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Also known as: Embryophyta, Metaphyta, Plantae
Written by

Rudolf Schmid,

Hans Lambers•All
Fact-checked by

The Editors of Encyclopaedia Britannica

Last Updated: May 13, 2025 • Article History

Key People:

Alexander von Humboldt

Sir Hans Sloane, Baronet

Henry Chandler Cowles

David Fairchild

Julius von Sachs

(Show more)
Related Topics:

angiosperm

plant movement

vegetation

woody plant

Progymnospermophyta
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grasslandHighveld grassland near Heidelberg, South Africa, southeast of
Johannesburg.

plant, (kingdom Plantae), any multicellular eukaryotic life-

form characterized by (1) photosynthetic nutrition (a characteristic
possessed by all plants except some parasitic plants and
underground orchids), in which chemical energy is produced
from water, minerals, and carbon dioxide with the aid
of pigments and the radiant energy of the Sun, (2) essentially
unlimited growth at localized regions, (3) cells that
contain cellulose in their walls and are therefore to some extent
rigid, (4) the absence of organs of locomotion, resulting in a more
or less stationary existence, (5) the absence of nervous systems,
and (6) life histories that show an alteration of haploid and diploid
generations, with the dominance of one over the other being
taxonomically significant.

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duckweedDuckweed (Lemna minor).

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giant sequoiaGiant sequoia tree in California.

Plants range in size from diminutive duckweeds only a few

millimetres in length to the giant sequoias of California that reach
90 metres (300 feet) or more in height. There are an estimated
390,900 different species of plants known to science, and new
species are continually being described, particularly from
previously unexplored tropical areas of the world. Plants evolved
from aquatic ancestors and have subsequently migrated over the
entire surface of Earth, inhabiting tropical, Arctic, desert, and
Alpine regions. Some plants have returned to an aquatic habitat in
either fresh or salt water.
PhotosynthesisDiagram of photosynthesis showing how water, light, and carbon
dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.
(more)

Plants play a vital role in the maintenance of life on Earth.

All energy used by living organisms depends on the complex
process of photosynthesis, which is mostly carried out by green
plants. Radiant energy from the Sun is transformed into organic
chemical energy in the form of sugars through the fundamental
series of chemical reactions constituting photosynthesis. In nature
all food chains begin with photosynthetic autotrophs (primary
producers), including green plants and algae. Primary producers,
represented by trees, shrubs, and herbs, are a prolific source of
energy in the form of carbohydrates (sugars) stored in the leaves.
These carbohydrates, produced in photosynthesis, are broken down
in a process called respiration; the smaller units of
the sugar molecule and its products fuel numerous metabolic
processes. Various parts of the plant (e.g., leaves) are the energy
sources that support animal life in different community habitats. A
by-product of photosynthesis, oxygen, is essential to animals.

The daily existence of human beings is also directly influenced by

plants. Plants furnish food and flavourings; raw materials for
industry, such as wood, resins, oils, and rubber; fibres for the
manufacture of fabrics and cordage; medicines; insecticides; and
fuels. More than half of Earth’s population relies on
the grasses rice, corn (maize), and wheat as their primary source of
food. Apart from their commercial and aesthetic value, plants
conserve other natural resources by protecting soils from erosion,
by controlling water levels and quality, and by producing a
favourable atmosphere.

The following article summarizes the morphological, physiological,

and ecological features of plants. The principal focus is on structure
and function, physiology, life histories, and ecology, and on how the
various plant groups have evolved, dispersed, and become adapted
to life on land. The features that define each major plant group and
the role they play in the wider ecosystem also are discussed.

Britannica Quiz

Pop Quiz: 13 Things to Know About Photosynthesis

Definition of the kingdom

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Plant cellCutaway drawing of a plant cell, showing the cell wall and internal
organelles.
2 of 2

Why plant diversity is so importantPlants have evolved into many diverse forms
that define and sustain ecosystems.(more)
See all videos for this article

The kingdom Plantae includes organisms that range in size from

tiny mosses to giant trees. Despite this enormous variation, all
plants are multicellular and eukaryotic (i.e., each cell possesses a
membrane-bound nucleus that contains the chromosomes). They
generally possess pigments (chlorophylls a and b and carotenoids),
which play a central role in converting
the energy of sunlight into chemical energy by means
of photosynthesis. Most plants, therefore, are independent in their
nutritional needs (autotrophic) and store their excess food in the
form of macromolecules of starch. The relatively few plants that are
not autotrophic have lost pigments and are dependent on other
organisms for nutrients. Although plants are nonmotile organisms,
some produce motile cells (gametes) propelled by whiplike flagella.
Plant cells are surrounded by a more or less rigid cell
wall composed of the carbohydrate cellulose, and adjacent cells are
interconnected by microscopic strands of cytoplasm called
plasmodesmata, which traverse the cell walls. Many plants have the
capacity for unlimited growth at localized regions of cell division,
called meristems. Plants, unlike animals, can use inorganic forms of
the element nitrogen (N), such as nitrate and ammonia—which are
made available to plants through the activities of microorganisms
or through the industrial production of fertilizers—and the
element sulfur (S); thus, they do not require an external source
of protein (in which nitrogen is a major constituent) to survive.

angiosperm life cycleLife cycle of a typical angiosperm. The angiosperm life cycle
consists of a sporophyte phase and a gametophyte phase. The cells of a sporophyte
body have a full complement of chromosomes (i.e., the cells are diploid, or 2n); the
sporophyte is the typical plant body that one sees when one looks at an angiosperm.
The gametophyte arises when cells of the sporophyte, in preparation for reproduction,
undergo meiotic division and produce reproductive cells that have only half the
number of chromosomes (i.e., haploid, or n). A two-celled microgametophyte (called a
pollen grain) germinates into a pollen tube and through division produces the haploid
sperm. An eight-celled megagametophyte (called the embryo sac) produces the egg.
Fertilization occurs with the fusion of a sperm with an egg to produce a zygote, which
eventually develops into an embryo. After fertilization, the ovule develops into a seed,
and the ovary develops into a fruit.(more)

The life histories of plants include two phases, or generations, one

of which is diploid (the nuclei of the cells contain two sets of
chromosomes), whereas the other is haploid (with one set of
chromosomes). The diploid generation is known as the sporophyte,
which literally means spore-producing plant. The haploid
generation, called the gametophyte, produces the sex cells,
or gametes. The complete life cycle of a plant thus involves
an alternation of generations. The sporophyte and gametophyte
generations of plants are structurally quite dissimilar.
The concept of what constitutes a plant has undergone significant
change over time. For example, at one time the photosynthetic
aquatic organisms commonly referred to as algae were considered
members of the plant kingdom. The various major algal groups,
such as the green algae, brown algae, and red algae, are now
placed in the kingdom Protista because they lack one or more of
the features that are characteristic of plants. The organisms known
as fungi also were once considered to be plants because they
reproduce by spores and possess a cell wall. The fungi, however,
uniformly lack chlorophyll, and they are heterotrophic and
chemically distinct from the plants; thus, they are placed in a
separate kingdom, Fungi.

No definition of the kingdom completely excludes all nonplant

organisms or even includes all plants. There are plants, for
example, that do not produce their food by photosynthesis but
rather are parasitic on other living plants. Some animals possess
plantlike characteristics, such as the lack of mobility (e.g., sponges)
or the presence of a plantlike growth form (e.g.,
some corals and bryozoans), but in general such animals lack the
other characteristics of plants cited here.

Despite such differences, plants share the following features

common to all living things. Their cells undergo complex metabolic
reactions that result in the production of chemical energy,
nutrients, and new structural components. They respond to internal
and external stimuli in a self-preserving manner. They reproduce
by passing their genetic information to descendants that resemble
them. They have evolved over geological time scales (hundreds of
millions of years) by the process of natural selection into a wide
array of forms and life-history strategies.
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plant evolutionSignificant events in plant evolution.

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Understanding adaptations in the evolution of plantsPlants, ranging from the
simple liverwort (a bryophyte) to the flowering plants (angiosperms), have evolved
structures enabling them to colonize the land of almost any habitat.(more)
See all videos for this article

The earliest plants undoubtedly evolved from an aquatic green

algal ancestor (as evidenced by similarities in pigmentation, cell-
wall chemistry, biochemistry, and method of cell division), and
different plant groups have become adapted to terrestrial life to
varying degrees. Land plants face severe environmental threats or
difficulties, such as desiccation, drastic changes in temperature,
support, nutrient availability to each of the cells of the plant,
regulation of gas exchange between the plant and the atmosphere,
and successful reproduction. Thus, many adaptations to land
existence have evolved in the plant kingdom and are reflected
among the different major plant groups. An example is the
development of a waxy covering (the cuticle) that covers the plant
body, preventing excess water loss. Specialized tissues and cells
(vascular tissue) enabled early land plants to absorb and transport
water and nutrients to distant parts of the body more effectively
and, eventually, to develop a more complex body composed of
organs called stems, leaves, and roots. The evolution and
incorporation of the substance lignin into the cell walls of plants
provided strength and support. Details of the life history are often a
reflection of a plant’s adaptation to a terrestrial mode of life and
may characterize a particular group; for example, the most highly
evolved plants reproduce by means of seeds, and, in the most
advanced of all plants (angiosperms), a reproductive organ called
a flower is formed.
Nonvascular plants
Definition of the category
1 of 2

red carpet mossRed carpet moss (Bryoerythrophyllum columbianum).

2 of 2

Compare and contrast liverworts and mosses in their reproduction, water

absorption, and energy generationBryophytes, such as mosses and liverworts, are
the most primitive plants.(more)
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Informally known as bryophytes, nonvascular plants lack

specialized vascular tissue (xylem and phloem) for internal water
and food conduction and support. They also do not possess
true roots, stems, or leaves. Some larger mosses, however, contain
a central core of elongated thick-walled cells called hydroids that
are involved in water conduction and that have been compared to
the xylem elements of other plants. Bryophytes are second
in diversity only to the flowering plants (angiosperms) and are
generally regarded as composed of three divisions: Bryophtya
(the mosses), Marchantiophyta (the liverworts),
and Anthocerotophyta (the hornworts).

Because bryophytes generally lack conducting cells and a well-

developed cuticle that would limit dehydration, they depend on
their immediate surroundings for an adequate supply of moisture.
As a result, most bryophytes live in moist or wet shady locations,
growing on rocks, trees, and soil. Some, however, have become
adapted to totally aquatic habitats; others have become adapted to
alternately wet and dry environments by growing during wet
periods and becoming dormant during dry intervals. Although
bryophytes are widely distributed, occurring in practically all parts
of the world, none are found in salt water. Ecologically, some
mosses are considered pioneer plants because they can invade bare
areas.

liverwort archegoniaEgg-producing archegonia of a common liverwort plant

(Marchantia polymorpha).

Bryophytes are typically land plants but seldom attain a height of

more than a few centimetres. They possess the
photosynthetic pigment chlorophyll (both a and b forms)
and carotenoids in cell organelles called chloroplasts. The life
histories of these plants show a well-defined alternation of
generations, with the independent and free-living gametophyte as
the dominant photosynthetic phase in the life cycle. (This is in
contrast to the vascular plants, in which the dominant
photosynthetic phase is the sporophyte.) The sporophyte
generation develops from, and is almost entirely parasitic on, the
gametophyte. The gametophyte produces multicellular sex organs
(gametangia). Female gametangia are called archegonia; male
gametangia, antheridia. At maturity, archegonia each contain
one egg, and antheridia produce many sperm cells. Because the
egg is retained and fertilized within the archegonium, the early
stages of the developing sporophyte are protected and nourished
by the gametophytic tissue. The young undifferentiated sporophyte
is called an embryo. Although bryophytes have become adapted to
life on land, an apparent vestige of their aquatic ancestry is that the
motile (flagellated) sperm depend on water to allow gamete
transport and fertilization.

Bryophytes are widely believed to have evolved from complex green

algae that invaded land around 500 million years ago. Bryophytes
share some traits with green algae, such as motile sperm, similar
photosynthetic pigments, and the general absence of vascular
tissue. However, bryophytes have multicellular reproductive
structures, whereas those of green algae are unicellular, and
bryophytes are mostly terrestrial and have complex plant bodies,
whereas the green algae are primarily aquatic and have less-
complex forms.
Representative members
Division Bryophyta
peat mossPeat moss (Sphagnum flexuosum).

Moss is a term erroneously applied to many different plants

(Spanish moss, a flowering plant; Irish moss, a red alga; pond moss,
filamentous algae; and reindeer moss, a lichen). True mosses are
classified as the division Bryophyta.

moss life cycleLife cycle of moss.

The moss gametophyte possesses leaflike structures (phyllids) that

usually are a single cell layer thick, have a costa (midrib), and are
spirally arranged on a stemlike axis (caulid). The moss
gametophyte is an independent plant and is the familiar, erect
“leafy” shoot. Multicellular rhizoids anchor the gametophyte to the
substrate. The sporophyte plant develops from the tip of the fertile
leafy shoot. After repeated cell divisions, the young sporophyte
(embryo) transforms into a mature sporophyte consisting of foot,
elongate seta, and capsule. The capsule is often covered by a
calyptra, which is the enlarged remains of the archegonium. The
capsule is capped by an operculum (lid), which falls off, exposing a
ring of teeth (the peristome) that regulates the dispersal of spores.
Division Marchantiophyta

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thalloidThalloid of the liverwort Marchantia with gemma cups.

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Study spore-producing liverworts and their anatomic features such as

gametes, thalli, and rhizoidsCharacteristic features of liverworts.(more)
See all videos for this article

Liverworts, the second major division of nonvascular plants, are

found in the same types of habitat as mosses, and species of the
two classes are often intermingled on the same site. The curious
name liverwort is a relic of the medieval belief in the “doctrine of
signatures,” which held that the external form of a plant provided a
clue to which diseased body organ could be cured by a preparation
made from that particular plant. There are two types of liverworts
(also called hepatics) based on reproductive features and thallus
structure. The more numerous “leafy” liverworts superficially
resemble mosses, but most notably differ in having lobed or divided
leaves that are without a midrib and are positioned in three rows.
Thalloid (thallose) liverworts have a ribbonlike, or strap-shaped,
body that grows flat on the ground. They have a high degree of
internal structural differentiation into photosynthetic and storage
zones. Liverwort gametophytes have unicellular rhizoids.
Liverworts have an alternation of generations similar to that of
mosses, and, as with mosses, the gametophyte generation is
dominant. The sporophytes, however, are not microscopic and are
often borne on specialized structures. They sometimes resemble
small umbrellas and are called antheridiophores and
archegoniophores.
Division Anthocerotophyta

hornwortHornwort (Dendroceros).

The third division of bryophytes comprises the hornworts, a minor

group numbering fewer than 100 species. The gametophyte is a
small ribbonlike thallus that resembles a thallose liverwort. The
name hornwort is derived from the unique slender, upright
sporophytes, which are about 3–4 cm (1.2–1.6 inches) long at
maturity and dehisce longitudinally into two valves that twist in
response to changing humidity, thereby releasing spores in small
numbers over a fairly long period of time.
Vascular plants
Definition of the category
1 of 2

tree fernTree fern (Cyathea medullaris).

2 of 2

bluebellsSpring flowering of bluebells (Hyacinthoides nonscripta) in a deciduous

forest of beech (Fagus sylvatica) and oak (Quercus) near Nairn, Scotland.(more)

Vascular plants (tracheophytes) differ from the nonvascular

bryophytes in that they possess specialized supporting and water-
conducting tissue, called xylem, and food-conducting tissue,
called phloem. The xylem is composed of nonliving cells
(tracheids and vessel elements) that are stiffened by the presence
of lignin, a hardening substance that reinforces the cellulose cell
wall. The living sieve elements that comprise the phloem are not
lignified. Xylem and phloem are collectively called vascular
tissue and form a central column (stele) through the plant axis.
The ferns, gymnosperms, and flowering plants are all vascular
plants. Because they possess vascular tissues, these plants have
true stems, leaves, and roots. Before the development of vascular
tissues, the only plants of considerable size existed in
aquatic environments where support and water conduction were
not necessary. A second major difference between the vascular
plants and bryophytes is that the larger,
more conspicuous generation among vascular plants is the
sporophytic phase of the life cycle.

The vegetative body of vascular plants is adapted to terrestrial life

in various ways. In addition to vascular tissue, the aerial body is
covered with a well-developed waxy layer (cuticle) that decreases
water loss. Gases are exchanged through numerous pores (stomata)
in the outer cell layer (epidermis). The root system is involved in
the uptake from the soil of water and minerals that are used by the
root system as well as the stem and leaves. Roots also anchor the
plant and store food. The stem conducts water and minerals
absorbed by the root system upward to various parts of the stem
and leaves; stems also conduct carbohydrates manufactured
through the process of photosynthesis from the leaves to various
parts of the stem and root system. Leaves are supported by the
stem and are oriented in a manner conducive to maximizing the
amount of leaf area involved in trapping sunlight for use in
photosynthesis.

Modifications of roots, stems, and leaves have enabled species of

vascular plants to survive in a variety
of habitats encompassing diverse and even extreme environmental
conditions. The ability of vascular plants to flourish in so many
different habitats is a key factor in their having become the
dominant group of terrestrial plants.

The vascular plants are divisible into the nonseed plants (lower
vascular plants, or cryptogams) and those that reproduce by seeds
(higher vascular plants, or phanerogams).
The ferns (Polypodiopsida) are a group of the lower vascular plants;
other groups include the whisk ferns (Psilotophyta), club and spike
mosses (Lycophyta), and horsetails (Sphenophyta, or Arthrophyta).
Collectively, the latter four groups are sometimes referred to as
pteridophytes, because each reproduces by spores liberated from
dehiscent sporangia (free sporing). Although the lower vascular
plants have adapted to terrestrial life, they are similar to
bryophytes in that, as an apparent vestige of their aquatic ancestry,
all produce motile (flagellated) male gametes (antherozoids, or
sperm) and must rely on water for fertilization to take place.
Nonseed plants
Class Lycopodiopsida

spike mossSpike moss (Selaginella).

This class is represented by three living orders, the principal

genera being Lycopodium (club mosses), Selaginella (spike
mosses), and Isoetes (quillworts). Extant members of
Lycopodiopsida occur in both temperate and tropical regions and
represent the survivors of a group of vascular plants that was
extremely diverse and numerous. As a group, the lycophytes were
prominent in the great coal-forming swamp forests of
the Carboniferous Period (358.9 million to 298.9 million years ago).
Although all living lycophytes are small herbaceous plants, some
extinct types were large trees. Lycophytes are differentiated into
stem, root, and leaf (microphylls). Sporangia are positioned on the
upper (adaxial) surface of the leaf (sporophyll). Some species form
distinct cones or strobili, whereas others do not.

More From Britannica

What Does Photosynthesis Produce?

Class Polypodiopsida
1 of 2

shield fernShield fern (Dryopteris dilatata).

2 of 2
Why do ferns prefer the shade?Ferns, like all tracheophytes, have vascular systems
to bring water up to their leaves.
See all videos for this article

Ferns are a diverse group of plants. Although they have a

worldwide distribution, ferns are more common in tropical and
subtropical regions. They range in size and complexity from small
floating aquatic plants less than 2 cm (0.8 inch) long to tall tree
ferns 20 metres (65 feet) high. Tropical tree ferns possess erect
columnar trunks and large compound (divided) leaves more than 5
metres (about 16 feet) long. As a group, ferns are either terrestrial
or epiphytic (growing upon another plant). Fern stems never
become woody (composed of secondary tissue containing lignin),
because all tissues of the plant body originate at the stem apex.
Subclass Polypodiidae

fern life cycleThe life cycle of the fern. (1) Clusters (sori) of sporangia (spore cases)
grow on the undersurface of mature fern leaves. (2) Released from its spore case, the
haploid spore is carried to the ground, where it germinates into a tiny, usually heart-
shaped, gametophyte (gamete-producing structure), anchored to the ground by
rhizoids (rootlike projections). (3) Under moist conditions, mature sperm are released
from the antheridia and swim to the egg-producing archegonia that have formed on the
gametophyte's lower surface. (4) When fertilization occurs, a zygote forms and
develops into an embryo within the archegonium. (5) The embryo eventually grows
larger than the gametophyte and becomes a sporophyte.(more)

Ferns of the subclass Polypodiidae, known as leptosporangiate

ferns, typically possess a rhizome (horizontal stem) that grows
partially underground; the deeply divided fronds (leaves) and the
roots grow out of the rhizome. Fronds are characteristically coiled
in the bud (fiddleheads) and uncurl in a type of leaf development
called circinate vernation. Fern leaves are either whole or variously
divided. The leaf types are differentiated into rachis (axis of a
compound leaf), pinnae (primary divisions), and pinnules (ultimate
segments of a pinna). Fern leaves often have prominent epidermal
hairs and large chaffy scales. Venation of fern leaves is usually
open dichotomous (forking into two equal parts).

Each frond is a potential sporophyll (spore-bearing leaf) and as

such can bear structures that are associated with reproduction.
When growth conditions are favourable, a series of brown patches
appear on the undersurface of the sporophylls. Each one of the
patches (called a sorus) is composed of many sporangia, or spore
cases, which are joined by a stalk to the sporophyll. The spore case
is flattened, with a layer of sterile, or nonfertile, cells surrounding
the spore mother cells. Each spore mother cell divides by reduction
division (meiosis) to produce haploid spores, which are shed in a
way characteristic to the ferns.

Each fern spore has the potential to grow into a green heart-shaped
independent gametophyte plant (prothallus) capable
of photosynthesis. In contrast to bryophytes, in which
the sporophyte is nutritionally dependent on the gametophyte
during its entire existence, the fern sporophyte is dependent on the
gametophyte for nutrition only during the early phase of its
development; thereafter, the fern sporophyte is free-living. In some
ferns the sexes are separate, meaning a gametophyte will bear only
male or female sex organs. Other species have gametophytes
bearing both sex organs. Features important in the identification of
ferns include such aspects of the mature sporophyte plant as
differences in the stem, frond, sporophyll, sporangium, and position
of the sporangium and the absence or presence, as well as the
shape, of the indusium (a membranous outgrowth of the leaf)
covering the sporangia.
Order Psilotales
whisk fernWhisk fern (Psilotum nudum).

Psilotales (whisk ferns) is an order represented by two

living genera (Psilotum and Tmesipteris) and several species that
are restricted to the subtropics. This unusual group of small
herbaceous plants is characterized by a leafless and rootless body
possessing a stem that exhibits a primitive dichotomous type of
branching: it forks into equal halves. The photosynthetic function is
assumed by the stem, and the underground rhizome anchors the
plant. The vascular tissue is organized into a poorly developed
central cylinder in the stem.
Order Equisetales
giant horsetail of EuropeGiant horsetail of Europe (Equisetum telmateia).

Equisetales (also called horsetails and scouring rushes) is an order

represented by a single living genus (Equisetum). It has
a worldwide distribution but occurs in greater variety in the
Northern Hemisphere. Like the lycophytes, this group was a
diverse and prominent group of vascular plants during
the Carboniferous Period, when some genera attained great size in
the coal-forming swamp forests. Known as sphenophytes, these
plants are differentiated into stem, leaf (microphylls), and root.
Green aerial stems have longitudinal ridges and furrows extending
the length of the internodes, and stems are jointed (articulated).
Surface cells are characteristically filled with silica. Branches,
when they occur, are borne in whorls at the node, as are the scale
leaves. Sporangia are borne in terminal strobili. Equisetales had its
origin in the Devonian Period (419.2 million to 358.9 million years
ago).
Order Marattiales
Known as giant ferns, the order Marattiales comprises a
single extant family with six genera and some 150 species of large
tropical and subtropical ferns with stout erect stems. The leaves
(fronds) may be very large, some reaching 4.5 metres (15 feet) or
more in length. The Marattiaceae generally are considered to be
one of the most primitive families of ferns still living.
Seed plants

1 of 2

gymnosperms and angiospermsTwo types of seed-bearing plants: gymnosperms

and angiosperms. (Left) The Lawson cypress is an evergreen gymnosperm, or “naked
seed” plant. It produces seeds in cones and bears needlelike leaves year-round. (Right)
The English elm is a broad-leaved and deciduous angiosperm, or flowering plant. It
produces seeds in fruits and drops its leaves in the autumn.(more)
2 of 2

Learn how advancements in the xylem, vascular tissue, and flower separate
angiosperms from gymnospermsGymnosperms dominated the plant world until
they were replaced by the more advanced flowering plants known as angiosperms.
(more)
See all videos for this article

Gymnosperms and angiosperms (flowering plants) share with ferns

a dominant, independent sporophyte generation; the presence of
vascular tissue; differentiation of the plant body into root, stem,
and leaf derived from a bipolar embryo (having stem and root-
growing apices); and similar photosynthetic pigments. Unlike ferns,
however, the seed plants have stems that branch laterally and
vascular tissue that is arranged in strands (bundles) around the
pith (eustele). Among seed plants, as in ferns, the stem tissues that
arise directly from the shoot apex are called primary tissues.
Primary tissues contribute to the longitudinal growth of the stem,
or primary growth. Secondary growth, resulting in an increase in
the width of the axis, is produced by meristematic tissue between
the primary xylem and phloem called vascular cambium.
This meristem consists of a narrow zone of cells that form new
secondary xylem (wood) and secondary phloem (secondary vascular
tissues).

rice grain anatomyThe outer layers and internal structures of a rice grain.

Major evolutionary advancements of these plants are demonstrated

by the generally more complex plant body and by reproduction
via seeds. Seeds represent an important
evolutionary innovation within the plant kingdom. Each seed has an
embryonic plant (sporophyte), food-storage tissue, and hardened
protective covering (seed coat). The seed thus contains and
protects the embryonic plant and, as the primary dispersal unit of
the seed plants, represents a significant improvement over the
spore, with its limited capacity for survival.

In comparing ferns and seed plants and their life histories, certain
significant differences are seen. The gametophyte in seed plants
has been reduced in size, usually consisting of a few to a dozen
cells. Thus, it is no longer itself a plant body, as in the bryophytes
and ferns. The gametophyte is not free-living but is embedded in
the sporophyte and thus less vulnerable to environmental stress
than the gametophytes of bryophytes and ferns. Finally, the spores
of seed plants are male and female, as are the sporangia that
contain them. The spores are not dispersed as in the bryophytes
and ferns but develop into gametophytes within the sporangia. In
the most advanced seed plants, the male gametes (sperm) are
carried to the egg by a later extension of the pollen grain called the
pollen tube. The advantage of this system is that the nonflagellated
sperm are no longer dependent on water to reach the egg.
1 of 2

wind pollinationWind pollination in grasses: yellow free-hanging anthers (pollen

producers) and white feathery stigmas (pollen collectors) of meadow fescue (Festuca
pratensis) provide maximum wind exposure.(more)

2 of 2

Insect pollinationOrange-tailed butterfly (Eurema proterpia) on an ash-colored aster

(Machaeranthera tephrodes). The upstanding yellow stamens are tipped with pollen,
which brushes the body of the butterfly as it approaches the center of the flat-topped
aster to feed on the nectar.(more)

Another terrestrial adaptation of the seed plants not found in ferns

is pollen dispersed by wind or animals. Pollen is a unit of genetic
material as well as part of the seed-formation process. The
dispersal of pollen by wind or animals, in addition to dispersal of
seeds, promotes genetic recombination and distribution of the
species over a wide geographic area.
Gymnosperms
conifer heightsThe heights of selected conifers and a highlight of the needle-and-
cone configuration of the Douglas fir (Pseudotsuga).(more)

The term gymnosperm (“naked seeds”) represents

four extant divisions of vascular plants whose ovules (seeds) are
exposed on the surface of cone scales. The cone-bearing
gymnosperms are among the largest and oldest living organisms in
the world. They dominated the landscape about 200 million years
ago. Today gymnosperms are of great economic value as major
sources of lumber products, pulpwood, turpentine, and resins.

Conifer stems are composed of a woody axis containing primitive

water- and mineral-conducting cells called tracheids. Tracheids are
interconnected by passages called bordered pits. Leaves are often
needlelike or scalelike and typically contain canals filled with resin.
The leaves of pine are borne in bundles (fascicles), and the number
of leaves per fascicle is an important distinguishing feature. Most
gymnosperms are evergreen, but some, such as larch and bald
cypress, are deciduous (the leaves fall after one growing season).
The leaves of many gymnosperms have a thick cuticle
and stomata below the leaf surface.

The tree or shrub is the sporophyte generation. In conifers, the

male and female sporangia are produced on separate structures
called cones or strobili. Individual trees are typically monoecious
(male and female cones are borne on the same tree). A cone is a
modified shoot with a single axis, on which is borne a spirally
arranged series of pollen- or ovule-bearing scales or bracts. The
male cone, or microstrobilus, is usually smaller than the female
cone (megastrobilus) and is essentially an aggregation of many
small structures (microsporophylls) that encase the pollen in
microsporangia.

sago palmSago palm (Cycas revoluta), a cycad.

The extant cycads (division Cycadophyta) are a group of

ancient seed plants that are survivors of a complex that has existed
since the Mesozoic Era (251.9 million to 66 million years ago). They
are presently distributed in the tropics and subtropics of both
hemispheres. Cycads are palmlike in general appearance, with an
unbranched columnar trunk and a crown of large
pinnately compound (divided) leaves. The sexes are always
separate, resulting in male and female plants (i.e., cycads are
dioecious). Most species produce conspicuous cones (strobili) on
both male and female plants, and the seeds are very large.

ginkgoLeaves and fruit of the female ginkgo, or maidenhair tree (Ginkgo biloba).

The ginkgophytes (division Ginkgophyta), although

abundant, diverse, and widely distributed in the past, are
represented now by a sole surviving species, Ginkgo
biloba (maidenhair tree). The species was formerly restricted to
southeastern China, but it is now likely extinct in the wild. The
plant is commonly cultivated worldwide, however, and is
particularly resistant to disease and air pollution. The ginkgo is
multibranched, with stems that are differentiated into long shoots
and dwarf (spur) shoots. A cluster of fan-shaped deciduous leaves
with open dichotomous venation occurs at the end of each lateral
spur shoot. Sexes are separate, and distinct cones are not
produced. Female trees produce plumlike seeds with a fleshy outer
layer and are noted for their foul smell when mature.

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joint pineJoint pine (Ephedra fragilis).

2 of 3

Gnetum gnemon
3 of 3

welwitschiaWelwitschia, or tumboa (Welwitschia mirabilis).

The gnetophytes (division Gnetophyta) comprise a group of three

unusual genera. Ephedra occurs as a shrub in dry regions in
tropical and temperate North and South America and in Asia, from
the Mediterranean Sea to China. Species of Gnetum occur as
woody shrubs, vines, or broad-leaved trees and grow in moist
tropical forests of South America, Africa, and Asia. Welwitschia,
restricted to extreme deserts (less than 25 mm [1 inch] of rain per
year) in a narrow belt about 1,000 km (600 miles) long in
southwestern Africa, is an unusual plant composed of an enormous
underground stem and a pair of long strap-shaped leaves that lie
along the ground. The three genera differ from all other
gymnosperms in possessing vessel elements (as compared with
tracheids) in the xylem and in specializations in
reproductive morphology. The gnetophytes have figured
prominently in the theories about gymnospermous origins of the
angiosperms.
Angiosperms
1 of 3

lotus flowerSacred lotus (Nelumbo nucifera).

2 of 3

talipot palmTalipot palm (Corypha umbraculifera) in bloom. The plant has the largest
branched inflorescence of any angiosperm.(more)
3 of 3

barley(Left) The barley spike, with rows of barley florets. (Right) Cross section of the
barleycorn.

Approximately 130 million years ago, flowering plants

(angiosperms) evolved from gymnosperms, although the identity of
the specific gymnospermous ancestral group remains unresolved.
The primary distinction between gymnosperms and angiosperms is
that angiosperms reproduce by means of flowers. Flowers are
modified shoots bearing a series of leaflike modified appendages
and containing ovules (immature seeds) surrounded and protected
by the female reproductive structure, the carpel or pistil. Along
with other features, angiospermy, the enclosed condition of the
seed, gave the flowering plants a competitive advantage and
enabled them to come to dominate the extant flora. Flowering
plants have also fully exploited the use of insects and other animals
as agents of pollination (the transfer of pollen from male to female
floral structures). In addition, the water-conducting cells and food-
conducting tissue are more complex and efficient in flowering
plants than in other land plants. Finally, flowering plants possess a
specialized type of nutritive tissue in the seed, endosperm.
Endosperm is the chief storage tissue in the seeds of grasses;
hence, it is the primary source of nutrition
in corn (maize), rice, wheat, and other cereals that have been
utilized as major food sources by humans and other animals.
Classification of angiosperms
seed germinationGermination of a monocot and a eudicot. (Top) In a corn seed
(monocot), nutrients are stored in the cotyledon and endosperm tissue. The radicle and
hypocotyl (region between the cotyledon and radicle) give rise to the roots. The
epicotyl (region above the cotyledon) gives rise to the stem and leaves and is covered
by a protective sheath (coleoptile). (Bottom) In a bean seed (eudicot), all nutrients are
stored in the enlarged cotyledons. The radicle gives rise to the roots, the hypocotyl to
the lower stem, and the epicotyl to the leaves and upper stem.(more)

Many of the flowering plants are commonly represented by two

basic groups, the monocotyledons and the dicotyledons,
distinguished by the number of embryonic seed leaves (cotyledons),
number of flower parts, arrangement of vascular tissue in the stem,
leaf venation, and manner of leaf attachment to the stem. However,
one of the major changes in the understanding of the evolution of
the angiosperms was the realization that the basic distinction
among flowering plants is not between monocotyledon groups
(monocots) and dicotyledon groups (dicots). Rather, plants thought
of as being “typical dicots” have evolved from within another group
that includes the more-basal dicots and the monocots together. This
group of typical dicots is now known as the eudicots, and
molecular-based evidence supports their having a single
evolutionary lineage (monophyletic). Other angiosperm groups,
such as the Magnoliids, do not fit the traditional paradigm of
monocot and dicot and are considered to have more-ancient
lineages.
dicotyledonous plantA typical dicotyledonous plant. (A dicotyledonous plant, or
dicot, is any flowering plant that has a pair of leaves, or cotyledons, in the embryo of
the seed.)(more)

The plant body of angiosperms consists of a central axis of two

parts, the shoot and the root. Shoots have two kinds of organs,
the stem and the leaves, while roots have one type of organ, the
root itself. Systems of classification are often based upon the
longevity of the portions of plant aboveground. Woody plants are
trees and shrubs whose shoots are durable and survive over a
period of years. They are further classified into deciduous
and evergreen plants. Deciduous plants drop their leaves at the end
of every growing season, whereas evergreens keep their leaves for
up to several years. Herbaceous plants have soft, flexible aerial
portions and commonly die back each year.

Another system of classification, based on the duration of the life

history, is particularly applicable to angiosperms of the temperate
region. Annuals are plants that complete the entire life history
(germinate from seeds, mature, flower, and produce seed) in one
growing season. Examples of annuals are corn, wheat,
and peas. Biennials complete their life history in two seasons,
blooming during the second
season. Beets, celery, cabbage, carrots, and turnips are biennials,
but their flowers are rarely seen because they are harvested during
the first season. Annuals and biennials are both generally
herbaceous plants. Perennials are plants that live from year to year.
Trees and shrubs are perennials, but some herbaceous plants are
also perennials.