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SEED PLANTS: ANGIOSPERMS
Chapter 23
The most obvious characteristic of an angiosperm is the flower.
The flower contains the reproductive parts of the plant.
The name angiosperm is derived from the Greek angeion, means vessel, and sperma means seed.
Angiosperms are divided into two classes, the Dicots (Magnoliopsida) and the Monocots (Liliopsida).
The paleoherbs are a small group of flowering plants which have traditionally been classified as dicots,
but which have many characters in common with monocots.
True dicots are called eudicots.
SEED
1.
2.
3.
4.
5.
Multicellular embryo
Food supplied by tissue
Multicellular seed coat
Diploid sporophyte
Product of fertilization
SPORE
Single cell
Food only in the cell
Covering not cellular
Haploid cell
Product of meiosis
DIVISION MAGNOLIOPHYTA
There are about 235,000 species of angiosperms.
It is the largest and most diverse group of photosynthetic organisms.
Angiosperms are heterosporangiate, producing pollen and ovules in different organs.
The pollen and ovule-bearing organs are usually produced together in a bisporangiate strobilus called a
flower.
Angiosperms differ from other seed plants in that they enclose their ovules (and seeds) within a carpel.
Other unique features of the angiosperms are the production of fruits and distinctive life-cycle features.
There are about 200 species of parasitic monocots and 2800 species of parasitic eudicots.
Most angiosperms are free living, but there are a few saprophytic and parasitic species.
Unique characteristics of the angiosperms are:
1.
2.
3.
4.
Flower
Close carpels.
Double fertilization producing endosperm and embryo.
Three-nucleate microgametophyte.
5. Eight-nucleate magagametophyte.
6. Stamens with pairs of pollen sacs.
7. Presence of sieve tubes and companion cells in phloem.
DEVELOPMENT OF GAMETOPHYTES
The most important reproductive adaptations of seed plants are:
1. Continued reduction of the gametophyte
2. Evolution of the seed.
3. Evolution of the pollen.
The reduced gametophyte can develop from spores retained within the sporangia of the parental
sporophyte.
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Retaining the female gametophyte within the sporangium protects it from environmental stresses.
The gametophyte obtains food from the sporophyte.
All seed plants have two different kinds of spores. They are heterosporous.
1. Megasporangium produces megaspores, which produces female gametophytes.
2. Microsporangium produces microspores, which produces male gametophytes.
In seed plants, the megaspores and the female gametophyte are retained on the parent sporophyte.
Layers of sporophyte tissue envelop the megasporangium. These tissues are called integuments.
The megasporangium, megaspore and integuments are called the ovule.
The female gametophyte develops inside the megaspore and produces one or more egg cells.
Megasporogenesis
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The ovule consists of a stalk or funiculus bearing the nucellus surrounded by one or two
integuments.
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The nucellus is equivalent to the megasporangium.
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Depending on the species, one or two ovules may arise from the plancentae.
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The integuments leave one small opening at one end of the ovule, the micropyle.
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Inside the ovule a single megasporocyte develops, undergoes meiosis, and produces four haploid
megaspores.
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Three of these degenerate, while the fourth undergoes three mitotic divisions to produce an eightnucleate embryo sac, or mature megagametophyte.
 Megasporocyte → meiosis → tetrads → functional megaspore
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Functional megaspore enlarges at the expense of the nucellus and the nucleus divides mitotically.
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At the end of the third mitotic division eight nuclei are arranged in two groups of four, one at the
micropylar end and the other at the chalazal end.
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Polar nuclei migrate to the center of the megagametophyte and become surrounded by a cell
membrane.
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At the chalazal end, the three nuclei become the antipodal cells after the formation of cell membranes
around them.
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The egg apparatus is formed at the micropylar end.
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At this point there are eight nuclei and seven cells.
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There are several patterns of embryo sac development.
Megasporangium → megaspores → female gametophytes → egg cell.
Microsporogenesis
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Four pollen sacs develop in each anther; two pollen sacs in each lobe.
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A layer of sterile cells surrounds the sporogenous tissue and nutritive cell.
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The sterile cells become the wall of the microsporangium or pollen sac.
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The nutritive tissue or tapetum is also located within the wall of the pollen sac.
 Sporogenous tissue → microsporocytes → meiosis → tetrad → microspores
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The spore wall may form after each nuclear division or after the second meiotic division.
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Pollen grains develop a resistant outer wall, the exine, and an inner wall, the intine.
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The exine is composed of sporopollenin and it is derived from the tapetum.
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The intine is made of cellulose and pectin and is laid down by the microspore protoplast.
Many pollen grains have three boat-shaped or pore-like thin areas of the pollen wall called apertures.
Microgametogenesis
→ Large tube nucleus (vegetative nucleus)
Microspore
→ Sperm cell
→ Small generative nucleus
→ Sperm cell
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The larger tube cells contain the generative cell.
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In most flowering plants, pollination occurs at the two-cell stage.
 Microsporangium → microspores → male gametophytes (pollen) → sperms.
POLLINATION
Pollen does not need a liquid for fertilization.
Pollination is the transfer of pollen to ovules.
Wind and animals carry out pollination.
Pollen grains are well protected by sporopollenin.
Sporopollenin contains only carbon, hydrogen and oxygen; it is an isoprene polymer.
FERTILIZATION AND DEVELOPMENT OF THE SEED
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Pollination is the transfer of pollen to the stigma.
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Fertilization is the fusion of gametes.
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It can occur in several different ways.
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The surface of the stigma is usually glandular and it is connected to the ovule by the transmitting
tissue.
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Once on the stigma the pollen grain germinates by forming the pollen tube.
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The pollen tube grows down the style and into the ovary via the micropyle.
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The pollen tube enters the ovule through the micropyle and penetrates one of the synergids releasing
the two sperm nuclei into it.
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Eventually one sperm nucleus enters the egg and the other joins the central cell containing the two
polar nuclei.
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The fusion with the central cell results in the fusion of three nuclei, the triploid primary endosperm
nucleus.
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The remaining synergid and antipodals degenerate.
Gnetum and Ephedra have double fertilization resulting in two embryos, one of which degenerates.
Double fertilization probably existed already in the common ancestor of angiosperms and gnetophytes.
The endosperm can be regarded as the modified extra embryo that became the nourishing tissue of the
first embryo.
Development of the seed
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Primary endosperm nucleus forms the endosperm.
Zygote develops into an embryo.
The integuments develop into the seed coat.
The ovary wall and related structures develop into the fruit.
Storage tissue.
Endosperm development may occur in a variety of way:
1. Nuclear: free nuclear divisions before wall formation.
2. Cellular: wall formation after citokinesis,
3. Helobial: first division is followed by wall formation but subsequent divisions are free nuclear.
In some groups of angiosperms, the nucellus develops into a food storage tissue, the perisperm.
Some seeds contain perisperm and endosperm.
In most eudicots and a few monocots, the embryo absorbs all the storage tissues before the seed becomes
dormant.
The cotyledons become the new storage tissue.
In gymnosperms the storage tissue is formed before fertilization and in angiosperms after fertilization.
In four phyla of gymnosperms, the megagametophyte provides the nutritive tissue.
In angiosperms it is the endosperm which is neither gametophyte nor sporophyte.
OTHER TYPES OF FEMALE GAMETOPHYTE DEVELOPMENT
The process of gametophyte development described above occurs in about 70% of the known flowering
plants.
The remaining 30% exhibit variation in which the female gametophyte has from 4 to 16 nuclei or cells at
maturity, and the endosperm may be 5n, 9n or even 15n.
APOMIXIS AND PARTHENOCARPY
Apomixis is the development of an embryo from a diploid cell of the nucellus (megasporangium).
Some embryos develop from a diploid (2N) nutritive cell or other diploid cell of the ovule.
There is no fertilization in apomixis, that is, no fusion of gametes.
Other structures are involved, e. g. ovary.
The fruit that develops is said to be parthenocarpic.
The new plant is a clone of the mother as in the case of asexual reproduction.
TRENDS OF SPECIALIZATION AND CLASSIFICATION IN FLOWERING PLANTS
First land plants appeared probably in the late Ordovician about 450 million years ago.
Rhynia appeared in the mid-Silurian record about 425 million years ago.
Trimerophytes first appeared in the early Devonian about 395 million years ago.
Ferns, lycopods, spenophytes and progymnosperms proliferated in the late Devonian and Carboniferous,
380 to about 280 million years ago.
Gymnosperms diversified during the Mesozoic, 250 to 65 million years ago.
Angiosperms first appeared about 130 million years ago in the early Cretaceous.
There are some debatable records dating as far back as the Jurassic (175 – 130 million years ago).
Botanists hypothesize that the primitive angiosperm had the following characteristic:
Simple leaves, and the flower had…
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Numerous unfused parts
Spirally arranged parts
Radial symmetry
Stamens and pistils present
Unilocular follicles containing 1 seed
EVOLUTIONARY TRENDS
In both monocots and dicots, evolutionary specialization has involved the following:
1. From few to many parts and indefinite number to few parts and definite number.
2. Reduction in the number of whorls from four to three, two or one in the most advanced. The floral
axis has become shortened so the spiral arrangement is not evident. Floral parts often have become
fused.
3. The ovary has changed from superior (hypogynous) to inferior (epigynous), and the perianth has
become differentiated into a distinct calyx and corolla.
4. Symmetry of floral parts has changed from radial (actinomorphic) to bilateral (irregular or
zygomorphic).
When on of the whorls is missing, the flower is said to be incomplete.
When either the stamens or pistils are absent, the flower is said to be imperfect that is unisexual.
When both male and female imperfect flowers occur on the same plant, the species is monoecious.
If a plant bears only male or female flowers, the species is said to dioecious.
Plants with complete flowers are hermaphroditic.
POLLNATION ECOLOGY
Coevolution is the process by which two or more species act as selective forces on one another and each
undergoes evolutionary change.
1. Plants evolved flower characteristics that attract insect.
2. Insects also evolved specialized anatomical structures of obtaining pollen and nectar and help in
pollination.
Bisexual flowers have the advantage that visiting insects can pick up pollen from the anthers and deliver
pollen from a neighboring plant at the same time.
In the early part of the Tertiary, 40 - 60 million years ago, specialized groups of flower-visiting insects
became more diverse.
Each group of flower visiting animal is associated with a set of flower characteristic related to the
animal's visual and olfactory senses.
Bee-pollinated flowers
Bees are the most important group of pollinators responsible for the pollination of more species than any
other group of pollinators.
Modern families of bees have existed for about 80 million years and evolved together with the
angiosperms.
Bees of both sexes feed on nectar and pollen and are adapted to carry them.
Bees can perceive ultraviolet light but cannot see red, which merges with the background.
Many species of bee are solitary and specialized to feed on one or a few species of plants.
Bee species often exhibit conspicuous adaptations that allow them to collect pollen and nectar.
There are about 20,000 species of bees, the majority of which visit flowers for food.
Bees exert a strong evolutionary selective force for the specialization of the plants they visit.
Bee flowers have showy blue or yellow petals often arranged in distinct patterns.
Many flowers have distinct markings that attract and guide the bee to the source of food; many of these
marking are invisible to humans.
The nectary is normally situated at the base of the corolla tube accessible to the mouthparts of bees but
not the chewing parts of beetles.
The bee flower usually has a landing platform of sorts.
Some orchids have developed a long passageway that forces the bee to follow a route in and out of the
flower and ensures that both anther and stigma come into contact with the bee's body.
Some orchids, genus Ophrys, resemble the female of a wasp, bee or fly and the males attempt to copulate
with them. In the process they pick up the pollinium and transfer it to another flower.
Beetle-pollinated flowers.
Beetles have a more developed sense of smell than the visual sense.
Flowers are usually white or dull in color with a strong scent, which could be fruity, spicy or foul.
The beetles may feed on nectar, petals, pollen or other flower parts.
Flowers have inferior ovaries deeply buried in floral tissue out of the reach of the beetles.
Moth and Butterfly pollinated flowers.
These flowers often have long corolla tube.
Flowers that coevolved with butterflies and diurnal moths resemble the bee flowers.
Moths and butterflies like bees are guided by a combination of sight and smell.
Some species of butterflies can see red and orange and pollinate flowers of those colors.
Moths are mostly nocturnal and their flowers are strongly scented.
Butterflies and moths have long mouthparts and they are used to suck nectar located at the base of long
tubular corollas or spurs.
Bird pollinated flowers.
Hummingbirds are the chief bird pollinators in North and South America.
Bird flowers have bright red and yellow color since birds have a keen sense of sight.
These flowers normally produce large volumes of nectar in tubes that are unavailable to small animals.
They have little odor because birds do not have a well developed sense of smell.
These flowers are large or form part of large inflorescences because they have to attract by sight and be
able to produce large amount
Bat pollinated flowers
These flowers produce copious amount of nectar and have dull colors and strong odors.
About 250 species of bat include some pollen, nectar or fruit in their diet.
These species of bats have long slender snouts with long extensible tongues often with brush-like tips.
Their front teeth are reduced in size or missing altogether.
The flowers tend to be large and robust and produce large amount of nectar.
Bats typically feed at night and the flowers are dull in color and open only at night.
Often these flowers are tubular and protect their nectar.
These flowers often hang down on long stalks below the foliage. Other flowers grow attached to the trunk
in a type of inflorescence called cauliflory.
Bat flowers are strongly scented with a fermenting or fruity odor.
Bats pollinate or disperse the seeds of about 130 genera of angiosperms.
Pollen consumed by bats has higher level of protein than does pollen of insect-pollinated flowers.
Interesting sites
http://biology.clc.uc.edu/courses/bio303/coevolution.htm
http://biology.clc.uc.edu/courses/bio106/pollinat.htm