Tetragon is one of the common names -- the more traditional name -- of Tetragonia tetragonoides; currently, the most trendy name is New Zealand Spinach. It is a spinach-like green, usually used in pottage; it has high levels of oxalate, so many sources do not recommend eating it raw. Young leaves and sprouts, however, are sometimes used as an accent flavor in salads without blanching.
It is not closely related to spinach (being of the Aizoaceae, or fig-marigold family), but does look and taste like spinach once it's on your plate. When alive, it grows as a vine-like mass, often growing in sandy soils and beaches; it is tolerant of salt and is fairly hardy, so it is also used as an ornamental ground-cover.
It is native to the shores of eastern Asia, Australia, and New Zealand. It has been historically eaten in these areas, and has been spread around the world for both food and ornamental use; it is also listed as an invasive species in many areas. It is currently found on all continents other than Antarctica.
It is nutritionally light, providing mostly vitamins and fiber. One cup (180 g) of cooked leaf provides 22 calories, 2.5 grams of dietary fiber, 438% of your daily vitamin K, 40% of manganese, 30% of vitamin B6, 30% of vitamin C, and 15% of your daily copper. Only the leaves and stems are eaten; it has yellow flowers and small spiky fruit that should not be sampled.
Tetragon gains its traditional name from its kite-shaped leaves. It is also known as Tetragonia, ice plant (careful, other plants of this name may not be edible), ever-bearing spinach, everlasting spinach, perpetual spinach, Della Nuova Zelanda, Botany Bay spinach, Cook’s cabbage, kōkihi, and sea spinach.