Deserts and Subtropics.

Deserts. The high-pressure and trade wind belts closer to the Tropics of Cancer and Capricorn receive very little rain, and plant growth there is extremely poor. The savannahs, both in the North ern and Southern Hemispheres, gradually give way, first to tropical semideserts and then to deserts.
By far the greatest desert in the world is the Sahara. The little rain it receives (about 100mm a year) falls usually in the form of sudden cloudbursts. In some parts there is no rain whatever for several   years.
The Sahara consists of low plateaus cut by dry river-beds. In it. central part there are several comparatively high uplands that receive a little more rain. Along the edges there are lowlands am depressions, below sea level in places.
In the broiling sun the rocky ground is greatly heated, reaching 70—80°C. At night, on the other hand, temperatures fall rapidly. These sharp changes from heat to cold cause the rocks to split and break into pieces, leaving heaps of loose stone and sand. There are vast areas in the Sahara covered with nothing but small stones. These stony plains alternate with sands covered with dunes and barkhans that shift with the wind (Fig. 141).
Sand-storms, or simooms, occur. Clouds of hot sand are lifted by the strong wind and driven across the desert, covering the sky and threatening to bury every living being that hadn't the time to hide.
Vegetation in the Sahara is very poor. There grows tough grass and long-rooted thorny scrub. There are plants of the tumbleweed type that break off near the ground and, rolling before the wind, scatter their seeds. Near springs and wells, where the underground water comes to the surface, there are oases of date-palm and acacia (Fig. 142).

Fig. 141. The advance of the sand on an oasis.

There is a great variety of lizards, snakes and scorpions, all of which can live without water for a long time. On the edges of the desert there are antelopes, hyenas and lions. Ostriches, birds that run very quickly but cannot fly, are also common.

one-humped camels in

search of grass. They cover great distances sometimes (see coloured picture of the desert). The meat and milk of the camel serve them as food, while from its hair, blankets and clothing are made. The inha­bitants of the oases lead a settled way of life and are engaged in agri­culture. One of the chief food-items is the date.
South Africa lies in the belt of south-east trade winds. These drop their moisture on the slopes of the Drakensberg Mountains, leaving the interior of the continent almost rainless. Here stretch dry steppes and semideserts. Plant growth consists of thorny scrub, aloe (a century plant with thick leaves), spurge (a plant with milky juice), juicy water-melons and pumpkins (Fig. 143). Antelopes and zebras very much enjoy eating these plants.

Fig. 142. A Sahara oasis. Near the pond grow date-palms.

The rainfall is especially low in the western part of South Africa, where a cold Atlantic current flows off coast. In this region the semi-desert gives way to the desert, where there is hardly any vegetation.
The Subtropics. The extreme north and south of Africa are sub­tropical. Summers are hot there, winters cool (10—15°C above zero).
The slopes of the Atlas Mountains facing the Mediterranean Sea have a subtropical climate with hot and dry summers and rainy win­ters. Hot dry winds called siroccos often blow from the Sahara brin­ging much dust. Evergreen forests and tough-leaved shrubs similar to those in Southern Europe grow here. Animal life includes the jackal, porcupine and macaca (a tailless monkey).

Fig. 143. Wild water-melons in the semidesert of South Africa (Kalahari).

In the south-west of South Africa the climate is also subtropical, with most of the rain falling in winter. There grow evergreen tough-leaved shrubs of a darkish blue-green colour, silver trees with silvery leaves, geraniums of all kinds, hyacinths and tulips. This is the home of many of our indoor plants.
The slopes of the Drakensbergs facing the Indian Ocean on the south-east of Africa have a wet subtropical climate and a rich ever­green vegetation.
There are many orchards and vineyards in the subtropical re­gions. Nature has been greatly changed by man. Besides natives, many people of European descent live here.
Questions and Assignments.
1.  Trace on your Atlas map  the succession of natural zones
in Africa along meridian 20'.

• Why do the deserts lying within the Tropics of Cancer and
  Capricorn in Africa extend to the very coasts of the Atlantic Ocean?
• Mark the desert and subtropical zones of Africa on the outline
  map.
• In what natural zones do the baobab, date-palm, silver tree,
  ficus and  tree-fern grow?
• What zones do the gorilla, rhinoceros, ostrich, giraffe and lion
  inhabit?