Evolution of flowers. With a spotty, incomplete fossil record of the early flowers, much of the understanding of flower evolution is inferred from modern flowers. Taxonomists for a century have defined angiosperm families on floral structure and separated “primitive” from “advanced” features. In this assessment, early, primitive flower characteristics are: an undifferentiated perianth with sepals and petals alike and separate; an indefinite number of parts in each floral whorl; spiral attachment superior ovaries; radial symmetry; and so forth.
Early carpels were leaf‐like and seeds were borne on the edges. In advanced flowers, the carpel is folded inward and the seeds are enclosed. Closed carpels have differentiated stigmas, styles, and ovaries. The pollen does not land on the ovules directly.
Pollination. Flowers and their pollinators coevolved; that is, two or more species act as selective forces on one another and each undergoes evolutionary change. Early flowers probably were wind pollinated, but the selective advantages of cross‐fertilization by animal pollinators must have been a powerful selective evolutionary force from the very beginning.
Specializations to ensure cross‐fertilization and attract pollinators include: colors in wavelengths visible to the pollinators; nectaries placed so that access requires passage across pollen sacs; odors; structural changes such as long corolla tubes and spurs filled with nectar.
Dispersal. Concomitant with the changes to insure fertilization are those that insure dispersal of the products of fertilization, such as the seeds and fruits. Fruits can be dry or fleshy, remain closed or split open at maturity, have hooks or spines that attach to fur or feathers. Seeds can have hard coats, colors, wings, plumes, and all manner of other clever ways to move the new generation away from the old—which is the underlying point of the whole process. Dispersal not only permits colonization of new areas by a species, but also prevents competition for water and minerals between parent and offspring at the home site.
Secondary metabolites (products). Chemical compounds produced by plants are either: 1.) primary products found in all plant cells that are necessary for life, such as amino acids, or 2.) secondary products found in some cells that are important for the survival or propagation of the plants that produce them. When the secondary products were first discovered they were thought to be waste products that plants neither were able to utilize nor get rid of so they were stored out of the way in the vacuoles. With further research it became apparent that the materials were not simply wastes, but had a purpose—to ward off insect attacks, to stop herbivores from eating the plants, or as a response to bacterial and other pathogens.
The toxicity of many of the products is not confined to insect attackers; humans who consume the plants also are affected. Alkaloids produced as secondary metabolites include: cocaine, caffeine, morphine, nicotine, and atropine—a potent pharmacological arsenal. Terpenoids are another class among which are the hydrocarbons, which plants release from their leaves in prodigious amounts and which contribute to air pollution. Terpenoids form the haze that makes the Great Smoky Mountains “smoky.” Rubber is a terpenoid as are taxol and menthol; so are the carotenoids of the plastids and sterols of the cell membranes. Phenolics are imporant secondary metabolites whose plant roles are still being discovered. The evolution of secondary metabolites gave flowering plants a biochemical means to cope with the environment—and added still another improvement over their neighbors.
Phylogeny. There are more questions than answers in the phylogeny of the angiosperms. Part of the problem lies in the lack of an adequate fossil record. The first clearly angiosperm fossil is from the Early Cretaceous and is an impression of a fully developed flower. Molecular RNA/DNA sequencing currently is being applied in new phylogenetic (cladistic) analyses to answer the question of angiosperm origins. As yet, no generally accepted answer exists, but several hypothesis are being hotly debated. The molecular data indicate the seed plants most closely related to the angiosperms are the gnetophytes and bennettitaleans, which, incidentally, is the same conclusion reached by some botanists using morphological and anatomical features 50 years ago. Others at the time favored the “seed ferns” as angiosperm ancestors. A second debate revolves around the nature of the first angiosperms. Were they woody or herbaceous? There are no clear answers in that debate either. Cladists in general favor a woody origin, but there are equally vociferous advocates for the herb hypothesis.
Classification. The long‐held separation of the angiosperms into two groups on the basis of the number of cotyledons in their seeds—monocots (one) and dicots (two)—is an artificial classification now being abandoned in favor of one based on molecular data, which recognizes evolutionary relationships. The 235,000 species of angio‐sperms are separated into three groups:
- Eudicots: 165,000 species; two cotyledons, leaves with net venation, primary vascular bundles in a ring, vascular cambium with secondary growth, pollen with three pores; flower parts primarily in fours or fives or multiples of four or five.
- Monocots: 65,000 species; one cotyledon, leaves with parallel venation, primary bundles scattered, vascular cambium rare, pollen with one pore; flower parts in threes or multiples of three.
- Magnoliids: 5,000 species; primitive characters, pollen with one pore, cells with ether‐containing oils; two subgroups: woody magnoliids and paleoherbs; most have fused carpels.
The monocots are a monophyletic group with a common ancestor based on their single cotyledon and a few other features. So, too, are the eudicots with their triaperturate pollen. The magnoliids, however, have no uniting feature and their evolutionary relationships are still being worked out.