Numbers of Things

Roughly speaking, evolution in the dicots has gone from many (no specific number) flower parts to five to some specific number fewer than five.  The flower parts in question are sepals (calyx lobes), petals (corolla lobes), stamens (and fertile anthers), carpels, and locules.  "Primitive" plants have many flower parts, while "advanced" plants have few flower parts.

For instance, plants of the advanced gesneriad family have (usually) five calyx lobes, five corolla lobes, two or four stamens, five carpels, and one locule.

These words primitive and advanced are terribly misleading, as if the Magnolia grandiflora in my back yard was descended from plants which stopped evolving a hundred million years ago.  I have to tell you, that primitive magnolia tree would clobber any number of really really advanced sinningias in a fair fight.  It's got plenty of tricks that lead it to victory over plants even more sophisticated than sinningias, such as grasses.

Instead of primitive and advanced, I much prefer the terms old and derived.  Sinningias are derived dicots, but by almost any measure (number of plants, total biomass, impact on the environment) the old genus Magnolia is way ahead of Sinningia (or any other gesneriad genus).  Magnolia leaves would cover and eventually kill the sinningias if I didn't keep removing them.  The sinningias are no threat at all to the magnolia.

The equation of "many flower parts" with primitive is also hazardous.  The daisy, a member of the advanced Asteraceae, has what appears to be lots of flower parts.  The fact that each petal belongs, in the strict botanical sense, to a separate flower doesn't change the fact that the members of the Asteraceae have evolved their way back to the good old multi-part reproductive unit.