Chapter 30. Plant Diversity II Seed Plants. Evolution of Seed Plants (angiosperms and gymnosperms). Seeds changed the course of plant evolution Enabling their bearers to become the dominant producers in most terrestrial ecosystems Seed plants arose about 360 million years ago.
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Plant Diversity II
Seeds changed the course of plant evolution
(mosses and other
Large sporophyte and
and other seedless
Reduced gametophyte dependent on sporophyte
(seed plants: gymnosperms and angiosperms).
Seedless Vascular Plant
gametophytes (n) in
inside these parts
inside these parts
in pollen cones
the flowering plant
(a) Unfertilized ovule. In this sectional view through the ovule of a pine (a gymnosperm), a fleshy megasporangium is surrounded by a protective layer of tissue called an integument. (Angiosperms have two integuments.)Ovules and Production of Eggs
Egg nucleus (n)
Sperm of seed plants does not require water or motility because the pollen tube conveys the sperm to the egg
When a pollen grain germinates it grows a pollen tube.
pollen grain) (n)
sperm nucleus (n)
Pollen grain (n)
(b) Fertilized ovule. A megaspore develops into a multicellular female gametophyte. The micropyle,the only opening through the integument, allowsentry of a pollen grain. The pollen grain contains amale gametophyte, which develops a pollen tubethat discharges sperm.
Gymnosperm seed. Fertilization initiatesthe transformation of the ovule into a seed,which consists of a sporophyte embryo, a food supply, and a protective seed coat derived from the integument.Seeds
Cycads: Large cones and palm-like leaves. Thrived during Mesozoic era.
Ginkgo biloba is the only extant species of this phylum
This phylum contains three genera of tropical or desert plants
An ovulate cone scale has two
ovules, each containing a mega-
sporangium. Only one ovule is shown.
each tree has
A pollen grain
forming a pollen
tube that slowly
haploid cells. One
survives as a
A pollen cone contains many microsporangia
held in sporophylls. Each microsporangium
contains microsporocytes (microspore mother
cells). These undergo meiosis, giving rise to
haploid microspores that develop into
Seeds on surface
of ovulate scale
pollen grain (n)
The female gametophyte
develops within the megaspore
and contains two or three
archegonia, each with an egg.
Fertilization usually occurs more than a year after pollination. All eggs may be fertilized, but usually only one zygote develops into an embryo. The ovule becomes a seed, consisting of an
embryo, food supply, and seed coat.
sperm nucleus (n)
By the time the eggs are mature,
two sperm cells have developed in the
pollen tube, which extends to the
female gametophyte. Fertilization occurs
when sperm and egg nuclei unite.
Egg nucleus (n)Life Cycle of a Pine
(b) Ruby grapefruit, a fleshy fruitwith a hard outer layer andsoft inner layer of pericarp
(a) Tomato, a fleshy fruit withsoft outer and inner layersof pericarp
(c) Nectarine, a fleshyfruit with a soft outerlayer and hard innerlayer (pit) of pericarp
(d) Milkweed, a dry fruit thatsplits open at maturity
(e) Walnut, a dry fruit that remains closed at maturityFruits
Typically consist of a mature ovary
Pericarp is the thickened wall of the fruit.
Wings enable maple fruits
to be easily carried by the wind.
Seeds within berries and other
edible fruits are often dispersed
in animal feces.
The barbs of cockleburs
facilitate seed dispersal by
allowing the fruits to
“hitchhike” on animals.Fruit adaptations can enhance seed dispersal
Anthers contain microsporangia.
Each microsporangium contains micro-
sporocytes (microspore mother cells) that
divide by meiosis, producing microspores.
pollen grains (containing
male gametophytes). The
generative cell will divide
to form two sperm. The
tube cell will produce the
Mature flower on
When a seed
into a mature
(in pollen grain)
In the megasporangium
of each ovule, the
megasporocyte divides by
meiosis and produces four
megaspores. The surviving
megaspore in each ovule
forms a female gametophyte
develops into an
embryo that is
with food into a
seed. (The fruit
ing the seed are
Seed coat (2n)
two sperm nuclei
are discharged in
Double fertilization occurs. One sperm
fertilizes the egg, forming a zygote. The
other sperm combines with the two polar
nuclei to form the nucleus of the endosperm,
which is triploid in this example.
sperm nuclei (n)
The Angiosperm Life Cycle
Early embryo has a rudimentary root and Cotyledons (seed leaves)
Pygmy date palm
Dog rose (Rosa canina), a wild rose
(no main root)
Taproot (main root)
Barley (Hordeum vulgare),
Pea (Lathyrus nervosus,
Lord Anson’sblue pea), a legume
Pollen grain with
Pollen grain with
(left) andmale flowers
multiples of three
Floral organs usually
in multiples of
four or five
(c) A flower pollinated by nocturnal animals. Some angiosperms, such as this cactus, depend mainly on nocturnal pollinators, including bats. Common adaptations of such plants include large, light-colored, highly fragrant flowers that nighttime pollinators can locate.
(a) A flower pollinated by honeybees. This honeybee is harvesting pollen and nectar (a sugary solution secreted by flower glands) from a Scottish broom flower. The flower has a tripping mechanism that arches the stamens over the beeand dusts it with pollen, some ofwhich will rub off onto the stigmaof the next flower the bee visits.
(b) A flower pollinated by hummingbirds.The long, thin beak and tongue of this rufous hummingbird enable the animal to probe flowers that secrete nectar deep within floral tubes. Before the hummer leaves, anthers will dust its beak and head feathers with pollen. Many flowers that are pollinated by birds are red or pink, colors to which bird eyes are especially sensitive.Evolutionary Links Between Angiosperms and Animals