The arid regions, especially the desert zones of the north, have their peculiar vegetation, such as yucca trees, hundreds of species of cacti, and many types of agave and mesquite bushes. A few drops of rain change the whole aspect of the country for some days, the whole desert being almost suddenly covered with flowers of the most brilliant colors. Among the cacti the most interest ing, perhaps, are the gigantic pitahaya, which bears excellent fruits, the strange organos (Cactus hexagonus) and the com moner varieties, yielding the ever present tuna whose alternate green, yellow and red colours correspond to the successive sea sons. Less numerous but economically more important are the agaves, of which the Agave americana, from which the pu/que is extracted, is the most popular, while other varieties give different types of mescal. Some plants of the Agave americana are said to yield 2,000 pounds of sap in six months. Most vegetable fibres used in Mexico come from some kind of agave, the ixtle fibre being produced by Agave uninvitata, while the Agave rigida var. elongata yields the famous henequen or Sisal hemp. This particularly valuable variety of agave is grown in the Yucatan peninsula and in the State of Tamaulipas only.
On the central and southern parts of the high plateau, or tierra fria, where there is a greater degree of humidity, relatively abundant trees are found on the slopes of the mountains, cereals of Eurasian origin can be cultivated, and all kinds of fruit trees may be grown, together with the local maize. It is hard to say where the exact limit between the tierra fria and the tierra helada may be determined because the deforestation of the high plateau has allowed the pine trees of the higher altitudes to invade rela tively warmer regions. In ascending the slopes of the moun tains, one finds an intermediate zone where different varieties of oaks (Quercus crassifolia, Q. reticulata, etc.) are found, together
with a pine tree of high altitude (Pinus oocarpa), the ocote pine, long and short leaved pines and the oyamel (Abies religiosa).
On the slopes of the mountains around the Valley of Mexico forests can be seen of the superb ahuehuete (Taxodium dis tichum). The extreme upward limit of trees seems to be be tween 12,500 and 13,500 ft., but juniper bushes occur as high as 14,000 ft.; above that, only grasses are found until one reaches the lower limit of the snow field. The same thing may be said of the limit between the tierra fria flora and that of the tierra templada. It is only gradu ally that one passes from the f or mer to the latter and it is only in the presence of the banana, coffee and orange trees that one can feel sure of having reached a lower level. For a long stage the ocote pine and many varieties of oaks are found together, especially on the eastern slopes, with the addition of magnolias, acacias, myrtles, mimosas and bamboos. There are still cacti when the level is reached where the first palm trees appear, but it is only when the latter is found in abundance that one is in the real tierra caliente.
The vegetation of the tierra caliente is of such variety as to bewilder the stranger. Trees of hundreds of species, themselves half-buried among palms and ferns of all kinds, form the frame on which hang closely interwoven curtains of lianes and epiphytes, while saprophytic and parasitic plants of infinite diversity are found at the foot and on the trunks of the trees, or on the decay ing vegetable matter that litters the soil of the forests. To open a way through such an entangled mass, even with a machete, is a slow and painful process, more especially, perhaps, because of the oppressive semi-darkness that surrounds the explorer. In the forests of Chiapas are found more than 5o species of cabinet wood, including the ebony, the mahogany, the rosewood and the Spanish cedar. Other valuable trees of the tierra caliente are the log-wood (Hematoxylon campechianum), the dividivi tree (Caesalpinia coriaria), the zapote Chico (Achras sapota), ex ploited in Campeche and Chiapas for chicle, as well as rubber trees. There are not less than 17 oil-bearing plants in Mexico and a much larger number of medicinal ones, yielding jalap, arnotto, ginger, licorice, sarsaparilla, ipecac and various gums.