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
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 frommicrospores.
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 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.
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.
Click on the images to view enlargements.
Phylum Gnetophyta (Gnetophytes)
Welwitschia has a deep taproot and a small exposed part with cones and leaves.
Flower parts are modified leaves. They develop within a bud.
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.
protect developing bud
The large colorful petals of many flowers function to attract pollinators.
Stamens are composed of an anther and a filament.
The anther contains microsporangia. Microspores and microgametophytes are produced within the anther.
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.
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.
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
Within the megasporangium:
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.
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.
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 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.
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.
Pollination and Coevolution
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-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.
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.
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.
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.
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.
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 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.
Identify each component on the diagram. Select from the list below.