The angiosperms burst upon the fossil record long ago (in human terms) with tremendous diversity already in place. Some of that fact rests with their site of evolution: places with lots of turnover such as tropical rain forests (where no dead plant parts remain unrecycled to become fossilized) and/or places with no sedimentation possible for fossilization (mountain slopes, etc.). Regardless of how they got here, controversies rage between what constitutes plesiomorphic and apomorphic among the angiosperms. Here we will take the Cronquist perspective.
| Plesiomorphic | Apomorphic |
|---|---|
| elongate axis | short axis |
| spiral | whorled |
| radial symmetry | bilateral |
| lots of parts | reduced numbers |
| free parts | fused parts |
| perianth large | perianth missing |
| beetle pollination | wind or bee pollination |
| bisexual | unisexual |
| hypogynous | epigynous |
| superior ovary | inferior ovary |
| single flowers | inflorescences |
| all flowers alike | division of labor |
In looking at this diagram, please observe the trends from plesiomorphic to apomorphic from the table above. The flower formulae given here show you these trends! The diagram below shows an arrangement of families that indicates patterns from more plesiomorphic (at bottom) to more apomorphic (at top). The goal of the diagram is to help you understand the table above...please do not memorize floral formulae...focus on the trends.
Finally, and this is important, please remember that, while a family may posess many plesiomorphic character states (and therefore be near the bottom of the diagram), it is not the ancestor of a family that is nearer the top of the diagram. Families near the top of this diagram merely have a greater preponderance of apomorphic characters...that's all! In human terms, we did not evolve from modern chimpanzees...rather, chimps and humans share a common ancestor.
In class, various examples of flowers are shown from each family to point out some basic characteristics of plant families. The goal of that was to expose you to some of the diversity in angiosperms and to reinforce the trends from primitive to advanced. Again, please do NOT get confused about this: The Asteraceae did not evolve from the existing Apiaceae...they probably share a common ancestor...both have diverged from that common ancestor.
The most plesiomorphic flower was something approaching a Magnolia flower. The common ancestor for all flowering plants probably had an elongate floral axis with many parts in each of the four whorls. The parts were probably large and more leaf-like than those of modern flowers. The filaments were probably more blade-like. The ovary was likely superior and the flower hypogynous. The flower was probably pollinated by beetles and provided much excess pollen and nectar to satisfy the animal. From some common ancestor like this evolved all modern floral types.
Key elements to take home with you from this portion of the course include that plants appear to be quite diverse in terms of floral structure. This makes floral structure an important area for studying meaningful character state changes. Second, this study shows four major lines of evolution among flowering plants. Of these two are monocot and two are dicot...
One line of dicot evolution culminates in the Lamiaceae (the mint family) and the other terminates in the Asteraceae (the daisy family). These two lines appear to have gone through a reduction in flower size, a reorientation through zygomorphy, and a division of labor in inflorescences to minimize display costs.
One line of monocot evolution has evolved in a parallel manner, floral reduction into a tight inflorescence, and display costs have gone to zero by moving to wind pollination (the Poaceae). The other line of monocot evolution has moved into zygomorphy in a big way and very specialized pollination mechanisms with bilateral symmetry, but it has not resulted in diminution of the individual flower (Orchidaceae).
This page © Ross E. Koning 1994.
The MLA citation style for this page would be:
Koning, Ross E. "Floral Evolution". Plant Physiology Website. 1994. http://koning.ecsu.ctstateu.edu/plant_biology/angioevolve.html (your visit date).
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