The Incompleat Naturalist: Basal Angiosperms I

Angiosperms, the flowering plants, appeared in the fossil record during the age of dinosaurs. Dinosaurs and flowering plants coevolved for tens of millions of years. When the dinosaurs were wiped out (with the exception of birds) 66 million years ago, flowering plants survived. There are more than 300,000 described species of angiosperms and they have a dizzying array of sizes, forms, structures and reproductive systems.

The standard story, taught for two centuries, is there are two kinds of angiosperms, monocots and dicots. This story says monocots have flower parts in multiples of three, parallel veins in their leaves and one cotyledon (the mono cot), that stores and transports nutrients to the plant embryo. Their counterparts, the dicots, have flower parts in multiples of four or five, net veins in their leaves and two cotyledons. There are numerous other differences between these groups but in the field these three characteristics can usually distinguish monocots from dicots.

Leaves of pawpaw (Asimina triloba) a basal angiosperm

Botanists knew about some weird flowering plants that did not quite follow those simple rules but new evidence has forced a major change in the classification of angiosperms. Monocots are still monocots but evolutionary plant biologists have blown up the dicots, largely based on DNA evidence. This new classification still has standard dicots but they are now called eudicots, the true dicots. Another group of flowering plants show characteristics of both monocots and eudicots and are classified as basal angiosperms. Basal angiosperms do have many characteristics of eudicots like the above-mentioned two cotyledons and net veins, but many have flower parts in multiples of three and their pollen structure is more similar to that of monocots.

Basal angiosperms branched off early from the main flowering plant evolutionary line. These plants have characteristics in common with some of the first flowering plants. I like to image herbivorous dinosaurs munching on basal angiosperms. Here are some of the primitive attributes shown by the basal angiosperms; flowers often have parts in threes or multiples of threes (a monocot-like trait), flowers show little differentiation between petals and sepals, they make numerous pollen producing stamens and numerous egg bearing carpels.

You might think these primitive flowering plants, these living fossils are rare, but they are not. It is true they make up only about 3% of flowering plants. It is also true that one order of basal angiosperms contains only one species and grows only on the Pacific island of New Caledonia. But other basal angiosperms are common, well known and produce striking flowers.

Florida anise, Illicium floridanum, is a basal angiosperm found in westernmost Florida and adjacent areas of Alabama, Mississippi and Louisiana. This small trees have bright green, lanceolate leaves. When crushed, the leaves smell of licorice or according to one source, freshly caught fish. In spring Florida anise bears striking maroon flowers. The flowers typically have flower parts in multiples of threes with an unruly batch of petals, a dense ring of purple stamens and in the center another ring of carpels. Florida anise like many deep purple flowers is fly pollinated.

The purple flower of Florida anise ( Illicium floridanum)

Pawpaw (Asimina triloba) is a basal angiosperm found throughout eastern North America. This small tree grows in the forest under-story, along watercourses and in North Carolina flowers in early spring. The flower is purplish-brown with three sepals and six petals. The flower produces a disagreeable odor that attracts carrion feeding insects like flies and beetles. Pawpaw has another interesting insect interaction. It serves as the host plant of the zebra swallowtail butterfly, Protographium marcellus. Female zebra swallowtails lay their eggs on leaves or branches of pawpaw; the caterpillars eat the leaves and retain toxic plant compounds when they metamorphose into the butterfly stage. This gives the butterfly some protection from predators.