The Aponogetons are a genus of tropical aquatic plants distributed from Africa and Madagascar through South Asia and Indonesia to Australia. The genus Aponogeton is somewhat isolated all by itself in the family Aponogetonaceae, which contains only this one species. Aponogeton fenestralis, the Madagascar Lace Plant, was among the first tropical plants in French and English aquariums, imported before 1860 and mentioned in J. Shirley Hibberd's Rustic Adornments for Homes of Taste.
The aponogetons form tubers and thickened rhizomes that store nutrients to see them through the dry seasons, when their pools and watercourses may dry up entirely. Unlike other purely aquatic plants that raise leaves above water level when it drops to a few inches, aponogetons keep below the surface, a virtue in aquaria, where a large aponogeton in too low a tank, however, will overspread the top with its leaves. In the aquarium many Aponogeton species and hybrids may still insist on passing through resting and growth phases. Cooler water during rest periods helps them pull through, but some like to be taken up and stored for a few months in slightly damp sand. They'll let you know by withdrawing their leaves one by one. The African aponogetons are particularly adapted in this way to an annual drought, so constant aquarium life doesn't suit them. But perhaps their resting phase should keep the Aponogetons off my beginner's list of plants.
Only a couple of species reproduce vegetatively, in that you can pull up a big clump and slice it or pull it apart, as you'd do with sword plants (Echinodorus) or day-lilies in your garden. Aponogeton tubers ("bulbs") more typically have a single growing point, from which all leaves will spring. You mustn't damage that growing point during dormancy, and you mustn't plant the tuber upside-down: instead you soak it for a couple of hours, lay it on damp sand to sprout and cover it with a glass bowl until you see the tell-tale sprout of green. Don't bury that growing point as you're planting the tuber; leave the top third of the thickened tuber standing free of the substrate.
Cooler temperatures are prefered: at temperatures over 80o or so, leaf growth slows or stops and rhizomes stop storing nutrients.
Aponogeton crispus is a tall plant, one of the rewards for having a high "show" tank; its stems (petioles) can be over six inches long and its narrow leaf blade another eighteen inches. Its filmy, slightly translucent leaves with their ruffled ("crispus") edges constrast dramatically with Anubias. They open faintly reddish and mature to green in the plants I have, which are variable in this aspect, though they were purchased together and came from the same source. A mark, I suppose, of a hybrid origin: most A. crispus in the market are actually hybrids. The Danish plantsmen Tropica developed an Aponogeton crispus x rigidifolia hybrid which has the long thin rhizome-like tuber of the rigidifolius parent rather than the thicker one of crispus, and there are A. crispus x undulatus plants too. Setting the plant behind a dense low growth or a chunk of root masks its long leaf-stalks. Conversely, plant it near a front corner, positioned where filter outflow will extend its leaves across the upper water levels as a framing motif, and look through the stems to a solid feature somewhat behind.
A. crispus comes from South India and Sri Lanka. It loves my very soft, slightly acidic water, but it's not finicky about pH readings in the 7s.
Aponogeton ulvaceous produces filmy long leaves from its round little tuber. When I was a child, one growing in a tiny flower pot completely filled a 5-gallon tank, much too small for such a plant. But the pair of dwarf gouramies loved it, and they spawned, an event that hooked me forever on keeping fish.