(4) Savanna.—As one proceeds away from the equator on a simplified globe, the scrubby brush lands gradually give place to broad grassy areas. Sometimes these are dotted with clumps of trees or bushes, the outposts of the forest and the scrub. In other places they are absolutely treeless except along the rivers. The "pampas" of Argentina and the "Llanos" of Venezuela are some of the best known savannas. In central Africa among the highlands and farther north in the great plains of the Sudan similar grasslands are developed on a vast scale, while in northern India and northern Burma they occur in large patches. The parts of such savannas where trees alternate with grass are almost ideal for big game. The trees furnish shelter, while the grass furnishes food for innumerable animals such as buffaloes, antelope, giraffes, zebras, elephants, and many smaller herbivorous species, and these in turn supply food for lions, tigers, leopards, and other carnivores. For man, however, the savannas are not so good. The grass, to be sure, furnishes food for cattle, although it is apt to be much tougher and less nutritious than the grass of more northern regions. In the long dry season, however, water can be procured only from wells of extreme depth. The sod is so tough that it is difficult to plow. Hence most of the natives of tropical grasslands wander from place to place with their cattle. A good example is the Kaffirs of South Africa, among whom prices are reckoned in so many cows. Many tropical highlands also have large areas of grasslands, and these are among the best parts of the tropics as may be seen in the plateau around Mexico City.
(5) Deserts.—Poleward from the grasslands the desert begins. Usually there is no sharp transition, for the grassland and sometimes the scrub gradually diminish as one passes into regions where the equatorial rain belt has less and less effect. On a simplified globe the deserts would be most pronounced between and from the equa tor, where the subtropical area of high pressure and the trade winds prevail alternately according to the season. On the actual earth, however, the deserts occupy these latitudes only on the western sides of the continents, while rainy monsoon areas lie on the east. To make up for this, as it were, the deserts extend into much higher latitudes in the interior of the continents, especially in Asia.
Both grasses and bushes are found in the deserts. The plants which support such wandering people as the Arabs consist of grasses and other small herbaceous forms which sprout quickly after the in frequent rains, remain green only a few weeks, and then wither and disappear so quickly that one would never know they had existed. In most deserts, however, there is also a more permanent type of vegetation, consisting of little bushes spaced far apart so that each has a large area where it can spread its roots -horizontally and thus get as much water as possible from each infrequent shower. Some types, which grow in hollows, form what may be called an inverted forest, for the roots reach far down to ground water, and are so large that they form as it were an underground forest, far bigger -than the small plants that rise above the surface. Throughout most of the desert,
however, the water table is so deep that plants are unable to reach it.
Although the total number of plants in a desert is small compared with moister regions, the number of species is large. Not only are there the relatively long-lived bushy types and the temporary grasses and the other herbaceous forms which grow up quickly after rains, but in the moist spots there are the same kinds as in regions of abun dant rain, while around the salt lakes there are forms similar to those that grow on the seashore. In addition to this the desert is full of highly specialized plants like the cactus adapted for storing large quantities of water. The cactus can retain water so long that speci mens which were pulled up by the roots and hung in a dry place for eight years still retained half as much water as at the beginning. The desert of northwestern Mexico is the home of a curious almost leafless bush somewhat larger than a currant bush. Its stout tapering stems are covered with a glossy bark and look hard and woody. When a twig is cut, however, the knife goes through it easily as if it were made of wax, and drops of sap begin to fall almost in a stream. The bark is waterproof, but wherever it is broken the stored water oozes out rapidly. Because of the necessity of adapting themselves to extreme aridity, many of the genuine desert plantS are peculiarly awkward in appearance. Their fat, hairy stems, their spines, and their fuzzy or leathery leaves seem uncouth compared with the grace ful vegetation of moister regions.
(6) Subtropical Dry Forest.—On the cooler borders of the desert, especially on the western side of the continents, the -vegetation in latitudes to or more consists of subtropical dry forest. This is also found on many mountains which rise within the desert itself. It is composed of small, gnarled, hard-leaved .trees or bushes which often form open park-like expanses through which it is easy to travel. In some places, however, they graduate into a tangle of bushes above which rise frequent trees. For example, along the southern coast of Asia Minor the lower mountains are clothed with scattered trees and occasional bushes which give an open, friendly aspect like a park. Higher up, however, toward the level where the coolness and moisture of the mountains cause them to be clothed with pine forests, there is a bushy belt almost impossible to cross. The subtropical trees which make up the dry forest, although not conifers, are likely to be evergreens like the laurel, olive, holly, and live oak. This is an ad vantage, because the winter temperature in these latitudes is often quite high and as the rain comes chiefly in winter, the trees can grow even at that season. This makes up in part for the dry summer when growth must cease.