|Sweet potato in flower in Hong Kong, China|
|Sweet potato roots|
The sweet potato (Ipomoea batatas) is a dicotyledonous plant that belongs to the family Convolvulaceae. Its large, starchy, sweet-tasting, tuberous roots are a root vegetable. The young leaves and shoots are sometimes eaten as greens. Ipomoea batatas is native to the tropical regions in America. Of the approximately 50 genera and more than 1,000 species of Convolvulaceae, I. batatas is the only crop plant of major importance—some others are used locally, but many are poisonous. The sweet potato is only distantly related to the potato (Solanum tuberosum) and does not belong to the nightshade family.
The genus Ipomoea that contains the sweet potato also includes several garden flowers called morning glories, though that term is not usually extended to Ipomoea batatas. Some cultivars of Ipomoea batatas are grown as ornamental plants; the name "tuberous morning glory" may be used in a horticultural context.
The plant is a herbaceous perennial vine, bearing alternate heart-shaped or palmately lobed leaves and medium-sized sympetalous flowers. The edible tuberous root is long and tapered, with a smooth skin whose color ranges between yellow, orange, red, brown, purple, and beige. Its flesh ranges from beige through white, red, pink, violet, yellow, orange, and purple. Sweet potato varieties with white or pale yellow flesh are less sweet and moist than those with red, pink or orange flesh.
In certain parts of the world, sweet potatoes are locally known by other names. In New Zealand English, the Māori term kūmara (also spelled kūmera) is commonly used. Although the soft, orange sweet potato is often called a "yam" in parts of North America, the sweet potato is botanically very distinct from a genuine yam (Dioscorea), which is native to Africa and Asia and belongs to the monocot family Dioscoreaceae. To add to the confusion, a different crop plant, the oca, Oxalis tuberosa (a species of woodbind), is called a "yam" in many parts of Polynesia, including New Zealand. To prevent confusion, the United States Department of Agriculture requires sweet potatoes labeled as "yams" to also be labeled as "sweet potatoes". The sweet potato is North Carolina's state vegetable.
- 1 Origin, distribution and diversity
- 2 Cultivation
- 3 Nutrient content
- 4 Culinary uses
- 5 Names
- 6 Notes
- 7 References
- 8 External links
Origin, distribution and diversity
The origin and domestication of sweet potato is thought to be in either Central America or South America. In Central America, sweet potatoes were domesticated at least 5,000 years ago. In South America, Peruvian sweet potato remnants dating as far back as 8000 BC have been found.
Daniel F. Austin postulated that the center of origin of I. batatas was between the Yucatán Peninsula of Mexico and the mouth of the Orinoco River in Venezuela. The 'cultigen' had most likely been spread by local people to the Caribbean and South America by 2500 BC. Strong supporting evidence was provided that the geographical zone postulated by Austin is the primary center of diversity. The much lower molecular diversity found in Peru–Ecuador suggests this region should be considered as secondary center of sweet potato diversity.
The sweet potato was also grown before western exploration in Polynesia. Sweet potato has been radiocarbon-dated in the Cook Islands to 1000 AD, and current thinking is that it was brought to central Polynesia around 700 AD, possibly by Polynesians who had traveled to South America and back, and spread across Polynesia to Hawaii and New Zealand from there. It is possible, however, that South Americans brought it to the Pacific, although this is unlikely as it was the Polynesians who had a strong maritime tradition and not the native South Americans. The theory that the plant could spread by floating seeds across the ocean is not supported by evidence. Another point is that the sweet potato in Polynesia is the cultivated Ipomoea batatas, which is generally spread by vine cuttings and not by seeds.
Sweet potatoes are now cultivated throughout tropical and warm temperate regions wherever there is sufficient water to support their growth. Due to a major crop failure, sweet potatoes were introduced to Fujian province of China in about 1594 from Luzon. The growing of sweet potatoes was encouraged by the Governor Chin Hsüeh-tseng (Jin Xuezeng). Sweet potatoes were introduced as a food crop in Japan, and by 1735 was planted in Shogun Tokugawa Yoshimune's private garden. It was also introduced to Korea in 1764.
According to the Food and Agriculture Organization (FAO) statistics, world production in 2004 was 127 million tonnes. The majority comes from China, with a production of 105 million tonnes from 49,000 km2. About half of the Chinese crop is used for livestock feed.
Per capita production is greatest in countries where sweet potatoes are a staple of human consumption, led by Papua New Guinea at about 500 kg per person per year, the Solomon Islands at 160 kg, Burundi and Rwanda at 130 kg and Uganda at 100 kg.
In the U.S., North Carolina, the leading state in sweet potato production, provided 38.5% of the 2007 U.S. production of sweet potatoes. In 2007, California produced 23%, Louisiana 15.9%, and Mississippi 19% of the U.S. total.
The town of Opelousas, Louisiana's "Yambilee" has been celebrated every October since 1946. The Frenchmen who established the first settlement at Opelousas in 1760 discovered the native Atakapa, Alabama, Choctaw, and Appalousa tribes eating sweet potatoes. The sweet potato became a favorite food item of the French and Spanish settlers and thus continued a long history of cultivation in Louisiana.
Mississippi has about 150 farmers growing sweet potatoes on about 8,200 acres (30 km2), contributing $19 million to the state's economy. Mississippi's top five sweet potato producing counties are Calhoun, Chickasaw, Pontotoc, Yalobusha, and Panola. The National Sweet Potato Festival is held annually the entire first week in November in Vardaman (Calhoun County), which proclaims itself as "The Sweet Potato Capital".
The town of Benton, Kentucky, celebrates the sweet potato annually with its Tater Day Festival on the first Monday of April. The town of Gleason, Tennessee, celebrates the sweet potato on Labor Day weekend with its Tater Town Special.
Sweet potatoes became popular very early in the islands of the Pacific Ocean, spreading from Polynesia to Japan and the Philippines. One reason[original research?] is that they were a reliable crop in cases of crop failure of other staple foods because of typhoon flooding. They are featured in many favorite dishes in Japan, Taiwan, the Philippines, and other island nations. Indonesia, Vietnam, India, and some other Asian countries are also large sweet potato growers. Sweet potato, also known as kelang in Tulu is part of Udupi cuisine. Uganda (the second largest grower after China), Rwanda, and some other African countries also grow a large crop which is an important part of their peoples' diets. The New World, the original home of the sweet potato, grows less than three percent (3%) of the world's supply. Europe has only a very small sweet potato production, mainly in Portugal. In the Caribbean, a variety of the sweet potato called the boniato is popular. The flesh of the boniato is cream-colored, unlike the more popular orange hue seen in other varieties. Boniatos are not as sweet and moist as other sweet potatoes, but many people prefer their fluffier consistency and more delicate flavor.
Sweet potatoes have been an important part of the diet in the United States for most of its history, especially in the Southeast. From the middle of the 20th century, however, they have become less popular. The average per capita consumption of sweet potatoes in the United States is only about 1.5–2 kg (3.3–4.4 lb) per year, down from 13 kg (29 lb) in 1920. Southerner Kent Wrench writes: "The Sweet Potato became associated with hard times in the minds of our ancestors and when they became affluent enough to change their menu, the potato was served less often."
|Producers (in million tonnes)
Data for year 2011
The plant does not tolerate frost. It grows best at an average temperature of 24 °C (75 °F), abundant sunshine and warm nights. Annual rainfalls of 750–1,000 mm (30–39 in) are considered most suitable, with a minimum of 500 mm (20 in) in the growing season. The crop is sensitive to drought at the tuber initiation stage 50–60 days after planting, and it is not tolerant to water-logging, as it may cause tuber rots and reduce growth of storage roots if aeration is poor.
Depending on the cultivar and conditions, tuberous roots mature in two to nine months. With care, early-maturing cultivars can be grown as an annual summer crop in temperate areas, such as the northern United States. Sweet potatoes rarely flower when the daylight is longer than 11 hours, as is normal outside of the tropics. They are mostly propagated by stem or root cuttings or by adventitious roots called "slips" that grow out from the tuberous roots during storage. True seeds are used for breeding only.
They grow well in many farming conditions and have few natural enemies; pesticides are rarely needed. Sweet potatoes are grown on a variety of soils, but well-drained, light- and medium-textured soils with a pH range of 4.5-7.0 are more favorable for the plant. They can be grown in poor soils with little fertilizer. However, sweet potatoes are very sensitive to aluminum toxicity and will die about six weeks after planting if lime is not applied at planting in this type of soil. Because they are sown by vine cuttings rather than seeds, sweet potatoes are relatively easy to plant. Because the rapidly growing vines shade out weeds, little weeding is needed. In the tropics, the crop can be maintained in the ground and harvested as needed for market or home consumption. In temperate regions, sweet potatoes are most often grown on larger farms and are harvested before first frosts.
In the Southeastern United States, sweet potatoes are traditionally cured to improve storage, flavor, and nutrition, and to allow wounds on the periderm of the harvested root to heal. Proper curing requires drying the freshly dug roots on the ground for two to three hours, then storage at 85–90 °F (29–32 °C) with 90 to 95% relative humidity from five to fourteen days. Cured sweet potatoes can keep for thirteen months when stored at 55–59 °F (13–15 °C) with >90% relative humidity. Colder temperatures injure the roots.
Yields of sweet potato crop
In 2010, the world average annual yield for sweet potato crop was 13.2 tonnes per hectare. The most productive farms of sweet potato breeds were in Senegal, where the nationwide average annual yield was 33.3 tonnes per hectare. Yields as high as 80 metric tonnes per hectare have been reported from farms of Israel.
|Nutritional value per 100 g (3.5 oz)|
|Energy||359 kJ (86 kcal)|
|Dietary fibre||3 g|
|Vitamin A equiv.||
|Percentages are roughly approximated using US recommendations for adults.
Source: USDA Nutrient Database
|Nutritional value per 100 g (3.5 oz)|
|Energy||378 kJ (90 kcal)|
|Dietary fiber||3.3 g|
|Vitamin A equiv.||
|Percentages are roughly approximated using US recommendations for adults.
Source: USDA Nutrient Database
Besides simple starches, raw sweet potatoes are rich in complex carbohydrates, dietary fiber and beta-carotene (a provitamin A carotenoid), while having moderate contents of other micronutrients, including vitamin B5, vitamin B6, manganese and potassium (right table). When cooked by baking, small variable changes in micronutrient content occur to include a higher content of vitamin C at 24% of the Daily Value per 100 g serving (right table), as well as an increase in polyphenol levels.
The Center for Science in the Public Interest has compared the nutritional value of sweet potatoes to other foods. Considering fiber content, complex carbohydrates, protein, vitamin A and potassium, the sweet potato ranked highest in nutritional value.
Sweet potato varieties with dark orange flesh have more beta carotene than those with light-colored flesh, and their increased cultivation is being encouraged in Africa, where vitamin A deficiency is a serious health problem. A 2012 study of 10,000 households in Uganda found that 50% of children who ate normal sweet potatoes suffered from vitamin A deficiency compared with only 10% of those on the high beta carotene variety.
Comparison of sweet potato to other food staples
The table below presents the relative performance of sweet potato to other food staples. While sweet potato provides less edible energy and protein per unit weight than cereals, it is a higher nutrient density source of certain vitamins and minerals than cereals.
|STAPLE:||Maize / Corn[A]||Rice[B]||Wheat[C]||Potato[D]||Cassava[E]||Soybean (Green)[F]||Sweet potato[G]||Sorghum[H]||Yam[Y]||Plantain[Z]|
|Component (per 100g portion)||Amount||Amount||Amount||Amount||Amount||Amount||Amount||Amount||Amount||Amount|
|Vitamin C (mg)||0||0||0||19.7||20.6||29||2.4||0||17.1||18.4|
|Pantothenic acid (mg)||0.42||1.01||0.95||0.30||0.11||0.15||0.80||-||0.31||0.26|
|Vitamin B6 (mg)||0.62||0.16||0.3||0.30||0.09||0.07||0.21||-||0.29||0.30|
|Folate Total (μg)||19||8||38||16||27||165||11||0||23||22|
|Vitamin A (IU)||214||0||9||2||13||180||14187||0||138||1127|
|Vitamin E, alpha-tocopherol (mg)||0.49||0.11||1.01||0.01||0.19||0||0.26||0||0.39||0.14|
|Vitamin K1 (μg)||0.3||0.1||1.9||1.9||1.9||0||1.8||0||2.6||0.7|
|Saturated fatty acids (g)||0.67||0.18||0.26||0.03||0.07||0.79||0.02||0.46||0.04||0.14|
|Monounsaturated fatty acids (g)||1.25||0.21||0.2||0.00||0.08||1.28||0.00||0.99||0.01||0.03|
|Polyunsaturated fatty acids (g)||2.16||0.18||0.63||0.04||0.05||3.20||0.01||1.37||0.08||0.07|
|A corn, yellow||B rice, white, long-grain, regular, raw, unenriched|
|C wheat, hard red winter||D potato, flesh and skin, raw|
|E cassava, raw||F soybeans, green, raw|
|G sweet potato, raw, unprepared||H sorghum, raw|
|Y yam, raw||Z plantains, raw|
|Nutritional value per 100 g (3.5 oz)|
|Energy||175 kJ (42 kcal)|
|Dietary fiber||5.3 g|
|Vitamin A equiv.||
Direct link to database entry 
|Percentages are roughly approximated using US recommendations for adults.
Source: USDA Nutrient Database
Amukeke (sun-dried slices of root) and inginyo (sun-dried crushed root) are a staple food for people in northeastern Uganda. Amukeke is mainly served for breakfast, eaten with peanut sauce. Inginyo is mixed with cassava flour and tamarind to make atapa. People eat atapa with smoked fish cooked in peanut sauce or with dried cowpea leaves cooked in peanut sauce.Emukaru (earth baked root) is eaten as a snack anytime and is mostly served with tea or with peanut sauce.
The young leaves and vine tips of sweet potato leaves are widely consumed as a vegetable in West African countries (Guinea, Sierra Leone and Liberia, for example), as well as in northeastern Uganda, East Africa. According to FAO leaflet No. 13 - 1990, sweet potato leaves and shoots are a good source of vitamins A, C, and B2 (riboflavin), and according to research done by A. Khachatryan, are an excellent source of lutein.
In Egypt, sweet potato tubers are known as "batata" (بطاطا) are a common street food in winter, where street vendors with carts fitted with ovens sell them to people passing time by the Nile or the sea. The two varieties used are the orange fleshed one as well as the white/cream fleshed one. They are also baked at homes as a snack or dessert, drenched with honey.
In China, sweet potatoes, typically of the yellow variety, are baked in a large iron drum, and sold as street food during winter.
In Korea, sweet potatoes are baked in foil or in open fire, typically during winter. In Korean, sweet potatoes are called "Goguma". In Japan, this is called yaki-imo (roasted sweet potato), which typically uses either the yellow-fleshed Japanese sweet potato or the purple-fleshed (Okinawan) sweet potato.
Sweet potato soup, served during winter, consists of boiling sweet potato in water with rock sugar and ginger. Sweet potato greens are a common side dish in Taiwanese cuisine, often boiled or sautéed and served with a garlic and soy sauce mixture, or simply salted before serving. They, as well as dishes featuring the sweet potato root, are commonly found at bento (Pe̍h-ōe-jī: piān-tong) restaurants. In northeastern Chinese cuisine, sweet potatoes are often cut into chunks and fried, before being drenched into a pan of boiling syrup.
In some regions of India, fasts of religious nature are an occasion for a change in normal diet, and a total absence from cooking or eating is held as elective while a normal diet for a fasting day is a light feast consisting of different foods from usual, amongst which sweet potato is one of the prime sources of sustenance. Sweet potato – shakarkand, शक्करकंद – is eaten otherwise, too, and a popular variety of preparation in most parts is roasted slow over kitchen coals at night and eaten with some dressing—primarily salt, possibly yogurt—while the easier way in the south is simply boiling or pressure cooking before peeling, cubing and seasoning for a vegetable dish as part of the meal. In Indian state of Tamil Nadu, it is known as 'Sakkara valli Kilangu'. It is boiled and consumed as evening snack. In some parts of India, fresh sweet potato is chipped, dried and then ground into flour; this is then mixed with wheat flour and baked into chapattis (bread). Between 15 to 20 percent of sweet potato harvest is converted by some Indian communities into pickles and snack chips. A part of the tuber harvest is used in India as cattle fodder.
In Japan, both sweet potatoes (called "satsuma-imo") and true purple yams (called "daijo" or "beni-imo") are grown. Boiled sweet potato is the most common way to eat it at home. Steaming sweet potatoes is also a common cooking method in Japan. Also, the use in vegetable tempura is common. Daigaku-imo is a baked sweet potato dessert. Because it is sweet and starchy, it is used in imo-kinton and some other wagashi (Japanese sweets), such as ofukuimo. Shōchū, a Japanese spirit normally made from the fermentation of rice, can also be made from sweet potato, in which case it is called imo-jōchū. Imo-gohan, sweet potato cooked with rice, is popular in Guangdong, Taiwan and Japan. It is also served in nimono or nitsuke, boiled and typically flavored with soy sauce, mirin and dashi.
In Korean cuisine, sweet potato starch is used to produce dangmyeon (cellophane noodles). Sweet potatoes are also boiled, steamed, or roasted, and young stems are eaten as namul. Pizza restaurants such as Pizza Hut and Domino's in Korea are using sweet potatoes as a popular topping. Sweet potatoes are also used in the distillation of a variety of Soju.
In Malaysia and Singapore, sweet potato is often cut into small cubes and cooked with yam and coconut milk (santan) to make a sweet dessert called bubur caca. A favorite way of cooking sweet potato is deep frying slices of sweet potato in batter, and served as a tea-time snack. In homes, sweet potatoes are usually boiled. The leaves of sweet potatoes are usually stir-fried with only garlic or with sambal belacan and dried shrimp by Malaysians.
In the Philippines, sweet potatoes (locally known as camote or kamote) are an important food crop in rural areas. They are often a staple among impoverished families in provinces, as they are easier to cultivate and cost less than rice. The tubers are boiled or baked in coals and may be dipped in sugar or syrup. Young leaves and shoots (locally known as talbos ng kamote or camote tops) are eaten fresh in salads with shrimp paste (bagoong alamang) or fish sauce. They can be cooked in vinegar and soy sauce and served with fried fish (a dish known as adobong talbos ng kamote), or with recipes such as sinigang. The stew obtained from boiling camote tops is purple-colored, and is often mixed with lemon as juice. Sweet potatoes are also sold as street food in suburban and rural areas. Fried sweet potatoes coated with caramelized sugar and served in skewers (camote cue) are popular afternoon snacks. Sweet potatoes are also used in a variant of halo-halo called ginatan, where they are cooked in coconut milk and sugar and mixed with a variety of rootcrops, sago, jackfruit and bilu-bilo (glutinous rice balls). Bread made from sweet potato flour is also gaining popularity. Sweet potato is relatively easy to propagate, and in rural areas that can be seen abundantly at canals and dikes. The uncultivated plant is usually fed to pigs.
In the mountainous regions of West Papua, sweet potatoes are the staple food among the natives there. Using the bakar batu way of cooking (free translation: burning rocks), rocks that have been burned in a nearby bonfire are thrown into a pit lined with leaves. Layers of sweet potatoes, an assortment of vegetables, and pork are piled on top of the rocks. The top of the pile then is insulated with more leaves, creating a pressure of heat and steam inside which cooks all food within the pile after several hours. In most parts of Indonesia, sweet potatoes are frequently fried with batter and served as snacks.
Candied sweet potatoes are a side dish consisting mainly of sweet potatoes prepared with brown sugar, marshmallows, maple syrup, molasses, orange juice, marron glacé, or other sweet ingredients. It is often served in America on Thanksgiving. Sweet potato casserole is a side dish of mashed sweet potatoes in a casserole dish, topped with a brown sugar and pecan topping. Sweet potato pie is also a traditional favorite dish in Southern U.S. cuisine. Sweet potato slices are fried in bacon drippings and eaten with the bacon on toast. Sweet potato fries or chips are another common preparation, and are made by julienning and deep frying sweet potatoes, in the fashion of French fried potatoes. Sweet potato fries are used with a variety of condiments such as Blue Cheese. Baked sweet potatoes are sometimes offered in restaurants as an alternative to baked potatoes. They are often topped with brown sugar and butter. Sweet potato butter can be cooked into a gourmet spread. Sweet potato mash is served as a side dish, often at Thanksgiving dinner or with barbecue. John Buttencourt Avila is called the father of the sweet potato industry in North America.
Before European contact, the Māori grew the small, yellow-skin, finger-sized kūmara known as taputini, hutihuti and rekamaroa that they had brought with them from east Polynesia. Modern trials have shown that the taputini was capable of producing well, but when American whalers, sealers and trading vessels introduced larger varieties in the early 19th century, they quickly predominated.
Currently there are three main varieties (red, orange and gold) grown in the subtropical northern part of the North Island near Dargaville and widely available throughout New Zealand year-round, where they are a popular alternative to potatoes. The red variety has dull red skin and purple-streaked white flesh, and is the most popular. The orange variety is the same as the American "Beauregard" variety. The gold kumara has pale, yellowish skin and flesh.
In the Solomon Islands, and neighboring Melanesian countries (as well as some parts of Polynesia), the sweet potato, along with the yam, also goes by the name common desert truffle. In Honiara, the capital city of the Solomon Islands, sweet potato is commonly referred to as "potato", whereas true potatoes are referred to as "English potato". Among the Urapmin people of Papua New Guinea, taro (known in Urap as ima) and the sweet potato (Urap: wan) are the main sources of sustenance, and in fact the word for "food" in Urap is a compound of these two words.
In Spain, sweet potato is called boniato. On the evening of All Souls' Day, in Catalonia (northeastern Spain) it is traditional to serve roasted sweet potato and chestnuts, panellets and sweet wine. The occasion is called La Castanyada. Sweet potato is also appreciated to make cakes or to eat roasted through the whole country.
In the Veneto (northeast Italy), sweet potato is known as patata mericana in the Venetian language (patata americana in Italian, meaning "American potato"), and it is cultivated above all in the southern area of the region; it is a traditional fall dish, boiled or roasted.
All parts of the plant are used for animal fodder.
Several selections are cultivated in gardens as ornamental plants for their attractive foliage, including the dark-leafed cultivars 'Blackie' and 'Ace of Spades' and the chartreuse-foliaged 'Margarita'.
Cuttings of sweet potato vine, either edible or ornamental varieties, will rapidly form roots in water and will grow in it, indefinitely, in good lighting with a steady supply of nutrients. For this reason, sweet potato vine is ideal for use in home aquariums, trailing out of the water with its roots submerged, as its rapid growth is fueled by toxic ammonia and nitrates, a waste product of aquatic life, which it removes from the water. This improves the living conditions for fish, which also find refuge in the vast root systems.
Although it is sometimes called a yam in North America, the sweet potato is not in the yam family, nor is it closely related to the common potato. The first Europeans to taste sweet potatoes were members of Columbus's expedition in 1492. Later explorers found many varieties under an assortment of local names, but the name which stayed was the indigenous Taino name of batata. This name was later transmuted to the similar name for a different vegetable, the ordinary potato, causing confusion from which it never recovered. The first record of the name "sweet potato" is found in the Oxford English Dictionary of 1775.
The Portuguese took the Taino name batata directly, while the Spanish also combined it with the Quechua word for potato, papa, to create the word patata for the common potato. In Argentina, Venezuela, Puerto Rico and the Dominican Republic it is called batata. In Mexico, Peru, Chile, Central America, and the Philippines, the sweet potato is known as camote (alternatively spelled kamote in the Philippines), derived from the Nahuatl word camotli. Boniato is another name widely used in mainland Spain and in Uruguay.
In Peru, the Quechua name for a type of sweet potato is kumar, strikingly similar to the Polynesian name kumara and its regional Oceanic cognates (kumala, umala, 'uala, etc.), which has led some scholars to suspect an instance of Pre-Columbian trans-oceanic contact.
In New Zealand, the most common variety is the Red (purple) cultivar, and is called kumara, though orange (Beauregard) and gold varieties are also available. Kumara is particularly popular as a roasted food or in contemporary cuisine, as kumara chips, often served with sour cream and sweet chili sauce. Occasionally shops in Australia will label the purple variety "purple sweet potato" to denote its difference to the other varieties. About 95% of Australia's production is of the orange variety named "Beauregard", originally from North America, known simply as "sweet potato". A reddish-purple variety, Northern Star, is 4% of production and is sold as kumara.
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|Wikimedia Commons has media related to Sweet potato.|
|Wikisource has the text of the 1911 Encyclopædia Britannica article Sweet Potato.|
- Edmond, Joseph Bailey; Ammerman, Gale Richard (1 November 1971). Sweet potatoes: production, processing, marketing. Major Feed and Food Crops in Agriculture and Food Series. Westport, Connecticut: The Avi Publishing Company.
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- Journal of Nanyang University, Volumes 5-6. Contributor 南洋大學. 新加坡南洋大學. 1971. Retrieved 24 April 2014.
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