Flowering plants, also called angiosperms, represent the most evolutionary advanced and successful plants on Earth. More than a quarter-million flowering plant species pervade all habitats found on Earth. The development of flowers signals the existence of other evolutionary adaptations that distinguish flowering plants from their nonflowering brethren and account for their overwhelming success.
To the person unfamiliar with botanical classification, identifying nonflowering plants can itself be a challenge. Myriad species from the grass in your lawn to the towering oak tree and many aquatic plants are considered flowering plants. Botanists classify nonflowering plants into three major categories: mosses, ferns and gymnosperms, the most familiar examples of which are conifer trees. These species represent a march through evolutionary progress that reached its height in the flowering plants.
Nonflowering Plant Reproduction
Before plants evolved flowers, they relied on mechanical means--and a good bit of chance--to carry spores or pollen. For example, ferns drop millions of spores in hopes that one will develop into prothallium, which produces reproductive cells that must be carried by water. Mosses reproduce similarly, relying on water to carry reproductive cells to other plants. With few exceptions, gymnosperms need wind to carry pollen to the exposed seeds on cones.
Flowering Plant Reproduction
One of the most artful adaptations afforded by the flower is the deliberate attraction of pollinators, helpful insects, birds and mammals that carry pollen from flower to flower. Flowers have developed shapes and colors that attract specific species, and the sweet nectar that they produce rewards pollinators. As pollinators delve into the flower to feed, pollen clings to their bodies and is carried from flower to flower, allowing for cross-pollination.
Ovary and Fruit
Nonflowering seed-producing plants are called gymnosperms, a term that translates to "naked seed" because the seeds are not protected by an ovary. In flowering plants, an ovary encloses the seeds, providing protection as they develop. When the seeds are fertilized, the ovary sometimes develops a fruit coat, and animals that consume the fruit and its seeds can aid in dispersal.
The seeds of flowering plants also develop differently than the seeds of nonflowering plants. Once fertilized, nonflowering seeds contain nutritive tissue grown from the maternal reproductive cell. In comparison, flowering plants utilize a process called double fertilization, where each pollen grain contains two sperm cells. One sperm cell fertilizes the egg cell while the other combines with a cell in the ovule to form endosperm, nutrient-rich seed contents that sustain the embryonic plant as it germinates. The endosperm contains more nutrients than you would find in gymnosperm seeds, making the endosperm more effective at sustaining the embryonic flowering plant.