Chapter 30 Plant Diversity II: The Evolution of Seed Plants
Overview: Feeding the World
• The seed arose about 360 million years ago.
• Seed plants, including gymnosperms and angiosperms, have
come to dominate modern landscapes and make up the great
majority of plant biodiversity.
• Agriculture, the cultivation and harvest of plants (especially
angiosperms), began 13,000 years ago.
• Humans began the cultivation of plants independently in
various regions, including the Near East, East Asia, Africa, and
• This was the single most important cultural change in the
history of humanity, and it made possible the transition from
hunter-gatherer societies to permanent settlements.
Concept 30.1 The reduced gametophytes of seed plants are protected
in ovules and pollen grains
• A number of terrestrial adaptations contributed to the success of seed
• These adaptations include the seed, the reduction of the
gametophyte generation, heterospory, ovules, and pollen.
• Bryophyte life cycles are dominated by the gametophyte generation,
while seedless vascular plants have sporophyte-dominated life
• The trend to gametophyte reduction continued in the lineage of
vascular plants that led to seed plants.
• Seedless vascular plants have tiny gametophytes that are
visible to the naked eye.
• The gametophytes of seed plants are microscopically small and
develop from spores retained within the moist sporangia of the
parental sporophyte. • In seed plants, the delicate female gametophyte and the young
sporophyte embryo are protected from many environmental stresses,
including drought and UV radiation.
• The gametophytes of seed plants obtain nutrients from their
parents, while the free-living gametophytes of seedless
vascular plants must fend for themselves.
Heterospory is the rule among seed plants.
• Nearly all seedless plants are homosporous, producing a single kind
of spore that forms a hermaphroditic gametophyte.
• Seed plants likely had homosporous ancestors.
• All seed plants are heterosporous, producing two different types of
sporangia that produce two types of spores.
• Megasporangia produce megaspores, which give rise to female
• Microsporangia produce microspores, which give rise to male
Seed plants produce ovules.
• In contrast to the few species of heterosporous seedless vascular
plants, seed plants are unique in retaining their megaspores within
the parent sporophyte.
• Layers of sporophyte tissue, integuments, envelop and protect the
• Gymnosperm megaspores are surrounded by one integument.
• Angiosperm megaspores are surrounded by two integuments.
• An ovule consists of the megasporangium, megaspores, and
• A female gametophyte develops from a megaspore and produces
one or more egg cells.
Pollen eliminated the liquid-water requirement for fertilization. • The microspores develop into pollen grains that are released from the
• Pollen grains are covered with a tough coat containing
• They are carried by wind or animals.
• The transfer of pollen to the vicinity of the ovule is called
• The pollen grain germinates and grows as a pollen tube into the
ovule, where it delivers one or two sperm into the female
• Bryophytes and seedless vascular plants have flagellated sperm cells
that swim a few centimeters through a film of water to reach the egg
cells within the archegonium.
• In seed plants, the female gametophyte is retained within the
• Male gametophytes travel long distances as pollen grains.
• The sperm of seed plants lack flagella and do not require a film
of water, as they rely on the pollen tube to reach the egg cell of
the female gametophyte within the ovule.
• The sperm of some gymnosperm species retain the ancestral
flagellated condition, providing evidence of this evolutionary
• The evolution of pollen contributed to the success and diversity of
Seeds became an important means of dispersing offspring.
• What is a seed?
• When a sperm fertilizes an egg of a seed plant, the zygote
forms and develops into a sporophyte embryo. • The ovule develops into a seed, consisting of the embryo and
its food supply within a protective coat derived from the
• The evolution of the seed enabled plants to resist harsh environments
and disperse offspring more widely.
• For bryophytes and seedless vascular plants, single-celled spores
are the only protective stage in the life cycle.
• Moss spores can survive even if the local environment is too
cold, too hot, or too dry for the moss plants themselves to
• Because of their tiny size, the spores themselves can be
dispersed in a dormant state to a new area.
• Spores were the main way that plants spread over Earth for the
first 100 million years of life on land.
• The seed represents a different solution to resisting harsh
environments and dispersing offspring.
• In contrast to a single-celled spore, a multicellular seed is a
much more complex, resistant structure.
• After being released from the parent plant, a seed may remain
dormant for days or years.
• Under favorable conditions, it germinates and the sporophyte
embryo emerges as a seedling.
Concept 30.2 Gymnosperms bear “naked” seeds, typically on cones
• The ovules and seeds of gymnosperms (“naked seeds”) develop on
the surfaces of modified leaves that usually form cones (strobili).
• In contrast, ovules and seeds of angiosperms develop in
enclosed chambers called ovaries.
• The most familiar gymnosperms are the conifers, cone-bearing trees
such as pine, fir, and redwood. The four phyla of extant gymnosperms are Cycadophyta,
Ginkgophyta, Gnetophyta, and Coniferophyta.
• There are four plant phyla grouped as gymnosperms.
• Phylum Ginkgophyta consists of only a single extant species, Ginkgo
• This popular ornamental species has fanlike leaves that turn
gold before they fall off in the autumn.
• Landscapers usually plant only male trees because the coats of
seeds produced by female plants produce a repulsive odor as
• Cycads (phylum Cycadophyta) have large cones and palmlike leaves.
• 130 species of cycads survive today.
• Cycads flourished in the Mesozoic era, which was known as the
“Age of Cycads.”
• Phylum Gnetophyta consists of three very different genera.
• Weltwitschia plants, from deserts in southwestern Africa, have
straplike leaves that are among the largest known leaves.
• Gentum species are tropical trees or vines.
• Ephedra (Mormon tea) is a shrub of the American deserts.
• The conifers belong to the largest gymnosperm phylum, the phylum
• The term conifer comes from the reproductive structure, the
cone, which is a cluster of scalelike sporophylls.
• Although there are only about 600 species of conifers, a few
species dominate vast forested regions in the Northern
Hemisphere where the growing season is short.
• Conifers include pines, firs, spruces, larches, yews, junipers, cedars,
cypresses, and redwoods.
• Most conifers are evergreen, retaining their leaves and
photosynthesizing throughout the year. • Some conifers, like the dawn redwood and tamarack, are
deciduous, dropping their leaves in autumn.
• The needle-shaped leaves of some conifers, such as pines and firs,
are adapted for dry conditions.
• A thick cuticle covering the leaf and the placement of stomata in
pits further reduce water loss.
• Much of our lumber and paper comes from the wood (actually xylem
tissue) of conifers.
• This tissue gives the tree structural support.
• Coniferous trees are amongst the largest and oldest organisms of
• Redwoods from northern California can grow to heights of over
• One bristlecone pine, also from California, is more than 4,600
years old, and may be the world’s oldest living organism.
The Mesozoic era was the age of gymnosperms.
• The gymnosperms probably descended from progymnosperms, a
group of Devonian plants that were heterosporous but lacked seeds.
• The first seed plants to appear in the fossil record were
gymnosperms dating from around 360 million years ago.
• Angiosperms arose more than 200 million years later.
• The two surviving clades of seed plants are gymnosperms and
• Early gymnosperms lived in Carboniferous ecosystems dominated by
seedless vascular plants.
• The flora and fauna of Earth changed dramatically during the
formation of the supercontinent Pangaea in the Permian.
• Climatic conditions became warmer and drier, favoring the
spread of gymnosperms.
• Many groups of organisms disappeared while others emerged. • Amphibians decreased in diversity and were replaced by reptiles,
which were better adapted to dry conditions.
• The lycophytes, horsetails, and ferns that had dominated in
Carboniferous swamps were largely replaced by gymnosperms.
• The change in organisms was so dramatic that geologists use the
end of the Permian, 251 million years ago, as the boundary between
the Paleozoic (“old life”) and Mesozoic (“new life”) eras.
• The terrestrial animals of the Mesozoic, including dinosaurs,
were supported by a vegetation consisting mostly of conifers
and cycads, both gymnosperms.
• The dinosaurs did not survive the mass extinction at the end of the
Mesozoic, but many gymnosperms persisted and are still an
important part of Earth’s flora.
The life cycle of a pine demonstrates the key reproductive
adaptations of seed plants.
• The life cycle of a pine illustrates the three key adaptations to
terrestrial life in seed plants:
1. Increasing dominance of the sporophyte.
2. The advent of the seed as a resistant, dispersal stage in the life
3. The evolution of pollen as an airborne agent bringing gametes
• The pine tree is the sporophyte.
• It produces its sporangia on scalelike sporophylls that are
packed densely on cones.
• Conifers, like all seed plants, are heterosporous.
• Male and female gametophytes develop from different types of
spores produced by separate cones: small pollen cones and large
• Most pine species produce both types of cones. • A pollen cone contains hundreds of microsporangia held on small
• Each cone produces microspore mother cells that undergo
meiosis to produce haploid microspores.
• Each microspore develops into a pollen grain containing a male
• A larger ovulate cone consists of many scales, each with two ovules.
• Each ovule includes a megasporangium.
• Ovulate cones produce megaspore mother cells that undergo meiosis
to produce four haploid cells, one of which will develop into a
• Surviving megaspores develop into female gametophytes,
which are retained within the sporangia.
• Two or three archegonia, each with an egg, develop within the
• During pollination, windblown pollen falls on the ovulate cone and
grows into the ovule through the micropyle.
• Fertilization of egg and sperm follows.
• The pine embryo, the new sporophyte, has a rudimentary root and
several embryonic leaves.
• The female gametophyte surrounds and nourishes the embryo.
• The ovule develops into a pine seed, which consists of an
embryo (new sporophyte), its food supply (derived from
gametophyte tissue), and a seed coat derived from the
integuments of the parent tree (parent sporophyte).
• It takes three years from the appearance of young cones on a pine
tree to the formation of mature seeds.
• The scales of ovulate cone separate and the seeds are typically
dispersed by the wind. • A seed that lands in a habitable place germinates, and its embryo
emerges as a pine seedling.
Concept 30.3 The reproductive adaptations of angiosperms include
flowers and fruits
• Angiosperms, commonly known as flowering plants, are vascular
seed plants that produce flowers and fruits.
• They are the most diverse and geographically widespread of all
plants, including more than 90% of plant species.
• There are about 250,000 known species of angiosperms.
• All angiosperms are placed in a single phylum, the phylum
The flower is the defining reproductive adaptation of angiosperms.
• The flower is an angiosperm structure specialized for sexual
• In many species of angiosperms, insects and other animals
transfer pollen from one flower to female sex organs of another.
• Some species that occur in dense populations, like grasses, are
• A flower is a specialized shoot with up to four circles of modified
leaves: sepals, petals, stamens, and carpals.
• The sepals at the base of the flower are modified leaves that are
usually green and enclose the flower before it opens.
• The petals lie inside the ring of sepals.
• These are often brightly colored in plant species that are
pollinated by animals.
• They typically lack bright coloration in wind-pollinated plant
• Sepals and petals are sterile floral parts, not directly involved in
reproduction. • Stamens, the male reproductive organs, are sporophylls that produce
microspores that will give rise to pollen grains containing male
• A stamen consists of a stalk (the filament) and a terminal sac
(the anther) where pollen is produced.
• Carpals are female sporophylls that produce megaspores and their
products, female gametophytes.
• At the tip of the carpal is a sticky stigma that receives pollen.
• A style leads to the ovary at the base of the carpal.
• Ovules are protected within the ovary.
Fruits help disperse the seeds of angiosperms.
• A fruit usua