A mammoth effort to uncover the biological relationships of the world's flora has also found that there was one common ancestor - a botanical Eve - to all green plants alive today.
In a further twist to botanical taxonomy, scientists have categorised fungi - traditionally grouped within the plant kingdom - as being closer to animals than to plants.
The findings are the result of work by 200 scientists from 12 countries, who for the last five years have reconstructed the evolutionary relationships between the Earth's entire flora using DNA technology.
Called the Green Plant Phylogeny Research Co-ordination Group, the scientists yesterday published what they have called the most complete "tree of life" for any group of living organisms.
One of the study's major findings is that the invasion of the land by plants more than 450 million years ago was led by aquatic plants emerging from freshwater and not, as again stated in biology textbooks, by marine plants emerging from the oceans.
Brent Mishler, a biology professor at the University of California, Berkeley, and a principal co-investigator on the project, said the new insights into how land plants evolved indicate one common ancestor for all the flowers, grasses, trees, ferns, shrubs and mosses of the world.
"Plants came out on to land probably many times, but only one lineage actually made it. This indicates there's an Eve - a common ancestor - in the primordial soup of green plants," Professor Mishler said.
"Until now, it was commonly believed that within the green plant group there were several lineages of land plants. For example, people believed that mosses were derived from a different aquatic ancestor than were flowers or ferns," he said.
The new tree of plant life was released yesterday at the 16th International Botanical Congress in St Louis, Missouri. It shows that the single plant kingdom of old now consists of four kingdoms - a kingdom of green plants, one each for the "red" and "brown" seawater plants of the oceans, and a fourth belonging to the fungi, which are now classified alongside animals.
The new insights into botanical relationships have emerged because of the ability of scientists to analyse vast amounts of DNA information quickly and accurately.
Genetic studies of flowering plants, for instance, have shown that they can no longer be simply divided into monocots (grasses) and dicots (flowers). For instance, some traditional dicots, such as magnolias and water lilies, are now classified as belonging to the same branch as the monocots.
"We can't really think of life on Earth in terms of only the animal kingdom and the plant kingdom anymore. In fact, there are five kingdoms of complex organisms on Earth today. One of the kingdoms, fungi, are more related to you than the tree on which they are growing," Professor Mishler said.
Increased understanding of plant life will allow scientists to better predict biological properties, which could be useful in discovering new substances such as drugs, he added.