Life as we know it would not exist without plants to convert sunlight and inorganic compounds into food energy. In Kingdom Plantae, plant species are classified according to their method of reproduction.

One group is the "seed plants," which can be divided into two subgroups called angiosperms and gymnosperms.

Angiosperm derives from the Greek words for "vessel" and "seed." Angiosperms include vascular land plants and hardwood trees with flowers and fruit. They reproduce by making seeds that are enclosed in an ovary.

Gymnosperm derives from the Greek words for "naked seeds." Gymnosperms include vascular land plants and softwood trees that do not have flowers and fruit. They are cone-bearing and reproduce by making naked seeds on cone scales or leaves.

Plant life evolved millions of years ago from primitive algae in the sea. Nonvascular mosses, liverworts and hornworts then arrived on the scene. These types of living species reproduce by fragmentation or spores. Next came seedless vascular plants like ferns and horsetails.

Plants with a vascular system were stronger and able to grow taller. Gymnosperms, like conifers and ginko biloba, appeared during the Paleozoic Era and reproduced by dispersing “naked seeds” not imbedded in flowers or fruit.

Angiosperms evolved later during the Mesozoic Era. Angiosperms adapted to a challenging terrestrial ecosystem by developing a complex vascular system, flowers and fruit. They reproduced by seed and spread quickly on land.

Gymnosperms and angiosperms are more highly evolved than nonvascular plants. Both are vascular plants with vascular tissue that live on land and reproduce by making seeds.

They are also classified as eukaryotes, meaning they have a membrane-bound nucleus.

Only angiosperms are known as flowering plants. Many have beautiful petals, fragrant blossoms and fruit that contains dozens of seeds. Angiosperms typically drop their leaves when the seasons change and chlorophyll production ceases.

By contrast, gymnosperms such as pine trees produce bare, uncovered seeds, usually in pine cones. Most gymnosperms have green, needle-like leaf structures; angiosperm leaves are flat_._ Angiosperm leaves are seasonal in their life cycle while gymnosperms are generally evergreen.

The flowers of angiosperms have male and female reproductive parts. Stamens are male sex structures that make pollen on their anthers.

Pollination occurs when pollen grains from the anther reach the pistil, which is the flower’s female structure. A pollen tube in a structure called the style helps the generative cell in pollen reach the ovarian embryo sac.

The generative cell in pollen splits into two sperm cells. One fertilizes the egg, and the other one helps make endosperm through a process known as double fertilization. Fertilized eggs mature into seeds protected inside fruit.

Sporophytes in gymnosperms make male and female gametophytes. For instance, male cones have male gametophytes (pollen), and they are smaller than cones with female gametophytes.

Wind carries pollen from male to female cones. The fertilized female gametophyte produces a seed on a scale inside the cone.

Pollination methods of angiosperms differ somewhat from those of gymnosperms.

Angiosperms rely on bird, bees and other pollinators, as well as abiotic factors such as wind and water. Gymnosperms rely solely on the wind to carry pollen between male and female reproductive parts.

Unlike angiosperms, some species of gymnosperms have been around since the days of the dinosaur. For example, cycads (in the division known as Cycadophyta) look like palm trees, but they are actually close relatives of Coniferophyta (conifers) and Ginkgophyta (the division that contains Ginkgo biloba).

Gnetophyta, like the Welwitschia mirabilis desert plant, have existed for at least 145 million years based on fossil evidence. The Welwitschia can live up to 1,500 years. DNA shows that it is closely related to conifers and other gymnosperms, although the plant also has flower parts. It has been speculated that angiosperms may have evolved from gnetophytes.