Basically . . . complicated!
In The Basics of Blooms, I introduced you to the essential anatomy of a simple flower and the terminology used to describe the parts. Even in a simple flower, a great variety of diverse forms occur in nature.
Take your simple flower petals, for example. Flower petals don't have to be separate; if they are curved inward, joined or fused, you will have a bell-shaped flower and these can come in almost any color. What would have been the petal tips can be rounded, jagged or pointed, but they are now called corolla lobes instead of petal tips. Your floral bell can be single and nodding, or arranged linearly on a stem and nodding, as in Lily-of-the-Valley. When flowers are arranged linearly on a stem, each flower can have a stem or they can each be stemless. If they are stemless, the flowers are said to be sessile, and the whole structure, or inflorescence, is called a spike. If each of the flowers sits on a little stem, the little stems are called pedicels, and the inflorescence is called a raceme.
But not all flowers are arranged linearly. Some are grouped in a globular arrangement, as in clover blooms, and some look like a rounded disk from the top, like the Cicuta maculata (spotted water hemlock) inflorescence shown in the thumbnail picture above, right. This type of inflorescence is known as a umbel, with the oldest flowers being closest to the perimeter and the youngest in the middle. Actually, this particular umbel is even more complex, as it is an umbel consisting of smaller umbels. This make it a compound umbel! The plant family Umbelliferae is noted for having plants with umbels. Carrots, parsley, celery and fennel are familiar plants in this family.
When a flower is . . . not a flower!
Many flowers we think of as "flowers" are not basic flowers at all, but inflorescences of a special kind. Plenty of plants in our gardens besides the Sunflower belong to this very large family. These plants are characterized by having composite inflorescences. The practically universal icon of this type of inflorescence is the Sunflower (Helianthus annuus), the State Flower of Kansas, USA (see photo at left). Other "flowers" like this are daisies, zinnias, marigolds and chrysanthemums. In the Sunflower, the two types of actual flowers, or florets, are easily seen. The "petals" are each actually flat, corolla-like appendages of the ray flowers, while the center of the blossom is composed of florets known as disk flowers.
Of course, we can't stop with a bloom as complex as the composite, as some plants in this family have inflorescences composed of inflorescences. For example, remember the raceme I mentioned earlier in this article? Just replace each of the simple flowers with one of these composite flowers, and you have something like what you see when the Hawaiian Silver Sword plant blooms.
What if your raceme is branched? Such a raceme is called a panicle, and if you really want to panic(le), put a composite blossom at the end of each little branch instead of a simple flower. Now, that is one complicated inflorescence! Such a structure is produced by the white panicle aster, Symphyotrichum lanceolatum.
From the sublime to the bizarre
I'll conclude this introduction to inflorescences by sharing one of my personal favorites, the type characteristic of the Araceae, or arum family (see photo to right). These inflorescences consist of a leaflike spathe that either partially or completely surrounds what is often a rod or bat-like structure called a spadix On the spadix are found the actual flowers, which are small like the disk flowers in a composite blossom. Here, however, the female blooms (or pistillate blooms) are often grouped separately from the male blooms (or staminate blooms). If the flowers are complete (that is, having both male and female parts), the pistils are receptive first, and then the anthers release the pollen later after the pistils are no longer receptive.
To close, here are a couple of important terms that apply to inflorescences; determinate and indeterminate. A determinate inflorescence has a fixed number of flowers on it and the flowers begin opening with the topmost ones first, as in Echinops. The indeterminate one keeps growing for a while with no predetermined end while the first flowers to open are the oldest ones The best example I know of for the indeterminate type is the banana (Musa) inflorescence.
Robert H. Mohlenbrock @ USDA-NRCS PLANTS Database / USDA SCS. 1989. Midwest wetland flora: Field office illustrated guide to plant species. Midwest National Technical Center, Lincoln.
Public Domain Clip Art
LariAnn Garner, Aroidia Research