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Seed plants
Seed plants are one of the largest groups of plants on land. A seed contains an embryo and nutrients, which are
enclosed inside a protective covering called a seed coat. Development of a new plant begins with the seed. There are
two main subgroups of seed plants: gymnosperms and angiosperms.
Gymnosperms
The term gymnosperm comes from a Greek word meaning “naked seed.” Gymnosperms do not produce true flowers
or fruit, and therefore the seeds of gymnosperms are not enclosed in flowers or fruit. The seeds of most gymnosperms
develop on the surface of the scales of female cones. Gymnosperms are thought to be more “primitive” than the
angiosperms (flowering plants). They reproduce fairly slowly. More than a year may pass between pollination and
fertilization, and it can take seeds up to 3 years to mature. Also, most gymnosperms are wind-pollinated.
Gymnosperms are all woody plants (trees, shrubs, or vines) and grow throughout most of the world, from 72 о north
latitude to 55о south latitude. They are dominant in many colder and arctic regions.
Gymnosperms are some of the tallest, most massive, and longest-lived plants in the world. Humans use several species
as ornamentals and sources of high-quality wood.
The gymnosperms consist of four major, related groups: conifers, cycads, ginkgos, and gnetales. Together, they
represent about 15 families, 75-80 genera, and about 820 species.
The conifers are the largest and most economically important group of gymnosperms. The
conifers include such plants as the pines, spruces, yews, hemlocks, firs, junipers, redwoods,
and many others. The plants in this group are called conifers because most of them bear
their seeds in specialized structures called cones (strobili). Cones protect ovules and seeds
and aid pollination and dispersal. Conifers produce two types of strobili: male or pollen
cones (the male strobilus) and female or seed cones (the ovulate strobilus).The male pollen
grains develop in small cones. The female cells develop in the ovules in large cones.
Conifers’ leaves are called needles.
The cycads are widely distributed through the tropical and subtropical regions.
They are characterized by a large crown of large pinnately compound leaves.
They are frequently confused with and mistaken for palms or ferns, but are only
distantly related to both. Cycads are woody, long-lived, unisexual plants. All
species have coralloid roots, which support symbiotic cyanobacteria that can fix
atmospheric nitrogen. The cycads and ginkgo are unique among seed plants in
having motile sperm; this is often taken as evidence of their evolutionary
primitiveness.
The ginkgos are represented by only a single living species Ginkgo biloba. It is a tree,
sometimes attaining large size, native to China but widely planted around the world. It
is undoubtedly one of the most distinct of all deciduous trees.
The bright green fan shaped leaves are unique among all
trees and turn a golden yellow in the fall. Ginkgo is often
referred to as a "living fossil" because nearly identical plants
are known from fossils nearly 200 million years old. The
fossil record shows that they were formerly a widespread,
abundant, and diverse group.
The gnetales are one of the most peculiar plant groups. They include three highly distinct groups totaling 68 species.
One group, the genus Ephedra, is composed of shrubs native to deserts and
semiarid areas. The second group, the genus Gnetum, is composed of climbing
vines (and one tree species) native to tropical rainforests. The third group contains
a single species, Welwitschia mirabilis. It lives in the desert of Southwest Africa,
produces two leaves that grow throughout the life of plant, and lives about two
thousand years.