 
Cordillera de Guanacaste 
Along the summit of this cordillera, formed by volcanic peaks that stretch in a straight 
line from Cerro El Hacha and Volcan Orosi in the northwest to Volcan Arenal in the 
southeast (see the section on Physical geography in the chapter on The physical envi- 
ronment), two life zones occur: premontane rain forest and lower montane rain forest 
(Holdridge, 1967; Tosi, 1969). However, for the simplified version of this classification 
system used in the Manual treatments, these are unified into just one category, bosque 
pluvial (rain forest). 
On Cerro El Hacha (500-617 m), the relatively low elevation and the rain shadow 
of the Orosi and Cacao volcanoes create conditions favoring species of the Pacific low- 
land deciduous dry forest and savanna woodlands, some of which are Agonandra 
macrocarpa, Ayapana amygdalina, Byrsonima crassifolia, Capparis frondosa, Celtis 
iguanaea, Clitoria guianensis, Curatella americana, Declieuxia fruticosa, Diospyros 
salicifolia, Dyssodia montana, Hippocratea volubilis, Melothria pendula, Serjania 
