Genera in northern Utah (genera
with native species will be listed in green)
The Asteraceae, or Composite family, is one of the easiest to recognize. Look for the clusters of flowers all sitting directly on the receptacle or, to put it more formally, look for involucrate heads. The inflorescence of the Asteraceae is so distinctive that the family was recognized as a distinct group early on and given a name, Compositae, that was so universally recognized that when the structure of family names was first formalized, it was agreed that Compositae would continue to be an accepted name. I prefer it, as does Welsh who uses it in A Utah Flora, but Shaw and Cronquist (lead author of the Intermountain Flora) prefer Asteraceae so, reluctantly, I go along with them.
The Asteraceae is one of the four largest families, having about 1100 genera and 25,000 species. It occurs throughout the world, but has its greatest diversity in the semi-arid tropics. It is not abundant in tropical rain forests. Most of its members are shrubs or semi-shrubs, but most of our species are herbaceous. There are a few trees in the family, but not many; there are even fewer epiphytes and true aquatics in the family. It is an economically important family, but more for weeds and garden plants than as a source of food. Infamous weeds in the family include dandelions and thistles; popular garden plants include dahlias, zinnias, chrysanthemums, asters, cosmos, and daisies. The best known food product in this part of the world is lettuce; artichokes, both green and Jerusalem, are also members of this family.
As mentioned earlier, the most distinctive feature of the Asteraceae is the inflorescence. This consists of several small flowers, termed florets, that are crowded together and sessile on a receptacle. The uninitiated and the occasional sleepy botanist often refer to this inflorescence as a flower and it can look like a single flower, but it is made up of few to many flowers. The technical term for this kind of inflorescence is a capitulum. The flower cluster is surrounded by bracts; bracts that surround a group of flowers are called involucral bracts, and the structure they form is called an involucre. Some members of the Asteraceae have only one row of involucral bracts; others have several rows. The arrangement and texture of the bracts is often important in identifying the genus.
In some species, some or all of the florets are subtended by their own bracts. These bracts are attached to the receptacle, hence they are called receptacular bracts. They are generally not visible unless you pull apart the capitulum. Species that do not have receptacular bracts are said to have a naked receptacle. If you have eaten artichokes, the green kind, the receptacular bracts are the tiny, spiny scales around the hairs at the base of the interior of the artichoke. The outer green leaves are the involucral bracts. After reading through the next two paragraphs, you ought to be able to tell what the hairs are.
In most members of the Asteraceae, the central florets have a radially symmetric tubular corolla with five short lobes. These florets are called disk florets because they form the central disk of the capitulum in typical daisies. The disk florets are usually surrounded by an outer ring of ray florets. These have bilaterally symmetric tubular corollas. Ray florets are usually pistillate or sterile, and have three or fewer teeth on the extended portion of the corolla (i.e., the ray). The best known weedy composite, the common dandelion, has a third kind of floret, a ligulate floret. These resemble ray florets in having a bilaterally symmetric corolla, but ligulate florets are bisexual and have five apical teeth at the end of the strap. Many beginners' texts, including Shaw's Vascular Plants of Northern Utah, use the term "ray florets" to cover both ray and disk florets. Thistles have yet another kind of floret, bilabiate florets. Bilabiate florets, like ray and disk florets, have a bilaterally symmetric tubular corolla, but they are not so extremely bilaterally symmetric, and have 34 lobes on the lower (long) lip and 1-2 lobes on the upper (shorter) lip. Like ligulate florets, bilabiate florets are usually bisexual. Although some of these distinctions may seem rather picky when you are first learning to identify members of the Asteraceae, it is important to realize that bilabiate, ray, and ligulate florets are not identical when one is trying to understand relationships within the family, or place a plant that one has never seen before. One way to help keep them straight is to redraw them in order from most primitive to most advanced.
Apart from their differences in corolla shape and sexuality, the four types of floret are very similar. The ovary is inferior and encloses a single seed. The calyx is so highly modified it is given a different name, the pappus. It may consist of capillary hairs (i.e., fine hairs) that may be plumose (i.e., have yet finer hairs on them, like down), bristles, awns, or hyaline scales. Or, it may be completely absent. Think for a moment about dandelions that have gone to seed and you will recognize the function served by the pappus in most Asteraceae: seed dispersal. The "parachute" of a dandelion consists of the capillary hairs that form the pappus. The "ropes" connecting the parachute to the portion of the ovary with the seed is an extension of the upper part of the ovary.
The androecium of Asteraceae comprises five stamens. In almost all species, the anthers are fused together, forming a ring around the style. This tube of anthers is an important part the pollen presentation mechanism of composites. The anthers (and pollen) mature before the stigmas and shed their pollen into the cylinder formed by the anthers. There is only one style, but it ends in two style branches which are stigmatic only on their inner, facing surfaces. At the time when the anthers are shedding their pollen, the style is short and its two branches are pressed together. As the style elongates, it grows up through the ring of anthers, with the branches still pressed together. The pollen is then pushed out of the anther cylinder by the tops of the style branches, which often have short hairs on their outer surfaces. This manes the pollen available to any passing insect but, because the style branches are still appressed to each other, none will land on the floret's own stigmas. Once the style has elongated to the point where the branches are completely above the anther ring, the style branches bend back, exposing the stigmatic surfaces but, by then, most of the floret's own pollen will have been dispersed. In other words, this pollen presentation mechanism is a means of preventing self-pollination. Why is this advantageous?
When dealing with large families, it is often helpful to become familiar with some of the larger subgroups such as subfamilies or tribes. There is a lot of discussion going on nowadays about the tribal limits. Part of the problem involves disagreement about the homologies between different structures, part the relative importance of different kinds of data. Some tribes, however, seem to be fairly solid, i.e., are supported by all or almost 'all synantherologists (i.e., people that work on the Asteraceae; what is the origin of the word?). Those that are well-represented in our flora are the Lactuceae, Cardueae, Senecioneae, Anthemideae, Inuleae, and Astereae. These are characterized briefly below for those who are interested. Remember, we also have representatives of some other tribes.
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