Since Eupatoriums are members of the Composite Family, the parts that at first glance look like flowers are actually heads composed of many flowers. The head at the left contains about 50 flowers. In that picture you can see an important feature common to all Eupatoriums -- no petal-like ray flowers are present, only tubular disk flowers. The spreading hairlike items are slender stigmas, two stigma branches arising from inside each corolla. When a flower head is composed only of disk flowers, it is said to be discoid.
For the picture at the right I have removed the slender, green, scale-like involucral bracts along one side of the head so you can see how the disk flowers are stacked so closely together atop a little hill-like, greenish thing, the receptacle. In most other Eupatorium species the receptacle is lower, or even flat. Such a high, "conic" receptacle is a special distinguishing feature for Blue Mistflower. In the same picture notice that the cylindrical, purple-topped corollas arise atop an oval, cream-colored affair -- the future fruits, or achenes.
At the left you can see better how the achene attaches to the corolla. Here the achene is more mature so the pappus bristles atop the achene are better developed, flaring from atop the achene. When the corolla falls off and the mature achene is released, these bristles will serve as "parachutes," and help the achenes travel into new areas on the wind. The whole flower at the left is a little more than ¼-inch high (7 mm).
A plant very similar to Blue Mistflowers, known as Ageratum, is often planted around peoples' homes. Though the plant at first glance is very similar, when you look closer you see important differences. For instance, you can see at the left that mistflowers have very conspicuous pappuses, but Ageratums have none. Mistflower plants spread by creeping rhizomes, while Ageratums do not. Also, mistflowers are native American plants, but Ageratums are introduced from the tropics.