Today we have a type of inflorescence (in the style of the recent post about racemes) and a whole plant family though, inevitably, it is going to get more confusing than it, at first, seems.
So here is the umbel. An umbel is an inflorescence (arrangement of flowers) that has flowers on single stems (pedicels) all arising from a common point. In some cases the pedicels are all the same length but, more commonly, they are of different lengths so the head is flat, with longer pedicels around the edge. A characteristic of the family is that the tiny flowers (which are usually white, yellow or more rarely pink) have five, often tiny petals, and the resultant fruits (mericarps) are held in pairs that split apart at maturity (schizocarps). They are often flat, as in parsnips. A characteristic of the family is that plants contain oils that may cause allergic reaction and celery and celery seeds are a common allergen. High oil content of the seeds may be one reason why the seeds famously have very short storage lives, the oils becoming rancid, which is why old seeds of parsley, parsnips and carrots germinate poorly.
Many commonly cultivated edible plants are in this family, including carrots, parsnips, celery, celeriac, chervil, parsley (below), dill, fennel and coriander. Parsley is a biennial and flowers in the second season.
Many ornamentals are umbels too, though bracts may cause some confusion (astrantia below and eryngium top) and often the umbels are compound, meaning that there are umbels of umbels, such as in common cow parsley.
The name umbel is derived from the Italian (or Latin) for shade umbra meaning shade and an umbrella (circa 1600) is a contraption that gives shade in warm climates, though here we use them more to keep dry. The spokes of an umbrella do have the floral ‘umbel’ structure.
So far so good and the family Umbelliferae is a good one and it is easy to recognise its members, even though it is closely related to the Araliaceae (ivy family).
But there is a problem because a family has to have a Type Genus, after which it is named. It is often the first named or the most important. So the type genus of Rosaceae is rosa and of Primulaceae it is primula. But there is no genus called umbel so Umbelliferae does not follow the rules.
It is for this reason that the daisy family Compositae no longer exists (though we still call them composites) and is now Asteraceae, after the genus aster – even though many of the perennial North American autumn asters themselves have been changed to symphyotrichum!
So Umbelliferae are now Apiaceae, named after the genus Apium – celery. Longwinded but we got there eventually!
By the way, apium is the Latin word for celery and the botanical name for parsley is petroselinum is from the Greek selinon meaning celery and petros meaning rock – some logic there since parsley grows in much drier places than celery which is a notoriously heavy drinker.