i think when you say "the spice fennel" you're referring to fennel seeds, either whole or ground. they taste very similar to a fresh bulb of fennel, but are usually much stronger in the licorice flavor, as jpmcgrew mentioned. sort of like how coriander seeds taste a lot like cilantro leaves.
you could use a little of them to get the fennel-ish flavor, and i'd add blanched celery to approximate the texture of the bulb.
hope this helps.