What Are Flowering Plants?

Flowering plants, known formally as angiosperms, are seed plants whose reproductive structures are organized into flowers and whose seeds develop enclosed within a fruit derived from the ovary. That enclosure is the defining character, and it separates them from gymnosperms such as conifers and cycads, whose seeds sit exposed on cone scales. With roughly 300,000 described species, angiosperms account for close to 90 percent of all land plant species and supply nearly all of the world's cultivated food crops, most timber outside the conifer belt, and a large share of the fiber and pharmaceutical raw material in use.

The group appears abruptly in the fossil record during the Cretaceous and diversified rapidly enough that Darwin called the pattern an abominable mystery. Modern phylogenetic work has resolved much of the branching order, placing Amborella and the water lilies near the base and grouping the bulk of the species into the eudicots and the monocots, the latter including the grasses, palms, orchids, and bamboos.

Floral Structure and Reproduction

A complete flower carries four whorls of modified leaves: sepals, petals, stamens bearing pollen-producing anthers, and one or more carpels enclosing the ovules. Fertilization in angiosperms is unusual in being double: one sperm nucleus fuses with the egg to form the embryo, and a second fuses with two polar nuclei to form the endosperm, the triploid tissue that nourishes the developing seed and that constitutes most of the edible mass of cereal grains. Floral form varies enormously across the group, from wind-pollinated grass florets stripped down to anthers and feathery stigmas to orchid flowers whose geometry admits a single insect species. A survey of flower diversity and angiosperm diversification treats the flower itself as the site of most of the evolutionary innovation in the group, since it is where the reliance on external mating vectors is resolved.

Pollination

Pollination is the transfer of pollen from anther to stigma, and because plants cannot move, it depends on animals, wind, or water. Insects, birds, and bats account for the majority of angiosperm pollination, and shifts between pollinator classes are repeatedly implicated in speciation, since a change in pollinator can isolate populations reproductively within a few generations. Analysis published in New Phytologist concluded that insect pollination has prevailed for most of angiosperm evolutionary history, with wind pollination arising later and repeatedly as a derived condition. The degree of specialization varies: work in AoB PLANTS on floral specialization and realized pollination accuracy shows that narrow floral morphology raises the precision of pollen placement while exposing the plant to risk if the partner species declines.

Growth Forms and Structural Diversity

Angiosperm body plans span herbs that complete a life cycle in weeks, succulents holding water in modified stems, aquatic species that flower underwater, parasitic forms that have lost photosynthesis entirely, and trees exceeding 100 meters. Vessel elements in the xylem, wider and shorter than the tracheids of gymnosperms, allow higher water transport rates and are part of the explanation for the group's ecological reach. Bamboos illustrate how far the monocot growth form can be pushed: they are woody grasses that grow from rhizomes at rates exceeding half a meter per day in some species, flower on multi-decade cycles, and yield culms with a strength-to-weight ratio that has made them a serious structural and composite material.

Applications

Flowering plants underpin activity across many fields, including:

• Agriculture and plant breeding, including cereal, legume, and fruit crop improvement
• Managed pollination services and pollinator health monitoring
• Pharmaceutical and natural product chemistry
• Timber, bamboo, and natural fiber construction materials
• Biofuel feedstock production from grasses and oilseeds
• Horticulture, landscape design, and urban greening
• Phytoremediation of contaminated soils