Seed plants possess several characteristics that have allowed them to dominate
the land. These include seeds, which protect the embryo until environmental
conditions are favorable for germination and growth, and pollen grains, which
transport sperm cells from plant to plant without the need for water. In addition,
seed plants have roots, which absorb water, and cuticles, which help maintain
the water inside their bodies.
Pollen arose more than 150 million years after the origin of seeds. Seed plants
diverged and flourished throughout the Carboniferous period as seed ferns; diversification
of ancestral pines and cycads followed. The Jurassic period was dominated by
cycads, cycadeoids, and primitive pines. This period is also called the Age
Most of the features used for classifying living gymnosperms involve reproductive
structures. Most gymnosperms bear seeds produced in cones. Pollination requires
that pollen be transferred from microstrobili to ovules. Ovules are generally
exposed (i.e., naked) or temporarily enclosed by sporophylls or other branches
from the main axis of the strobilus.
The Pinophyta, Cycadophyta, Ginkgophyta, and Gnetophyta are four divisions of
gymnosperms with living representatives. Pines are abundant in northern coniferous
forests and are planted throughout the Southern Hemisphere. Bristlecone pines
are long-lived. Other conifers dominate similar habitats in the Southern Hemisphere.
Pine leaves form on short shoots called fascicles, which usually consist of
two to five needles with deciduous, nonphotosynthetic, scalelike leaves at the
base. The fascicles themselves are intermittently deciduous, usually lasting
for less than 5 years.
Cycads are slow-growing gymnosperms of warmer climates. Their reproduction is
similar to that of the pines, except that their sperm are motile by means of
many flagella and their megastrobili can be massive. All cycads are dioecious.
There is only one living species of Ginkgo, a tree with fan-shaped, dichotomously
veined leaves. Ginkgo is dioecious and produces seeds that stink.
There are three distinct genera of gnetophytes. Ephedra species are monoecious
or dioecious and native to northern drier areas. These plants are mostly shrubby
and otherwise resemble horsetails in having whorled branches and small, essentially
functionless leaves. Their cones consist of paired bracts, some of which contain
microsporangia. Their life cycle is similar to that of conifers. Gnetum species
are tropical vines or trees with broad leaves; their reproduction resembles
that of Ephedra. Welwitschia is a bizarre, dioecious plant of southwest African
deserts. It produces two large, strap-shaped leaves with basal meristems. The
leaves arise from a concave, bark-encrusted, trunkless stem from which a taproot
extends into the ground.
Gymnosperms, especially pines and their relatives, are an important source of
lumber, wood pulp, and resin. Resin is used to make turpentine and rosin. Wood
pulp is used as raw material for making paper.
Angiosperms, which are the dominant plants on earth, have a flower that includes
seeds in a carpel. The main force behind the rapid evolutionary radiation of
angiosperms may have been pollination by insects and the availability of habitats
left open by the disappearance of many gymnosperms. The first flowers were probably
pollinated by beetles; later angiosperms attracted butterflies and bees.
Another hypothesis that explains the rapid evolution of angiosperms involves
the influence of dinosaurs. Large, high-browsing dinosaurs mysteriously gave
way to smaller, low-browsing dinosaurs by the end of the Mesozoic era. The low-browsing
dinosaurs probably devastated gymnosperms by eating young seedlings, which allowed
low-growing, herbaceous angiosperms to survive and diversify.
Flowering plants provide us with food, clothing, shelter, and medicine. Cereals
such as corn, wheat, and rice are members of the grass family, whose edible
grains can be stored for long periods. Modern wheats contain two, four, or six
sets of chromosomes, and they probably evolved from a natural cross between
ancient einkorn wheat (a diploid) and a wild grass (also a diploid), followed
by chromosome doubling in some germ cells thousands of years ago. This eventually
yielded tetraploid emmer wheat. Emmer wheat also crossed with a wild grass to
produce the first hexaploid wheats, which today are used to make bread and other
Corn probably evolved from an ancient grass relative, teosinte. Both corn and
teosinte have 20 chromosomes, they sometimes grow in the same fields, and they
can crossbreed. About 7,500 years ago, an environmental stress may have selected
teosinte plants with large kernels, and early farmers may then have cultivated
these individual plants. Breeding experiments led to the development of hybrid
corn, which results from crosses between separate inbred lines to yield bountiful
Rice is perhaps the most widely consumed modern cereal crop, and fossils of
the plants date back some 130 million years. Today, rice grows in a wide range
of environments. In 1961, a rice seed bank was started in the Philippines to
preserve different varieties and prevent reliance on a few types. Seed and pollen
banks have since been founded for other valuable plant species.
Although the Green Revolution increased food production in many countries, overpopulation
and poor distribution of food still remain problems around the world.