Fruits contribute to the diet, chiefly by adding vitamins. Many fruits are very good sources of provitamin A, or carotene, often associated with yellow color. (There are no other external clues about the nutritional value of fruit.) Others supply more than adequate amounts of vitamin C. They may contain small amounts of biotin and pantothenic acid, but the principal B vitamins-thiamine, riboflavin, and niacin-are usually not found in useful amounts. Nuts are usually good sources of vitamin E and often of the B vitamins, and they are sources of proteins and oils as well. The fruits with the best nutritional content are as follows:

• Citrus fruits of all kinds. They are excellent sources of vitamin C, and the orange and other orange-colored citrus fruits are fair sources of provitamin A and good sources of calcium.

• The avocado. It is a very good source of highly digestible oil containing unsaturated fatty acids. Some varieties are good sources of phosphorus and riboflavin and excellent sources of vitamin A. Some varieties also contain useful amounts of protein.

• The mango. It is usually an excellent source of provitamin A but a variable source of vitamin C; as mangoes mature, vitamin C content decreases.

• The guava. It is an excellent source of vitamin C and iron and a good source of niacin.

• The passion fuit. It is an excellent source of pro-vitamin A and perhaps the best fruit source of niacin. Its vitamin C content is fair.

• The papaya. It is a good source of provitamin A and ascorbic acid and, surprisingly, a fair source of calcium.

• The coconut. One of the best fruits with respect to protein, the coconut is an excellent source of oil, which unfortunately is mostly composed of saturated fatty acids. Its phosphorus content is high, and its iron con-tent is excellent, but its vitamin B content is very low for a seed.

• The macadamia nut. An outstanding source of protein, oil, phosphorus, iron, thiamine, and riboflavin and a fair source of calcium and niacin. This is perhaps the most nutritious of all nuts.

• The canistel. A fair source of protein and provitamin A, this fruit is one of the richest sources known of sugars.

The problems associated with introducing a new fruit into a region are numerous. The first problem is finding high-quality germplasm for introduction. A good begin-ning is to ask farmers, gardeners, nurserymen, agricul-tural extension agents, professors, and scientists at agricultural experiment stations around the tropical world about indigenous species or varieties desired. Most persons interested in rare and unusual fruits maintain a network of useful connections. One has to become part of this network. Joining organizations such as the Rare Fruit Council International (see later) is one way of getting in touch with knowledgeable people.

It is not enough to merely locate the species or variety desired. It is also necessary to obtain propagation material and perhaps propagation facilities. And one may be asked for materials or services in exchange. Perhaps the best way to get material is to find a person who grows the plant and knows it well. Museums, bo-tanical gardens, agricultural experiment stations, and universities are also good places to look.

Successful introduction is also a matter of securing the right kind of propagating materials at a favorable time. The seeds of many tropical fruits need to be planted within a few days of being removed from the fruit, for they die when dried. Budwood or scions for grafting must be transferred quickly. Bare-rooted trees are often difficult or impossible to transfer. Therefore, careful planning and attention to transportation details are necessary.