Evolution of Flowering Plants (Angiosperms)
Flowering plants are classed as Angiosperms. Angio comes from the Greek for “vessel”. So an Angiosperm has its seeds in a vessel, in this case an ovary, which is part of the fruit. Angiosperms bear flowers and fruits, not cones. Pollen is transferred by wind, water and animals such as insects, birds and bats. Seeds may be dispersed in fruit, carried by animals or by wind.
Conifers are classed as Gymnosperms because they have “naked seeds” which are not surrounded by an ovary. Gymno comes from the Greek for “naked”. They bear pollen cones and seed cones, not flowers. Pollen is transferred by wind. The seeds are dispersed by wind or water but in some cases animals.
In evolutionary history the Ginkgo, the most “primitive” Gymnosperm, may have evolved 250 million years ago. Conifers then became the most dominant Gymnosperm until the Mesozoic era ended 65 million years ago. Angiosperms first evolved about 140 million years ago and then underwent an explosive diversification so that 65 million years ago they became the dominant plant form. There are now maybe 350,000 Angiosperm species compared with only 1000 Gymnosperms.
The evolution of the flower was, therefore, the key step in the spread of flowering plants across the world. Pollen transfer by animals as well as wind led to an acceleration of evolution in an ‘arms race’ in which insects co-evolved rapidly with flowers. Flowers became more complex to attract specific pollinators. Fruit dispersal by animals as well as wind led to further flower evolution.