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Seed Plants

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Life Cycle of Seed Plants

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.

Gymnosperms

The four phyla of gymnosperms are cycads, ginkgo, gnetophytes, and conifers.

Gymnosperms have naked seeds. The seeds of angiosperms are contained within a fruit.

Phylum Coniferophyta (Conifers)

Conifers are the largest group of gymnosperms. They include evergreen trees such as pine, cedar, spruce, fir, and redwood trees.

They have naked 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 Pine

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. The integument 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 naked seeds (no fruit). Angiosperms (discussed below) are plants in which the seeds are enclosed within a fruit.

Phylum Cycadophyta (Cycads)

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.

ginkgo.jpg (321798 bytes) ginko leaves.jpg (247000 bytes)

Click on the images to view enlargements.

Phylum Gnetophyta (Gnetophytes)

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 Angiosperms

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.

EudicotsMonocots
may be woody or herbaceousherbaceous
flower parts in multiples of four or fiveflower parts in multiples of three
net-veined leavesparallel-veined leaves
vascular tissue in the stem forms ringsbundles of vascular tissue are scattered throughout the stem
two cotyledons (seed leaves)one cotyledon

Life Cycle

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.

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.

Pistil

All of the female reproductive structures form the pistil. This includes the stigma, style, and ovary.

Each chamber within a pistil is called a carpel. It evolved from a leaf that contained sporangia on its edges. Over evolutionary time, the leaf became curled to enclose the sporangia as seen in carpels today.

A simple pistil is also called a carpel because it has only one chamber.

A compound pistil contains several carpels that have become fused as a result of evolutionary change.

Ovary

The bottom portion of a pistil is the ovary. It contains ovules. As the reproductive process proceeds, the ovary enlarges and becomes the fruit and the ovules become seeds.

Below: Lily Pistil and Close-Up of Ovary

Below: Cross Section of a Lily ovary X 40

Development of Gametophytes

The diagram below shows the parts of the life cycle that are located within the ovary and within the anther. Structures within the ovary are labeled in red and structures within the anther are in blue. The sporophyte is represented by black. Refer to this diagram as you read the discussion below.

Within the megasporangium:

A sporangium is a structure that produces spores (see the diagram of an ovule below). Two protective layers called integuments surround the megasporangium of flowering plants (angiosperms). The entire structure including the integuments is the ovule and is destined to become the seed. The integuments will become the seed coat.

The diploid cell within the megasporangium will divide by meiosis to produce four megaspores. 

Three of the megaspores disintegrate.

The remaining megaspore nucleus divides 3 times to produce a cell with 8 nuclei. Cell walls form around them producing a gametophyte that has 7 cells and 8 nuclei. One of the cells contains two nuclei (see diagram below).. These two nuclei, called polar nuclei, will be discussed later. The micropyle is the opening in the integuments near the egg cell. Eventually, sperm pass through this opening.

Within the microsporangium:

An anther has 4 microsporangia (pollen sacs). Each contains many microsporocytes that will divide by meiosis to produce 4 microspores each.

The diagrams below show a cross-section of an anther at three different stages of development. Initially, microsporangia contain diploid cells. The sporangia and cells are part of the sporophyte (2N) plant.

Microspores are produced by meiosis.

The microspore produces a 2-celled microgametophyte.

Microsporangia rupture to release pollen.

Pollination is the transfer of pollen to the stigma.

Pollen

Pollen contains two nuclei, a generative nucleus and a tube nucleus. A membrane surrounds the generative nucleus and so it is technically a cell, but it contains very little cytoplasm. The generative cell is contained within the larger tube cell.

Below: Lily pollen X 200

Pollination and Fertilization

After landing on the stigma of a flower (pollination), the tube cell elongates to produce a pollen tube, which grows from the stigma through the style and through the micropyle to the egg. The generative cell will divide by mitosis to produce two sperm. As in gymnosperms, the sperm of angiosperms are contained within the pollen tube and therefore do not require water.

Below: Germinated pollen

Fertilization

Double Fertilization: One sperm fertilizes the egg the other one combines with the two polar nuclei forming a triploid (3N) cell.

The zygote grows by mitosis to form an embryo.

The 3N cell divides by mitosis and becomes endosperm, a food-containing material for the developing embryo.

The ovary, sometimes with other floral parts, develops into a fruit. It usually contains seeds.

Embryonic Development

The suspensor anchors and transfers nutrients to the developing embryo.

In eudicots, two heart-shaped cotyledons develop and absorb endosperm, which will be used as food when the seed germinates.

Monocot cotyledons do not store endosperm. Instead, when the seed germinates, the cotyledon absorbs and transfers nutrients to the embryo.

Below: Capsella embryo X 40. Capsella is a eudicot. The two cotyledons (seed leaves) can be seen. When the seed germinates, the cotyledons will provide the embryo with stored food. Shortly after germination, their food stores are used up and the cotyledons are shed. The shoot apical meristem is the terminal bud of the shoot (above-ground portion). The root apical meristem becomes the growing tip of the root.

Dormancy

Dormancy is a state in which the metabolic rate (rate of chemical reactions within the cell) slow down. The tissues within a seed become dormant, and as a result, they require very little food, oxygen, or water. 

The seeds of some species can survive for many years when they are dormant. Some seeds will not germinate until after a period of dormancy.

Seed Dispersal

hooks and spines, float (coconuts), parachutes

Seeds may be dispersed when animals eat the fruits. For example, squirrels bury them for later consumption but do not always retrieve all of them.

Germination

Germination usually requires sufficient water, warmth, and oxygen.

Pollination and Coevolution

Gymnosperms are wind-pollinated.

Angiosperms are wind- or animal-pollinated.

Origin of pollination vectors

Pollinators carry plant pollen to other plants. Pollination was by wind when plants first invaded the land 400 million years ago.

Pollen is a good source of proteins and insects evolved in response to the new food supply. Plants benefited by becoming pollinated, insects benefited by receiving food.

Both species evolved to facilitate the pollination relationship. This is referred to as coevolution.

Plants evolved ability to secrete nectar, a liquid rich in sugar, proteins, and lipids. Nectar functions to attract pollinators.

Flowers attract specific pollinators

Flowers are typically shaped so that their pollinators can gain access to the nectar but other species cannot. Some examples of strategies that flowers use to attract pollinators and to limit their access to only their pollinators are discussed below.

Birds are attracted to red flowers. Bees can see colors that humans cannot. Moth-pollinated flowers are white and bloom at night.

Many insects are attracted to odors. For example, stapelia smells like rotting meat and is pollinated by flies.

Shape is important in limiting access. Flowers are often shaped to limit access. For example, hummingbird-pollinated flowers are long, and shaped like the bill of a hummingbird.

Wind Pollination

Wind-pollinated flowers are small, have no petals and little color and do not produce nectar.

Below: Many grasses are wind pollinated. The flowers are typically small and not very colorful because they do not need to attract animal pollinators.

Fruits

The ovary of flowering plants becomes the fruit. Seeds are contained within the fruit. Gymnosperms do not produce fruit. 

The wall of the ovary thickens to become the pericarp of the fruit.

Fruits can be either fleshy or dry. Peaches, tomatoes, and oranges are fleshy fruits. Nuts and grains are dry fruits.

Asexual Reproduction

Stems

New plants can grow from horizontal stems.

Aboveground horizontal stems are called stolons (runners).

Underground horizontal stems are called rhizomes.

White potatoes are underground stems. They eyes are buds and can be used to produce new plants.

Roots

Sweet potatoes are modified roots and can be used to produce new plants.

The roots of some trees (apple, cherry) produce suckers (small plants) that can produce a new tree.

Cutting

Cut stems can be treated with hormones to encourage root growth.

Stems can be grafted to plants that have roots.

Axillary buds can be grafted to another plant to produce new branches from the grafted bud.

Tissue Culture

Plant tissue is grown on culture medium and treated with hormones to stimulate the cells to grow into plants.

Many plants can be produced from a few cells.

Genetic Engineering

Genetic engineering is concerned with modifying the DNA of organisms. Plants have been produced that are resistant to freezing, infections, insect pests, herbicides, and spoilage.

Transgenic plants contain DNA from a different species.

Comparisons Between Plants

Mosses have a dominant gametophyte (haploid) and a small dependent sporophyte (diploid), and are adapted to moist environments. Bryophytes and seedless plants have flagellated sperm that require water).

Ferns (seedless plants) have larger, more dominant sporophytes but still require wet conditions because of the tiny gametophyte and swimming sperm.

Gymnosperms and angiosperms are widespread and well adapted to land because of their large sporophytes and the coverings of the spores, gametophytes, gametes, zygotes, and embryos. Their sperm do not require water.

Activities

Identify each component on the diagram. Select from the list below.

terrestrial plants

bryophytes (nonvascular plants)

vascular plants

seedless plants

seed plants

gymnosperms

angiosperms

monocots

eudicots

Draw the sporophyte and gametophyte of moss, fern, pine, and an angiosperm. Note that gymnosperms and angiosperms have male and female gametophytes.

Moss Gametophyte

Moss Sporophyte

Fern Gametophyte

Fern Sporophyte

Pine Microgametophyte

Pine Megagametophyte

Pine Sporophyte

Angiosperm Microgametophyte

Angiosperm Megagametophyte

Angiosperm Sporophyte (any)