Reply #12: in this context, the term composite means...

[mike_c]

..."belonging to the aster family," which used to be called the Compositae. Each "flower" is more properly an inflorescence-- a large structure made of many tiny individual flowers. They're composites because there are two different kinds of flower in each inflorescence-- those with showy petals and those without. Many of the ones around the edge have a showy yellow petal, while the ones in the center all have greatly reduced or absent petals-- each of those little brown pin-cushiony bits is an individual, separate, tiny flower.

Flowers form at the ends of short stalks called recepticles. These are often not visible, but they're there nonetheless. A recepticle with it's flower petals and sepals forms at the end of a stem, either a major terminal stem or a smaller branch stem. A composite inflorescence-- like this one-- is a big cluster of these. In fact, each individual flower is a modified stem end, and the petals and sepals are modified leaves. Flowers form from tissues that would otherwise give rise to longer stems and more leaves in response to internal hormonal signals-- this is easily demonstrated by either adding the hormones artificially, thus inducing flower formation early, or by blocking the hormone, which causes stem and leaf production to continue instead of flower formation.

(On edit-- I feel compelled to interject that I've glossed over several important bits here, including the long-standing mystery surrounding the exact nature to the flower inducing hormone(s). it might also be worth reminding you that all normal cells have a complete copy of the genetic information-- all the genes for forming flower parts, and stems, and roots, and so on. Which ones get "turned on" in any individual cell is part of the developmental process.)

The process of determining whether a given newly formed cell at the growing tip of the stem will become a stem cell, or a leaf, or will be modified into part of a flower, is under control of plant hormones, also called growth regulators (which are themselves ultimately under genetic control). This is called "cell determination." At the time a new cell forms at the growing tip, it is like a stem cell in animals-- it can become any kind of tissue, but what kind it becomes is controlled by the hormones that it experiences during that early development. Once determined, it "becomes" either a mature stem cell, or a leaf cell, or several other types of cell, including various different flower cells. This is called "cell differentiation," and it's usually followed by cell growth (but not always).

So what's happening in this plant is that the hormonal signal is getting scrambled, and too many new cells are getting their "flowerness" or "leafness" turned on, and in inappropriate parts of the plant body. Once determined, they differentiate normally, producing characteristic structures-- leaves, flowers, etc-- but in places where those specific structures would not normally occur.

OK, I'll quit now. Remember, you asked.... :-)