Ranunculus is a very large genus in the Ranunculaceae family. Not all of them are tuberous and could be considered "bulbs".
Ranunculus asiaticus, commonly known as 'Persian Buttercup' is native to the eastern Mediterranean region in southwestern Asia, southeastern Europe (Crete, Karpathos and Rhodes), and northeastern Africa. Photograph 1 of commercially supplied tubers on a 10 mm grid by David Pilling. The free ends of the tubers point downwards, and the ends that are joined go uppermost in the ground. Before planting tubers are soaked for 24 hours and plump up, as shown in photos 2 and 3. Photo 4 shows the resulting shoots a couple of weeks later.
Photos showing the resulting flowers in Spring. Illustration from the Gottorfer Codex, a collection paintings on vellum depicting flowers of the garden of Schloss Gottorf, created between 1649 and 1659.
Seed related photos.
Ranunculus calandrinioides, a summer dormant buttercup native to the Atlas mountains in Morocco, has a thick rootstock with fleshy roots. It is easily grown under cold glass. Grow it as hard as possible (ie don't keep it frost free under glass) and give it as much light as possible. I have also found that it keeps much more compact and much more beautiful if kept bone dry until mid winter. In this way growth does not commence above ground until early spring and is much less lax. (Watered in early autumn, after a summer rest it comes into growth well before Christmas.) Photos by Tony Goode.
Ranunculus ficaria is a common European species, dwelling in cool and shady places. The first photo was taken by Angelo Porcelli. The second photo was taken by Jim McKenney who wrote: "Shown here are two forms, both presumably forms of Ranunculus ficaria. The one on the right is typical of the several feral forms which infest local woodlots and gardens here in Maryland. The one on the left is a large-flowered form which grows in a local garden. This large-flowered form has been identified as Ranunculus ficaria ssp ficariaeformis."
Ranunculus ficaria 'Brazen Hussy' is a cultivar with dark brown to black leaves and shiny bright yellow satiny flowers. Some pbs list members say it can be a bit of a thug spreading over time and others have trouble keeping it going. If moved, a few tubers are often inadvertenly left behind to keep the patch going. It is dormant for part of the year. Photos taken by Mary Sue Ittner in a Portland, Oregon garden.
Ranunculus kotchii is not an accepted species name but plants are grown under this name. Tuber photo by Peter Taggart.