CAOUTCHOUC or RUBBER is the coagulated latex or sap of various trees and other plants which grow mainly within the tropical forests. The most important of these belong to the genus Hevea, one species of which, Hevea brasiliensis, grows extensively in the Amazonian lowlands. This tree, which reaches a height of 100 feet, thrives best in districts which are regularly inundated, but it will grow elsewhere, provided it has a deep rich soil which is constantly moist. It does not flourish where there is a dry season, nor do sandy soils suit it. It only lives with difficulty in regions where the thermometer falls below 65° F. and probably finds its most favourable environment where the mean annual temperature is between 80° F. and 90° F. Several other species of Hevea produce rubber under somewhat similar conditions. The " manicoba " (Manihot Glaziovii) is also rubber-producing. It is a smaller tree than H. brasiliensis, and, while demanding a high temperature, thrives best on sloping land with a stony and well-drained soil. A poorer quality of rubber known as " mangabeira " is obtained from Hancornia speciosa, a tree which belongs rather to the savanna regions of the tropics, though it will also thrive where there is a heavy rainfall. Castilloa elastics grows in Central America where it flourishes best when it does not experience a minimum temperature of less than 60° F., and an annual precipitation of over 60 inches with a dry season of less than four months. It prefers a clay soil or a mixture of sand and clay, but will not grow in a marshy district or where the subsoil is very wet. One of the chief rubber-producing plants of Asia is Ficus elastics which frequently begins life as an epiphyte, but after some years strikes into the ground on its own account. It grows best in forested regions on the lower slopes of hills, and requires abundant rains with only a short dry period ; it can stand a certain amount of cold, and a slight frost does not injure it seriously. In Africa, rubber is mainly obtained by tapping certain trees and vines, notably Funtumia elastics, and various species of the genus Landolphia. Funtumia elastics grows both in regions which are always wet, and in regions which have a dry season. Of the Landolphia, L. ozvariensis grows in the forests of the Congo basin, while L. Heudelotti prefers the drier savanna lands of Sudan.
COFFEE.—There are two varieties of coffee in common use, Collea arabica and Collea liberica. The former, which is com mercially the more important, is grown in upland regions, while the latter prefers the lowlands. The coffee plant requires a heavy rainfall, and appears to thrive best where the annual precipitation is at least 60 or 70 inches. Its cultivation is seldom carried far
beyond the tropics, within which a temperature varying from 60° F. to F. appears most favourable to it. At the same time it seems able to stand occasional low temperatures, and the thermometer may even descend to freezing point without damaging more than the young wood. On the other hand, at a temperature of 95° F.
at most, the plant begins to suffer. Coffee is an exhausting crop and demands a rich soil. Hence it finds its most favourable environment on land from which virgin forest has recently been cleared. The height at which Collea arabica thrives best varies in the different regions in which it is grown, but probably the most favourable situations lie between 1,000 and 5,000 feet.
CACA0.—The cacao tree (Cacao theobroma) demands a warmer and more humid climate than coffee and grows best where the temperature falls between and Its natural habitat is the wet evergreen forest in districts subject to frequent inunda tions, and unlike coffee it does not thrive in upland regions. It flourishes on the banks of rivers where the alluvium, which they have brought down, is mixed with humus. When the plant is cultivated it requires, for a time, the shelter of shade trees.
RICE.—Of rice, which forms one of the chief foods of the human race, there are many varieties, but they may all be broadly divided into two main groups—upland and swamp. Of these, the latter are the more important. The saying that rice grows best with its feet in the water and its head in the fire, indicates the nature of the climate required. A high temperature is essential, and the plant must also be irrigated to a depth of several inches at certain stages of its growth. The best soils are clays or clay loans, such as are frequently found along the banks, and at the deltas, of rivers. These districts have the additional advantage of being flat and easily cultivated. In tropical regions, moreover, they are often under water at certain seasons of the year, and the rice plant then gets the necessary amount of water, while the land is fertilised by the silt which is deposited at these times. As rice is an exhausting crop, recourse would otherwise have to be had to manure, which is practically unobtainable in many places where rice is grown. The plant can also be cultivated on lands which are unflooded if irrigation is possible. Upland rice which requires much less moisture can be raised at a considerable height above sea-level.