There are more species of flowering plants on Earth than any other type of plant, a fact that demonstrates flowering plants' reproductive success when compared to other types of plants. According to botanists Pam Soltis, Doug Soltis and Christine Edwards, there are well over 25 million flowering plant species, making them the most diverse and successful plants on Earth.
Plants originated millions of years ago as aquatic organisms. Suspended in water, they had a constant supply of water, nutrients and a means to move reproductive cells from one organism to another. As plants moved to land, their primary challenge has been to meet the same needs in a terrestrial environment. As the most recently evolved plants, flowering plants have succeeded by best meeting the challenges of a land-based existence.
Mosses are the most primitive land plants in existence today. Tiny and without roots or vascular systems, they need to be constantly near a source of water and nutrients to survive. Plant reproduction also relies on water to carry sex cells to each other. One of the earliest developments that led to the flowering plants' success was the evolution of a vascular system that could carry nutrients and photosynthetic products from a root or leaf to a location dozens--even hundreds--of feet away. Flowering plants have the most advanced vascular systems of any plant.
Seeds and Flowers
Early vascular plants, the ferns, still reproduced with spores, requiring water to carry sex cells to each other and ensuring success by producing millions of spores with the hope that one or two would survive to adulthood. According to Gerhard Leubner of the University of Freiburg, development of the seed "is one of the most dramatic innovations during land plant evolution" and largely to credit for the success of flowering plants today. However, some non-flowering plants also produce seeds yet have not had the same reproductive success as flowering plants. Flowers protect seeds inside of an ovary rather than leaving them exposed and subject to damage. Flowering plant seeds also contain an extra substance called endosperm that provides for the nutritional needs of the seedling, giving it extra time to become established compared to non-flowering plants.
According to Stephen P. Broken of Yale University, flowering plant success is primarily attributed to their co-evolution with animals. Spore-producing plants rely on water for reproduction, while non-flowering seed plants need wind to carry pollen to a seed. Flowering plants evolved mutually beneficial relationships with pollinating animals, providing nectar and pollen as a food source in exchange for the animal transferring pollen from plant to plant. The brightly colored and sweetly scented flowers borne by flowering plants signal possible pollinators that food sources can be found within the flower.
The development of vascular systems lets plants reach skyward, and the evolution of seeds allowed them to survive in habitats where water was scarce. These developments occurred before the first appearance of flowering plants, but flowering plants perfected the vascular system and the seed. The first appearance of flowers guaranteed their success, removing chance from the reproductive equation and replacing it with the deliberate attraction of pollinators.