The Morning Glory family (Convolvulaceae) is found worldwide in all climates but the culinary ones all belong to the genus Ipomoea which inhabits only tropical and subtropical regions. A few of the other genera produce seeds which are sometimes ingested but not for the purpose of nutrition.Sweet Potatoes & U.S. "Yams"
Sweet potato vines are members of the Morning Glory family native to Central America and the Caribbean. They are unrelated to true potatoes which are Nightshades. They produce an edible "storage root" which is non-toxic and under proper "curing" conditions converts part of its starches to sugars making it sweet.
Water Spinach -
[Swamp Cabbage; Ong Choy - variously spelled (Cantonese); Pak Hung,
Pak Bung (Thai); Kang Kong (Malay, Filipino); Kang Kung, Rau Muong
(Vietnam); Toongsin Tsai (Mandarin); Chinese Watercress, Water
Convolvulus, Water Morning-glory; Ipomoea aquatica also
Ipomoea raptans (not common)]
This semi-aquatic plant is a controlled substance within the USA. Importing plants or seeds and/or growing plants without a permit are all illegal - see USDA Plant Profile. Growing it or even transporting it is totally illegal in some states.
Here in California it is a quarantined crop but may be transported without a permit within the state. Enough growers have permits it is in good supply, and that's a good thing because if it wasn't our large Asian population might try sneaking it into our waterways like they did with snakehead fish.
There are several varieties of this vegetable, including a long leafed
variety that can be grown in damp soil (Ching Quat) and a wider leafed
variety that requires free water (Pak Quat). The wider leaf variety is
preferred in Asia but is rarely available here, probably to keep the
growing areas farther from our waterways. The photo specimens are about
19 inches long.
Details and Cooking.
These tropical vines, native to Africa and Asia, are quite unrelated to the Sweet Potato vines and are in the same clade as lilies (monocots). They produce root tubers which are generally toxic, some less so, some more so, but it's best to avoid eating most of them raw. Some require tedious pounding and leaching to make them edible but these are not sold as vegetables in the USA. The varieties sold here just require a little cooking for detox.