A picture is worth more than a thousand words: the leaves of various Hydrophytum sp. in the formicarum - moselayanum complex placed side-by-side. Noticeably, the lateral veins are more prominent in the formicarum and leaf thickness increases towards moselayanum. 
Clockwise from the top: (1) Commonly traded species in US (2) Sp from Papua, ex Botanical Gdn, Europe (3) Sp from Northern Australia ex Botanical Gdn (4) 2 Sp from Triton Bay, Irian Jaya.
The commonly traded species has been identified as H. moseleyanum (syn. H. papuanum) and is also almost identical to the species from a Botanical Gardens in Europe collected in 1960s or 1970s from New Guinea. They have matt lime-green semi-succulent leaves and silvery brown tubers. The species from Northern Australia is also similar but is more deciduous under the same cultivation regime. It shreds leaves easily when dry. Finally, 2 species from Irian Jaya looks more intermediate just like the other H. aff. moseleyanum from the Philippines and Bogor, Java. These 2 irian plants differs in having longer pointed leaves.
Maybe it is more practical to define this group as a formicarum-moseleyanum complex rather than to pin a name to it.
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I started off by introducing Hydnophytum with affinities to H. moseleyanum without introducing H. moseleyanum. Why? The reason will be obvious in part 5. Here, i have taken the liberty to label those which have the H. moseleyanum look but are found outside and farway from New Guinea. 
You may now notice a trend of featuring Hydnophytum from west to east. I am trying to show the variation of Hydnophytum species complex; some may be distinctive enough to be a species on its own while others are just too fluid and form a continuous change from formicarum to moseleyanum from Sumatra to New Guinea.
Let's start off with the better known Hydnophytum formicarum (photo above). This plant is in cultivation in both private and botanical collections for sometime. Various distinctive forms or varieties are available from specialist nurseries time to time and they are mostly labelled as H. formicarum. Personally, these are probably variations of one species complex or have evolved into different species from west to east. Base on regional flora, Hydnophytum represented by one species which is H. formicarum in Andaman Islands, Peninsular Thailand & Malaysia, Sumatra. Observations of wild H. formicarum plants in Peninsular Malaysia and its offshore islets in the South China seas yield little variation. Thai plants are also similar. The leaves are leathery and have prominent veins, tubers are invariably dark brown with ant entrance holes irregularly placed. The "spination" or "bristles " covering the tubers are root stubs which stop growing. Tuber ridges and lobings are irregular. Having seen many seedlings from the same mother plant, the caudex is shaped more by environment conditions then genetics. More to come... in Part 2.
This is probably one of the better known impatiens amongst caudiciform and succulent growers, little known in the wider ornamental plant growing community. Werner Rauh has chosen this species to be included in his book Succulent and Xerophytic Plants of Madagascar Volume One, Strawberry Press (1995). It is aptly named Impatiens tuberosa. It is a perennial which sheds its stems during the dry season when it goes dormant. With the onset of the next growing season, new stems will emerge from the dormant buds on the stubby irregular above ground tuber. The plant comes from Montagnes des Francais near Antsiranana and Windsor Castle in Northern Madagascar on limestone cliff. It is likely to be found growing in cracks with accumulated peat with other Euphorbia species from the same area. The flowering plant above is only 3 mths old from seed! Perhaps its rapid growth is an adaption to the strongly seasonal climate with a short rainy monsoon. The caudice is not obvious at this stage. But it is clear from the photo below that the stem will abscise just above the tiny bud:
