Manual de Plantas de Costa Rica 159 
cicola, Plocosperma buxifolium, and Tradescantia petricola, are restricted to this kind 
of substrate. In sharp contrast, due to the effects of annual inundations from the Temp- 
isque watershed, the lower part of this valley has several more or less permanent marsh- 
lands with a rich aquatic flora, adding greatly to the overall diversity of the area. 
The Palo Verde wetlands, among the most important of the region (see Crow et al., 
1987), harbor about 100 species of aquatic plants (ca. 29% of the total for the country; 
Crow, 2002). In terms of shrubs, this area is dominated by Acacia farnesiana, A. gua- 
nacastensis, Mimosa pigra, and Parkinsonia aculeata, and in terms of herbs, by Aesch- 
ynomene sensitiva, Cyperus giganteus, Echinochloa polystachya, Sesbania herbacea, 
Thalia geniculata, and Typha domingensis. Many aquatic herbs are abundant in this 
area, including free-floating plants (Azolla microphylla, Pistia stratiotes, Salvinia min- 
ima, Spirodela polyrhiza, Wolffiella welwitschii), bottom-rooted plants with floating 
stems (Ludwigia sedoides, Marsilea deflexa, Neptunia natans, Nymphaea ampla), and 
largely submerged plants (Bacopa repens, Ceratophyllum muricatum, Mayaca fluvi- 
atilis, Najas guadalupensis, Utricularia foliosa). During the dry season, when the water 
level goes down, other species such as Croton argenteus, Hydrolea spinosa, Ipomoea 
carnea, and Solanum campechiense become abundant and conspicuous. For further in- 
formation about these wetlands, see Crow & Rivera (1986), Hernandez & Gémez-Lau- 
rito (1993), and Crow (2002). 
A few elements from the Guanacaste dry forest plains that reach south along the 
coastal strip into the lower end of the Central Valley (at 8300-900 m elevation) are Acro- 
comia aculeata, Aphelandra scabra, Capparis frondosa, C. indica, Cedrela salvador- 
ensis, Cydista heterophylla, Gyrocarpus jatrophifolius, Hiraea reclinata, Lippia car- 
diostegia, Loeselia glandulosa, Luehea alternifolia, Lycoseris grandis, Machaerium 
biovulatum, Nissolia fruticosa, Onoseris onoseroides, Poiretia punctata, Pterocarpus 
michelianus, Senna pilifera, Sloanea terniflora, Tecoma stans, Thouinidium decandrum, 
Trichilia americana, and Xylophragma seemannianum. While not so extensive as those 
of Guanacaste, a few pockets of dry forest or lowland savannas are also found along the 
Rio Grande de Tarcoles (near Cerro Rayos), in the valleys of the Candelaria and Térraba 
(near Boruca) rivers, and on the Osa and Burica Peninsulas. As pointed out previously 
here, the occurrence and diversity of dry forest gradually diminish southeastward, as av- 
erage annual precipitation increases and seasonality decreases. In general terms, this pat- 
tern makes an obvious mark on the floristic landscape all along the Pacific coast. 
Nicoya Peninsula. Being comprised principally of tropical moist and premontane 
wet forest (Tosi, 1969), the Nicoya Peninsula has a distinct flora, in which elements of 
both dry and wet climates come together. Furthermore, the terrain is quite irregular, 
with the highest peak reaching 983 m and supporting frequent fog. 
Several species reflect the higher moisture of this area in combination with season- 
ality, and form part of a distinctive floristic pattern, e.g., Acacia polyphylla, Anthurium 
