In the vicinity of Matias Romero open pine-oak forest (_Pinus caribaea_ and _Quercus_ sp.) is found on some ridges as low as 250 meters above sea level. On the Plains of Chivela in the southern part of the central region the vegetation takes on a semi-arid appearance, especially in a savanna on the plains. Clumps of small trees and bushes, consisting of _Croton nivea_, _Cordia cana_, _Jacquinia aurantiaca_, _Calycophyllum candidissimum_, and _Cassia emarginata_, are scattered on a grassy plain, from which rise widely-spaced palms of an unknown species (Pl. 3, fig. 2). Pacific Coastal Plain The vegetation of the Pacific lowlands definitely is semi-arid in character. Most of the trees are deciduous, thorny, and short. During the dry season the landscape presents a barren appearance, but shortly after the first summer rains dense green foliage appears (Pl. 4, figs. 1 and 2). Between Juchitan and La Ventosa few trees are more than two meters high (Pl. 5, fig. 1). In many areas the trees and bushes form an almost impenetrable tangle, whereas on especially rocky soils or on slopes those plants are more widely spaced. Abundant and widespread species of trees on the Plains of Tehuantepec include _Acacia cymbispina_, _Prosopis chilensis_, _Caesalpinia coriaria_, _Caesalpinia eriostachys_, _Celtis iguanaea_, _Cordia brevispicata_, _Jatropha aconitifolia_, and _Crescentia alata_. Montane Vegetation In order to illustrate the interruption of subtropical and temperate types of vegetation by the lowlands of the Isthmus of Tehuantepec, it is necessary to digress for a moment from the isthmus and consider the types of vegetation present on the adjacent highlands. On the higher peaks, such as Cerro de Zempoaltepetl, above about 2500 meters is fir forest (_Abies religiosa_); lower on the slopes are extensive pine forests, which on some slopes are mixed with oak or replaced entirely by oaks. Subtropical cloud forest, characterized by relatively cool temperatures and high humidity, is found at elevations usually between 1000 and 1800 meters on the windward slopes of the Sierra Madre Oriental in Veracruz and northern Oaxaca and on the northern and southern slopes of the Chiapan-Guatemalan Highlands. None of these forest types is continuous across the Isthmus of Tehuantepec. _The Sierra de los Tuxtlas_