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کاربر برگزیده علوم گیاهی
Seed Plants
Life Cycle of Seed Plants
Note - Many of the photographs below are linked to larger photographs. Click on a photographs if you wish to view an enlargement.
Seed plants are heterosporous- they have 2 different spore sizes: megaspores and microspores.
The generalized life cycle of plants has been modified (below) to illustrate plants which have separate male and female gametophytes (megagametophyte and microgametophyte) produced by different sized spores (megaspores and microspores).
The evolutionary trend from nonvascular plants to seedless vascular plants to seed plants has been a reduction in the size of the gametophyte. In seed plants, the gametophyte is usually microscopic and is retained within the tissues of the sporophyte.
The megasporangium is surrounded by layers of sporophyte tissue called the integument. The integument and structures within (megasporangium, megaspore) are the ovule.
Microspores germinate within the sporophyte tissue and become pollen grains. The microgametophyte is contained within the tough, protective coat of the pollen grain.
The entire microgametophyte (pollen grain) is transferred to the vicinity of the megagametophyte by a process of pollination. Wind or animals usually accomplish this transfer.
When pollen reaches the female gametophyte, it produces an elongate structure (pollen tube) that grows to the egg cell. Sperm are transferred directly through this tube to the egg. The advantage of this process is that sperm do not have to swim long distances as they do in seedless plants.
Seeds
Seeds contain the sporophyte embryo, food for the embryo, and a protective coat.
The embryo within the seed is dormant; it can survive for long periods without additional food or water. When conditions become favorable, the embryo resumes growth as the seed germinates.
GymnospermsThe generalized life cycle of plants has been modified (below) to illustrate plants which have separate male and female gametophytes (megagametophyte and microgametophyte) produced by different sized spores (megaspores and microspores).
The evolutionary trend from nonvascular plants to seedless vascular plants to seed plants has been a reduction in the size of the gametophyte. In seed plants, the gametophyte is usually microscopic and is retained within the tissues of the sporophyte.
The megasporangium is surrounded by layers of sporophyte tissue called the integument. The integument and structures within (megasporangium, megaspore) are the ovule.
Microspores germinate within the sporophyte tissue and become pollen grains. The microgametophyte is contained within the tough, protective coat of the pollen grain.
The entire microgametophyte (pollen grain) is transferred to the vicinity of the megagametophyte by a process of pollination. Wind or animals usually accomplish this transfer.
When pollen reaches the female gametophyte, it produces an elongate structure (pollen tube) that grows to the egg cell. Sperm are transferred directly through this tube to the egg. The advantage of this process is that sperm do not have to swim long distances as they do in seedless plants.
Seeds
Seeds contain the sporophyte embryo, food for the embryo, and a protective coat.
The embryo within the seed is dormant; it can survive for long periods without additional food or water. When conditions become favorable, the embryo resumes growth as the seed germinates.
The four phyla of gymnosperms are cycads, ginkgo, gnetophytes, and conifers.
Gymnosperms have ***** seeds. The seeds of angiosperms are contained within a fruit.
Phylum Coniferophyta (Conifers)Gymnosperms have ***** seeds. The seeds of angiosperms are contained within a fruit.
Conifers are the largest group of gymnosperms. They include evergreen trees such as pine, cedar, spruce, fir, and redwood trees.
They have ***** seeds produced in cones.
The leaves of conifers are needle-like and are adapted for dry conditions such as hot summers or freezing winters. Needles lose water slower than broad, flat leaves and therefore do not need to be shed during seasons when water is scarce, so most conifers are evergreen.
Conifers include the oldest and largest trees in the world. There are 4500-year-old bristlecone pines in Nevada. Redwoods in California may be greater than 90 meters tall and 2000 years old.
Reproduction in PineThey have ***** seeds produced in cones.
The leaves of conifers are needle-like and are adapted for dry conditions such as hot summers or freezing winters. Needles lose water slower than broad, flat leaves and therefore do not need to be shed during seasons when water is scarce, so most conifers are evergreen.
Conifers include the oldest and largest trees in the world. There are 4500-year-old bristlecone pines in Nevada. Redwoods in California may be greater than 90 meters tall and 2000 years old.
Microsporangia, Megasporangia
Spores (mega and micro) are produced by meiosis. Microspores are produced within protective structures called microsporangia; megaspores are produced within megasporangia.
Below: In pine, microsporangia are found within pollen cones.
Male gametophytes are produced from microspores.
The photograph below is a cross section of a pine microsporangium. The arrow points to a single microgametophyte (pollen grain). The wings aid it in being dispersed by the wind.
The pollen grains are eventually released from the sporophyte and carried by the wind to the vicinity of the egg. It will then produce sperm by mitosis because it is haploid.
Female gametophytes are produced from megaspores.
Female reproductive structures in pine are located in the seed cones (below).
Seed cones contain ovules. The structure diagrammed below is an ovule and will develop into a seed. Theintegument will become the seed coat.
Megasporocytes (megaspore mother cells) are cells contained within the ovule produce four megaspores by meiosis.
Three of the megaspores die.
The remaining one develops into a female gametophyte without being released from the megasporangium.
Female gametophytes function to produce eggs.
Below: Pine Megagametophyte. Two archegonia can be seen in this photograph.
Summary of Pine Life Cycle
Pollination and Fertilization
The male gametophyte (pollen grain) consists of two cells. One small and is called a generative cell. The other, larger cell is a tube cell. The generative cell will later divide to produce two sperm.
Pollination refers to the transfer of pollen to the vicinity of the egg. The two wing-like structures on the pollen grain aid in enabling the pollen to be carried by the wind.
After being transported by wind to a seed cone, the tube cell grows toward the egg, producing a pollen tube. The two sperm produced by the generative cell enter the pollen tube and move toward the egg.
Water is not required for reproduction. During pollination, the entire male gametophyte is transferred from the pollen cone to the seed cone. The sperm are not flagellated, so they remain within the tube cell and rely on the growth of a pollen tube to deliver them to the egg cell.
Seeds
The fertilized egg (zygote) develops into an embryo which is contained within the seed.
In moss and ferns, spores were carried by the wind and functioned to disperse the species. Seeds function as a mechanism of dispersal in seed plants.
Seeds contain food and a protective coat.
Gymnosperms are plants with ***** seeds (no fruit). Angiosperms (discussed below) are plants in which the seeds are enclosed within a fruit.
Phylum Cycadophyta (Cycads)Spores (mega and micro) are produced by meiosis. Microspores are produced within protective structures called microsporangia; megaspores are produced within megasporangia.
Below: In pine, microsporangia are found within pollen cones.
Male gametophytes are produced from microspores.
The photograph below is a cross section of a pine microsporangium. The arrow points to a single microgametophyte (pollen grain). The wings aid it in being dispersed by the wind.
The pollen grains are eventually released from the sporophyte and carried by the wind to the vicinity of the egg. It will then produce sperm by mitosis because it is haploid.
Female gametophytes are produced from megaspores.
Female reproductive structures in pine are located in the seed cones (below).
Seed cones contain ovules. The structure diagrammed below is an ovule and will develop into a seed. Theintegument will become the seed coat.
Megasporocytes (megaspore mother cells) are cells contained within the ovule produce four megaspores by meiosis.
Three of the megaspores die.
The remaining one develops into a female gametophyte without being released from the megasporangium.
Female gametophytes function to produce eggs.
Below: Pine Megagametophyte. Two archegonia can be seen in this photograph.
Summary of Pine Life Cycle
Pollination and Fertilization
The male gametophyte (pollen grain) consists of two cells. One small and is called a generative cell. The other, larger cell is a tube cell. The generative cell will later divide to produce two sperm.
Pollination refers to the transfer of pollen to the vicinity of the egg. The two wing-like structures on the pollen grain aid in enabling the pollen to be carried by the wind.
After being transported by wind to a seed cone, the tube cell grows toward the egg, producing a pollen tube. The two sperm produced by the generative cell enter the pollen tube and move toward the egg.
Water is not required for reproduction. During pollination, the entire male gametophyte is transferred from the pollen cone to the seed cone. The sperm are not flagellated, so they remain within the tube cell and rely on the growth of a pollen tube to deliver them to the egg cell.
Seeds
The fertilized egg (zygote) develops into an embryo which is contained within the seed.
In moss and ferns, spores were carried by the wind and functioned to disperse the species. Seeds function as a mechanism of dispersal in seed plants.
Seeds contain food and a protective coat.
Gymnosperms are plants with ***** seeds (no fruit). Angiosperms (discussed below) are plants in which the seeds are enclosed within a fruit.
Cycads re cone-bearing palm-like plants found mainly in tropical and subtropical regions today. They were very numerous in the Mesozoic Era.
Phylum Ginkgophyta (Ginkgo)
There is only one species left. It survived due to Chinese planting them along roadsides.
Click on the images to view enlargements.
Phylum Gnetophyta (Gnetophytes)Click on the images to view enlargements.
Welwitschia has a deep taproot and a small exposed part with cones and leaves.
Angiosperms
Angiosperms are flowering plants. They are the largest group of plants with about 90% of all plant species.
They evolved from gymnosperms during the Mesozoic and became widespread during the Cenozoic.
The seeds of angiosperms are covered by a fruit. In many species, the fruit helps with dispersal of the seeds by attracting animals to consume them.
Flowers may have contributed to the enormous success of angiosperms. The flowers of many species attract animal pollinators which carry pollen to other individuals of the same species.
Diversity of AngiospermsThey evolved from gymnosperms during the Mesozoic and became widespread during the Cenozoic.
The seeds of angiosperms are covered by a fruit. In many species, the fruit helps with dispersal of the seeds by attracting animals to consume them.
Flowers may have contributed to the enormous success of angiosperms. The flowers of many species attract animal pollinators which carry pollen to other individuals of the same species.
The oldest lineages of angiosperms are divided into three clades. The remaining lineages contain most flowering plants alive today. They are monocots, eudicots, and magnoliads.. The table below lists characteristics monocots and eudicots.
Life CycleEudicots | Monocots |
may be woody or herbaceous | herbaceous |
flower parts in multiples of four or five | flower parts in multiples of three |
net-veined leaves | parallel-veined leaves |
vascular tissue in the stem forms rings | bundles of vascular tissue are scattered throughout the stem |
two cotyledons (seed leaves) | one cotyledon |
The life cycle of flowering plants is similar to that of gymnosperms. It involves alternation of generations. A diploid sporophyte alternates with a haploid gametophyte.
Flower
Flower parts are modified leaves. They develop within a bud.
Flower parts evolved as modified leaves attached to a stem tip called a receptacle.
Monocots have flower parts in multiple of threes; eudicot parts are in multiples of fours or fives.
Below: Lily reproductive structures. These structures are described below.
Below: The stamens (anthers and filaments) and pistil (stigma, style, and ovary) have been removed from the receptacle.
Sepals
protect developing bud
Petals
The large colorful petals of many flowers function to attract pollinators.
Stamens
Stamens are composed of an anther and a filament.
The anther contains microsporangia. Microspores and microgametophytes are produced within the anther.
Ovules
Ovules are structures that will become seeds. They contain outer protective coverings called integuments and a megasporangium within the integuments. Within the megasporangium, megaspores are produced by meiosis. The megaspores produce megagametophytes, which, in turn, produce eggs.
A bud is a structure on a stem within which growth (cell division) occurs.
In many plants the same bud that previously formed leaves stops producing leaves and starts producing a flower.
Flower parts evolved as modified leaves attached to a stem tip called a receptacle.
Monocots have flower parts in multiple of threes; eudicot parts are in multiples of fours or fives.
Below: Lily reproductive structures. These structures are described below.
Below: The stamens (anthers and filaments) and pistil (stigma, style, and ovary) have been removed from the receptacle.
Sepals
protect developing bud
Petals
The large colorful petals of many flowers function to attract pollinators.
Stamens
Stamens are composed of an anther and a filament.
The anther contains microsporangia. Microspores and microgametophytes are produced within the anther.
Ovules
Ovules are structures that will become seeds. They contain outer protective coverings called integuments and a megasporangium within the integuments. Within the megasporangium, megaspores are produced by meiosis. The megaspores produce megagametophytes, which, in turn, produce eggs.