Transfer of pollen grain from anther to stigma via biotic or abiotic vectors.
Delivery of sperm to ovule by pollen tube and fertilization of the egg.
Development of ovule into a seed containing embryo and food supply.
Development of ovary into a fruit containing seeds for dispersal.
Development of the embryo into a plant upon seed germination.
Key Features of Angiosperm Life Cycle
The sporophyte is the dominant generation in angiosperms.
Gametophytes are reduced to a few cells and depend on the sporophyte for nutrients.
The angiosperm life cycle is characterized by flowers, double fertilization, and fruits.
Flower Structure and Function
Flowers are reproductive shoots of angiosperm sporophytes.
They consist of four whorls of floral organs attached to the stem at the receptacle.
Carpels and stamens are reproductive organs; petals and sepals are sterile.
Carpels
Female reproductive organs (megasporophylls).
A carpel has a long style with a stigma at the top that captures pollen.
At the base of the style is an ovary containing one or more ovules.
Flowers can have single or multiple carpels; multiple carpels may fuse together.
A single carpel is a simple pistil; fused carpels form a compound pistil.
Stamens
Male reproductive organs (microsporophylls).
A stamen consists of a stalk called a filament topped by an anther.
Anthers contain microsporangia (pollen sacs) that produce pollen.
Petals
Are brightly colored to attract pollinators.
Sepals
Resemble leaves and enclose/protect unopened flower buds.
Complete vs. Incomplete Flowers
Complete flowers contain all four floral organs.
Incomplete flowers lack one or more floral organs (e.g., grass flowers lack petals).
Some incomplete flowers are sterile (lacking stamens and carpels).
Clusters of flowers are called inflorescences.
Angiosperm Life Cycle Overview
Major stages:
Gametophyte development
Pollination
Double fertilization
Seed development
Gametophyte Development
Gametophytes have evolved to become smaller and dependent on the sporophyte.
Angiosperm gametophytes consist of a few cells surrounded by sporophyte tissues.
Female Gametophyte (Embryo Sac) Development
Develops within each ovule, inside the megasporangium.
Two integuments (protective tissues) surround the megasporangium, except at the micropyle (gap).
One cell in the megasporangium undergoes meiosis, producing four haploid megaspores.
Only one megaspore survives; the others degenerate.
The surviving megaspore divides, producing one large cell with eight haploid nuclei.
Membranes divide the cell, forming the embryo sac.
The eight nuclei form:
An egg and two synergid cells near the micropyle.
Three antipodal cells at the opposite end of the embryo sac (function unknown).
Two polar nuclei in the cytoplasm of the embryo sac.
Synergid cells attract and guide the pollen tube to the embryo sac.
Male Gametophyte Development in Pollen Grains
Diploid cells in the microsporangia (pollen sacs) of anthers undergo meiosis to produce microspores.
Each microspore undergoes mitosis to produce two cells: the generative cell and the tube cell.
A pollen grain (male gametophyte) consists of a spore wall surrounding a generative cell and a tube cell.
Pollination
Occurs when a pollen grain is transferred to a receptive stigma.
The pollen grain germinates and produces a pollen tube.
The nucleus of the generative cell divides to produce two sperm as the pollen tube grows down the style.
The pollen tube grows toward the micropyle in response to chemical attractants released by synergids.
One synergid dies when the pollen tube reaches the ovary, making a path into the embryo sac.
The tube nucleus degenerates, and the sperm cells are discharged into the embryo sac.
Double Fertilization
Fertilization, the fusion of gametes, occurs after the two sperm reach the female gametophyte.
One sperm fertilizes the egg, forming the zygote.
The other sperm combines with the polar nuclei, giving rise to the triploid (3n) food-storing endosperm.
Double fertilization ensures endosperm only develops in ovules containing fertilized eggs.
Seed Development
Each ovule develops into a seed after double fertilization.
The ovary develops into a fruit, enclosing the seeds and aiding in wind or animal dispersal.
The seed stockpiles proteins, oils, and starch reserves.
When a seed germinates, the embryo develops into a new sporophyte.
Pollination Mechanisms
Pollen can be transferred by wind, water, or animals.
Approximately 80% of angiosperm pollination is biotic, using animal pollinators.
Among abiotically pollinated species, 98% rely on wind and only 2% on water.
Coevolution
Coevolution is the joint evolution of two interacting species in response to selection imposed by each other.
Many angiosperms have coevolved with their pollinators (e.g., insects with long proboscises and flowers with longer floral tubes).
Seed Development and Structure
Seeds begin to form after successful pollination and double fertilization.
A mature seed consists of a dormant embryo surrounded by stored food and protective layers.
Endosperm Development
Endosperm development usually precedes embryo development.
Endosperm nutrients can be used by the seedling after germination in most monocots and many eudicots.
In other eudicots, the food reserves of the endosperm are exported to the cotyledons before the seed fully develops.
Structure of the Mature Seed
The embryo and its food supply are enclosed by a hard, protective seed coat formed from the integuments.
The seed enters a state of dormancy, which slows growth and metabolism.
A mature seed is typically 5–15% water by weight.
Seed Dormancy
Breaking seed dormancy occurs in response to environmental cues to ensure optimal growth conditions.
Seeds of many desert plants germinate only after heavy rainfall that saturates the soil.
Seeds can remain dormant for days to decades; most are viable for at least a year or two.
The oldest seed to grow a viable plant was a 2,000-year-old date palm seed.
Most soils accumulate a bank of ungerminated seeds that can rapidly repopulate an area following disturbance.
Fruit Structure and Function
A fruit is the mature ovary of a flower that protects the seeds and aids in dispersal.
Fertilization triggers hormonal changes that trigger fruit formation; unpollinated flowers do not develop fruit.
The ovary dries out at maturity in some fruits; in others, the ovary is fleshy and sweet.
Fruit Classification
Fruits are classified by their developmental origins:
Simple fruits develop from a single or several fused carpels.
Aggregate fruits develop from a single flower with multiple separate carpels.
Multiple fruits develop from a group of flowers called an inflorescence.
Accessory fruits include other floral parts in addition to the ovary.
Fruit Ripening
Fruit ripening usually corresponds to the completion of seed development.
In dry fruits, ripening involves tissues drying out.
Fleshy fruits become softer, develop bright colors (red, orange, or yellow), and convert starch and organic acids to sugar to attract animal dispersers.
Seed Dispersal
Seeds must be dispersed away from the parent plant to successfully compete for nutrients and light.
Seeds and fruits are dispersed by abiotic (water or wind) or biotic (animals) mechanisms.
Asexual Reproduction in Flowering Plants
Asexual reproduction produces offspring from a single parent without fusion of egg and sperm.
The resulting offspring is a clone that is genetically identical to the parent.
Asexual reproduction is common in plants, and for some species, the predominant mode of reproduction.
Mechanisms of Asexual Reproduction
Fragmentation occurs when a parent plant separates into parts that develop into whole plants.
Adventitious shoots from the root system of a parent plant can form separate shoot systems (e.g., aspen trees).
Advantages and Disadvantages of Asexual vs. Sexual Reproduction
Asexual reproduction (vegetative reproduction) involves offspring arising from mature vegetative fragments of the parent plant.
Asexual reproduction can be beneficial in a stable environment because all of the parent’s genes are efficiently passed on to the offspring.
However, clones are vulnerable to local extinction if there is an environmental change.
Sexual reproduction generates genetic variation that makes evolutionary adaptation possible.
However, only a fraction of seedlings survive.
Some flowers can self-fertilize to ensure that every ovule will develop into a seed.
Many species have evolved mechanisms to prevent “selfing”.
Mechanisms That Prevent Self-Fertilization
Many angiosperms have mechanisms that make it difficult or impossible for a flower to self-fertilize.
This contributes to genetic variation by ensuring sperm and egg are from different parents.
Dioecious species avoid self-fertilization by having staminate and carpellate flowers on separate plants.