Ever since recognizable ancestors of today's plant families first appeared in North America at the end of the Cretaceous period some 65 million years ago, the floristic face of the continent has been constantly shaped and reshaped, first by grand forces of climate and then by humans.
By 2 million years ago, today's more varied botanical pattern of cold-weather coniferous forests, sunny grasslands, dark deciduous woods, swamps, marshes, beaches and deserts was more or less in place -- only to be transformed within a 500-year blink of the eye by a wave of exotic plants that followed human migrants from other parts of the world.
This continually shifting and evolving tableau has remained so volatile, and so full of constant surprises, that botanists have never yet been able to pin it down long enough to achieve a long-sought goal: describing and cataloging the estimated 20,000 wild plant species of North America on a comprehensive scale.
One problem is that much of the floristic world of North America is still unexplored. Nearly 50 new species a year were discovered from 1975 to 1990, for instance, and the rate of discovery in earlier decades was so high that scientists trying to corral them in one compendium of knowledge simply became overwhelmed.
But lately they have persevered, and now the elusive goal is being achieved at last.
In a major event for the world of botany, the first two volumes of an ambitious 14-volume work called "Flora of North America North of Mexico" are to be published on Friday by Oxford University Press, with the other volumes to follow over the next 12 years. Coordinated by the Missouri Botanical Garden in St. Louis, the work is being carried out by hundreds of botanists at 30 institutions in the United States and Canada.
Not least among the revelations of the introductory volume, which sets the context for those to come, is the extent to which plant species that evolved separately in other parts of the world are now indelibly established in the North American wild. They comprise one-fifth to one-third of all plant species.
Volume Two is the first installment in the more detailed work of describing and classifying the plants themselves. It covers the continent's ferns and gymnosperms, the group of plants that includes conifers. The other volumes will cover all the remaining types of plants.
For each plant, the "Flora" will provide the scientific and common names as well as illustrations, key identifying features, distribution maps, descriptions of the plant's family relationships, summaries of its habitat and geographic ranges, chromosomal data, climatic factors affecting the species' ecology and other information pertinent to the plant.
The project, which got under way in 1983, is costing about $1 million a year in grants from the National Science Foundation, private foundations and contributions of staff members' time by the participating institutions.