Plant Life Cycles and Reproduction

Life Cycle of Angiosperms

The angiosperm life cycle involves diverse methods of producing seeds that are enclosed in tissues.
Angiosperms are the most diverse group of plants, ranging from tiny Wolffia, which are the smallest flower-bearing plants on Earth at only 2 mm long, to some of the tallest trees. Angiosperms include grains, grasses, flowers, fruits, and vegetables. Angiosperms thrive in climates from tropical to Arctic and adapt to varying amounts of precipitation and extremes of temperature.

Despite the vast diversity of angiosperms, members of this phylum have similar characteristics:

  • ovules (seeds) that are encased and protected by the carpel (fruit)
  • double fertilization
  • flowers that bear male and female reproductive organs

Angiosperms can be classified into one of two groups—monocots or eudicots. A monocot is a flowering plant that produces only one cotyledon, or seed leaf. These plants, such as peas, beans, and daisies, have flower parts that exist in groups of threes. For example, a monocot may have six petals on its flowers. Additionally, a monocot has vascular bundles (xylem and phloem) that are scattered throughout the stem and leaf veins that run in parallel with each other. In contrast, a eudicot is a type of flowering plant that produces two cotyledons and has flower parts in groups of four or five. The leaf veins of eudicots form a net-like pattern throughout the leaf. Examples of eudicots include wheat, corn, grass, and bananas.

As a flower blooms, an angiosperm's life cycle begins. The flower contains both female (ovule) and male (anther) parts. The anther is the male part of the flower that produces microspores and develops pollen. The anther carries diploid cells (containing two copies of chromosomes), which produce haploid cells (containing one copy of chromosomes). The haploid cells give rise to male gametophytes (the plant stage with only one copy of chromosomes) through the process of meiosis (sex cell division) and produce sperm cells. The ovule contains cells that undergo meiosis, producing cells that, in turn, develop into female gametophytes.

Types of Angiosperms

Type of Angiosperm Characteristics Examples
Basal angiosperm About 100 species
Branched from the lineage before dicots (seed with two embryonic leaves)
Woody stems
Indistinct sepals and petals
Water lilies, star anise
Magnoliid 8,000 species
Flowering trees
Woody evergreens
Magnolias, laurels, black-pepper trees
Monocot Flower parts in multiples of three
Leaf veins run in parallel
Scattered vascular bundles
Corn, wheat, rice, oats, tulips, spider plants, irises, daffodils
Eudicot Flower parts in multiples of four or five
Network of leaf veins
Vascular bundles in a ring
African violets, geraniums, pansies, dandelions, dogwoods, sunflowers, asters, roses

Angiosperms are plants that produce flowers. These structures are used for reproduction. They are classified by how many seed leaves they produce, either one or two. They can also be identified by the number and arrangement of their flower parts.

Flower Parts

Flowers provide the location for seed production in angiosperms.
Reproduction for most, but not all, angiosperms depends on flowers. Some angiosperms, both monocots (those with only one seed leaf, or cotyledon) and eudicots (those with two cotyledons), reproduce by asexual means. For those plants whose reproduction depends on flowers, each flower part is an integral part of the mechanism that fulfills the plant's main function: producing offspring. A flower is a determinate shoot, part of the plant that lives until seeds and fruit have been produced and then dies.

Flowers are a combination of both sterile tissues (petals and sepals) and reproductive tissues (carpels and stamens). A sepal is a leaflike structure that protects a flower bud. Flowers are constructed in whorls (sometimes referred to as verticils), which are arrangements of sepals, and petals. The innermost whorl is the pistil, the female organ of a flower. The next whorl contains the stamen, which is the male, pollen-producing reproductive organ of a flower. The third whorl is the corolla, which are the petals; the calyx, which are the sepals, forms the outermost whorl.

The pistil consists of an ovary, a style, and a stigma. The ovary is where ovules or egg cells are located. The style is a tube for delivering pollen (sperm cells) to the ovules. The stigma is the part of a flower's carpel to which pollen sticks.

The stamen consists of multiple anthers and filaments. The filament is the stalk of the stamen that supports the anther. Although there is only one pistil, there are multiple stamens. This gives flowers a greater opportunity for pollinators to carry pollen to other flowers and cross-pollinate them.

Petals are sterile parts of a flower, having no role in producing seeds. However, they do attract pollinators with their color and scent. Fragrance is produced in oils that petals produce. Color is a major attractant, and different colors draw different pollinators. Bees see green, blue, and ultraviolet. Flies prefer yellow, hummingbirds appear to like reds, and butterflies like yellow, orange, pink, and red. Night pollinators, mainly bats, are less interested in the color of petals and more drawn to the scent. Some bats, such as the lesser long-nosed bat and the Mexican long-tongued bat, need to have sharp senses because they pollinate the saguaro cactus flowers that bloom only one evening a year.

The calyx protects the flower bud before blooming. This is composed of sepals, the green whorl where the flower attaches to the stem. The sepals protect the bud before it blooms and the base of the flower once it blooms. The pedicel is the stalk that attaches the flower to the stem, much like the petiole that attaches a leaf to a branch.
The color of the petals and the scent of the flower are key attractors for pollinators, animals that transfer pollen from one flower to another.

Pollination and Fertilization

Pollination is the means by which flowering plants are fertilized. Angiosperms undergo double pollination, meaning two sperm cells are required for fertilization to occur.
Pollination, the fertilization of an ovule (egg cell) by pollen (sperm cell), occurs in two ways: self-pollination and cross-pollination. Self-pollination occurs when pollen transfers from the anther to the stigma of the same flower. Examples of flowers that shed pollen directly onto the stigma include peas, potatoes, peaches, wheat, oats, and a few legumes. Cross-pollination occurs when the pollen of one flower is taken to a flower of the same species and fertilizes the egg of the second flower. Common plants that cross-pollinate include grapes, plums, apples, dandelions, and maple trees. Cross-pollination requires a vector (an agent to aid in the transfer of the pollen). These vectors are called pollinators and include bees and wasps, ants and beetles, moths and butterflies, small mice, and even lizards. An abiotic means of pollination is the wind.

Flowers that need cross-pollination need a means of attracting pollinators. Pollination by insects is done for about 65% of all angiosperms. The most common pollinators include bees and wasps. Bee pollination is a mutually beneficial process. Flowers attract bees by offering nectar, a sweet liquid, and pollen as bee food. Although bees do use pollen as food, the amount eaten is small compared to the amount carried to the next flower. Bright blue and yellow flowers attract bees more than red or white flowers, and some flowers, such as the dandelion, have ultraviolet markers that guide bees to their nectar. Wasps use great amounts of energy and need energy-rich foods, such as figs. Fig wasps are responsible for pollinating nearly 1,000 fig species, which are common food sources for macaws, toucans, and monkeys in tropical forests.

Pollination by wind occurs about 20% of the time. Wind pollination works for flowers that have little or no scent.

Angiosperms require double fertilization, a process that involves two sperm cells. One sperm fertilizes the egg cell; the second sperm fertilizes the central cell (the cell whose division produces the egg). Pollination of the egg forms a zygote, which becomes the plant seed. The product of the second fertilization becomes the endosperm, which is the protective coating around an angiosperm seed. Coconut meat, the bran coating of wheat, beans, and peanuts are endosperm, as are the fruit of an apple, orange, or pear.
The flower marks the sporophyte generation of angiosperms. The sporophyte is diploid, meaning it has two copies of chromosomes (genetic material). The flower contains the male anther and the female ovary. Cells in each of these structures undergo meiosis, or cell division that reduces the number of chromosomes by half. The resulting microspores (male) and megaspores (female) are haploid, meaning they have only one copy of chromosomes. The microspores develop into pollen, while the megaspores develop into eggs. When the pollen contacts the egg, fertilization occurs, forming a diploid zygote. Angiosperms require double fertilization, meaning that one pollen grain (sperm) must fertilize the egg, while another fertilizes the central cell, which becomes the endosperm. The endosperm nourishes the developing embryo. Angiosperm life cycles require double fertilization to produce a seed protected by an endosperm.

Seed Formation and Dispersal

Seeds must be carried away from the parent plant in order to have the best chance of reaching maturity, and there are various mechanisms for dispersing seeds.
The basic parts of a seed vary depending on the parent plants, but most have the following components:
  • an embryo that consists of a root structure (radicle or hypocotyl) and seed leaves (cotyledons)
  • the endosperm, the protective coating around the seed, which provides nutrition to the embryo (for many plants, the embryo is encased by an endosperm)
  • a protective seed coating that surrounds the embryo and endosperm

Once a seed forms, it must reach fertile soil and germinate, meaning it begins to put out a shoot. The process of spreading seeds is called dispersal, and seeds disperse by various means. The seeds of dandelions, maples, and cottonwoods are carried by the wind to a new location. Seeds of water lilies and coconuts are usually dispersed by water movement. Frequently, animals play a role in distributing seeds. Some seeds attach to animal fur or pelts and are carried away from parent plants before falling off or being rubbed off, for example when the animal scratches itself or brushes against a tree. Many seeds are ingested when animals eat fruit, berries, grass seeds, or nuts. The seeds pass through the animals' digestive systems and are deposited in fecal matter. Other animals, such as squirrels, collect nuts and bury them for later consumption, sometimes not returning to retrieve them, which leaves those seeds to germinate underground.

Many seeds do not germinate immediately and may lie dormant for days, weeks, and even years. Seeds planted in desert soil, for example, may lie dormant until there is sufficient rain to activate growth. Seeds of eucalyptus and banksia require a forest fire for their seeds to sprout. In this situation, the fire clears some of the forest land and makes room for new growth to receive sunlight, gain nutrition from the ash, and have space in which to flourish. Seed dormancy depends on the presence of certain hormones that limit germination. In addition, dormant seeds are dehydrated and require sufficient water for germination to occur. Germinating seeds need oxygen in the soil, which explains why turning over or aerating soil frequently produces new plant growth. Seeds also need ambient temperatures, ranging from 5°C to 40°C, for growth.

In many situations, angiosperm sporophytes, those that have a double set of chromosomes, produce seeds and fruit within a single growing season, usually a period of 60 to 90 days. In the case of fruit trees, however, this process is much longer, as fruit trees rarely become mature sporophytes in fewer than five years.
As the endosperm surrounds the developing embryo, it becomes the fruit. Fruits can contain one or many seeds. Seeds are dispersed via animals, wind, water, or expulsion. Seed dispersal by animals includes when animals eat seeds and deposit them elsewhere, or when seeds stick to animals' fur or feathers and drop off later.