How Do Vascular Plants Reproduce

Learning Objectives

Compare and contrast the life cycles of angiosperms (flowering plants), gymnosperms (conifers), non-seed vascular plants (ferns), and nonvascular plants (mosses)
Describe the structures and functions of the flower, seed, and fruit in the angiosperm life bike
Explicate the procedure, locations, and significance of angiosperm gametogenesis and fertilization, including double fertilization
Explain the process and significance of seed maturation, dormancy, and formation
Predict mechanisms of pollination based on flower characteristics and dispersal based on fruit characteristics

Sexual reproduction in plants: Alternation of Generations

The text beneath is adapted from OpenStax Biological science 32.1

Plants have two singled-out multicellular stages in their life cycles, a phenomenon called alternation of generations(in contrast to thehaplontic and diplontic life cycles). These 2 stages are the multicellular, haploid gametophyte and the multicellular diploid sporophyte. This is very different from virtually types of animal reproduction where there is simply i multicellular stage: a diploid organism which produces single-celled haploid gametes.

Before we revisit this life wheel, a reminder of some terms:

Gamete: a mature haploid male or female germ cell that is able to unite with another of the opposite sex in sexual reproduction to form a zygote
Spore: a minute, typically one-celled, reproductive unit capable of giving ascension to a new individual without sexual fusion

Gametes are always haploid, and spores are ordinarily haploid (spores are ever haploid in the plant alternations of generations life cycle).

In the alternation of generations life bike, illustrated below, there is a mature multicellular haploid phase and a mature mulitcellular diploid stage. The multicelluar haploid stage (the gametophyte) produces gametes via mitosis which fuse to course a diploid zygote. The zygote develops into a mature multicellular diploid individual (the sporophyte), which produces haploid spores via meiosis. The haploid spores then develop into a mature multicellular haploid individual.

Notation the multicellular stages are named for what they produce, not what they come up from. The gametophyte makes gametes, and the sporophyte makes spores.

Alternation of Generations. Paradigm credit: Menchi, Wikimedia Commons. https://en.wikipedia.org/wiki/File:Sporic_meiosis.png

Though all plants brandish an alternation of generations life bicycle, in that location are pregnant variations in different lineages of plants, consistent with their evolutionary history:

In seedless not-vascular plants, orbryophytes (mosses), the haploid gametophyte is larger than the sporophyte (the establish structure that you meet is the gametophyte); this is a gametophyte-dominated life bike. The sporophyte is fastened to and dependent on the gametophyte. (By "dominated" we mean "the stage of the plant yous tin can see by heart.")
In seedless vascular plants (ferns), the sporophyte is larger than the gametophyte (the constitute construction that you run across is the sporophyte), but the gametophyte is free-living and contained from the diploid sporophyte.
The life wheel of angiosperms (flowering plants) and gymnosperms (conifers) is dominated by the sporophyte stage (the constitute structure that y'all encounter is the sporophyte), with the gametophyte remaining attached to and dependent on the sporophyte (reverse of bryophytes).
Though they both have sporophyte-dominated life cycles, angiosperms and gymnosperms differ in that angiosperms take flowers, fruit-covered seeds, and double fertilization, while gymnosperms do not have flowers, take "naked" seeds, and do not take double fertilization (more on this afterwards).

The video beneath describes reproduction in gametophyte-dominant nonvascular plants (eg, mosses):

The video below describes reproduction in sporophyte-ascendant vascular plants (eg, gymnosperms and angiosperms):

Reproduction in angiosperms

The information below is adapted from OpenStax Biological science 32.1

We'll look more closely at reproduction in angiosperms, which are unique among plants for three defining features: they have flowers, they takefruit-covered seeds, and they reproduce via a procedure calleddouble fertilization.

Flowers are adaptations to attractpollinators
Fruits are adaptations to facilitateseed dispersal
Double fertilization is an adaptation to invest resources for nourishment of the developing embryo, in a unique way compared to other plants

All the data beneath is specific to angiosperms, unless otherwise noted.

Blossom Structure

A typical flower has four "layers," illustrated and described beneath from external to internal structures:

The outermost layer consists of sepals, green, leafy structures which protect the developing flower bud before it opens.
The next layer is comprised of petals, modified leaves which are usually brightly colored, which help attract pollinators.
The third layer contains the male reproductive structures, the stamens. Stamens are equanimous ofanthers andfilaments. Anthers incorporate the microsporangia,the structures that produce the microspores, which go along to develop into male person gametophytes. Filaments are structures that support the anthers.
The innermost layer contains one or more than female reproductive structures, the carpel. Each carpel contains a stigma, style, and ovary. The ovaries comprise themegasporangia, the structures that produce themegaspores, which go on to develop into female person gametophytes. The stigma is the location where pollen (the male person gametophyte) is deposited by wind or by pollinators. The way is a construction that connects the stigma to the ovary.

The parts of the flower include the sepal, petals, stamens, and carpels. Image credit: OpenStax Biology, modification of work by Mariana Ruiz Villareal

The Pollen Grain: the Male Gametophyte

Pollen is the male gametophyte in angiosperms and gymnosperms. Pollen is often described in everyday language as plant sperm, but this is non the instance! As the male gametophyte, pollen is a multicellular, haploid stage that produces the sperm.

Pollen development occurs in a structure chosen themicrosporangium(micro = minor), located within the anthers. The microsporangia (plural of microsporangium) are pollen sacs in which the microspores develop into pollen grains.

Shown is (a) a cross section of an anther at two developmental stages. The immature anther (top) contains four microsporangia, or pollen sacs. Each microsporangium contains hundreds of microspore mother cells that volition each give rise to four pollen grains. The tapetum supports the development and maturation of the pollen grains. Upon maturation of the pollen (bottom), the pollen sac walls split open up and the pollen grains (male person gametophytes) are released. (b) In these scanning electron micrographs, pollen sacs are ready to burst, releasing their grains. Epitome credit: OpenStax Biological science; credit b: modification of work by Robert R. Wise; calibration-bar data from Matt Russell)

As a spore, the microspore is haploid, but it is derived from a diploid cell. Within the microsporangium, the diploid microspore mother prison cell divides by meiosis to requite ascension to 4 microspores, each of which will ultimately form a pollen grain, illustrated below. This process is similar to product of gametes in animals (note that haploid gametes in plants are produced by mitosis from a haploid gametophyte). Upon maturity, the microsporangia outburst, releasing the pollen grains from the anther where they take the opportunity to be transported to stigmas by current of air, water, or an animal pollinator.

Mature pollen grains contain two cells: a generative prison cell and a pollen tube jail cell (see, I told you pollen is multicellular!). The generative cell is contained within the larger pollen tube cell. When the pollen grain reaches a stigma, it undergoes a process calledgermination (which is not the aforementioned as seed germination). During pollen formation, the tube prison cell forms a pollen tube through the style to the bottom of the ovary, the generative jail cell migrates through it to enter the ovary for fertilization. During its transit inside the pollen tube, the generative cell divides to form 2 male gametes (sperm cells). Both sperm cells are required for successful fertilization in angiosperms.

Pollen develops from the microspore female parent cells. The mature pollen grain is composed of two cells: the pollen tube cell and the generative cell, which is inside the tube prison cell. The pollen grain has two coverings: an inner layer (intine) and an outer layer (exine). The inset scanning electron micrograph shows Arabidopsis lyrata pollen grains. (Prototype credit: OpenStax Biology pollen micrograph: modification of work by Robert R. Wise; calibration-bar data from Matt Russell)

Due to its protective covering that prevents desiccation (drying out) of the sperm, pollen is an important adaptation in facilitating colonization of land past plants. Pollen allows angiosperms and gymnosperms to reproduce away from water, unlike mosses and ferns which require water for sperm to swim to the female person gametophyte.

The Embryo Sac: The Female person Gametophyte

While the details may vary between species, the general development of the female gametophyte, or embryo sac, has two distinct phases. First, a single cell in the diploid megasporangium (mega = large), located inside the ovules,undergoes meiosis to produce 4 megaspores. Merely one megaspore survives, again like to gamete production in animals.

In the second stage of female gametophyte evolution, the surviving haploid megaspore undergoes mitosis without complete cell division to produce an eight-nucleate, seven-cell female gametophyte, the embryo sac, illustrated below. Two of the nuclei (the polar nuclei) move to the center of the embryo sac and fuse together, forming a single, diploid primal cell. This key jail cell later fuses with a sperm to form the triploid endosperm, which will ultimately provide nourishment for the developing embryo (analogous to yolk in animal eggs). Three nuclei position themselves on the terminate of the embryo sac opposite the micropyle (the site where sperm enter the embryo sac) and develop into the converse cells, which later degenerate to provide nourishment to the embryo sac. The nucleus closest to the micropyle becomes the female gamete, or egg cell, and the ii side by side nuclei develop into synergid cells. The synergids help guide the pollen tube for successful fertilization. Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the fertilized ovule forms the other tissues of the seed.

A construction called the integument protects the megasporangium and, later, the embryo sac. The integument will develop into the seed coat subsequently fertilization and protect the unabridged seed. But like the evolution of pollen, the evolution of the seed was an important adaptation assuasive plants to colonize land abroad from water due to the protection of the embryo within the plant. (Thus the seed is coordinating to the amniotic egg in animal reproduction.) The integuments, while protecting the megasporangium, do not enclose it completely, merely go out an opening chosen the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization. The ovule wall will go part of the fruit.

As shown in this diagram of the embryo sac in angiosperms, the ovule is covered by integuments (dark green) and has an opening chosen a micropyle. Within the embryo sac are three antipodal cells, 2 synergids, a central cell, and the egg cell. Image credit: OpenStax Biological science.

Double Fertilization

The text below was adapted from Openstax Biological science 32.2

The miracle of double fertilization, or two fertilization events, is unique to angiosperms and does not occur in any other type of found or other organism.

As described in a higher place, after pollen is deposited on the stigma, it germinates and grows through the style to accomplish the ovule. The pollen tube cell grows to grade the pollen tube, guided to the micropyle by chemic signals from the synergid cells. The generative jail cell travels through the tube to the egg and divides mitotically to course two sperm cells. One sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei, forming a triploid cell that develops into the endosperm, which serves as a source of diet for the developing embryo. Together, these ii fertilization events in angiosperms are known equally double fertilization, illustrated below. After fertilization is complete, no other sperm tin enter. The fertilized ovule forms the seed, and the ovary become the fruit, normally surrounding the seed.

In angiosperms, ane sperm fertilizes the egg to form the 2n zygote, and the other sperm fertilizes the primal cell to form the triploid (3n) endosperm. This is chosen a double fertilization. Image credit: OpenStax Biology

After fertilization, the zygote enters a temporary period of development (shown below). Information technology first divides to form two cells: the upper prison cell, or upmost jail cell, and the lower cell, or basal cell. The division of the basal jail cell gives ascent to the suspensor, which eventually makes connection with the maternal tissue. The suspensor does not become part of the future found, but instead provides a route for nutrition to be transported from the mother institute to the growing embryo. In this way the suspensor is a blazon of "extra-embryonic" tissue and is analogous to the umbilical cord in placental mammals. The apical cell as well divides, giving rise to the proembryo (the actual embryo that will develop into a plant). The endosperm accumulates starches, lipids, and proteins, then nourishes the developing cotyledons (embryonic leaves). The cotyledons will serve as an energy store for later embryo evolution. The seed then loses upwards to 95% of its h2o and embryonic development is suspended: the seed enters a catamenia of dormancy for dispersal, and growth is resumed only when the seed germinates. In one case development is reactivated, the developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.

Shown are the stages of embryo development in the ovule of a shepherd'southward purse (Capsella bursa). After fertilization, the zygote divides to form an upper concluding cell and a lower basal jail cell. (a) In the kickoff stage of evolution, the terminal cell divides, forming a globular pro-embryo. The basal prison cell also divides, giving rise to the suspensor. (b) In the second phase, the developing embryo has a heart shape due to the presence of cotyledons. (c) In the 3rd stage, the growing embryo runs out of room and starts to bend. (d) Eventually, it completely fills the seed. (credit: OpenStax Biological science, modification of work by Robert R. Wise; scale-bar data from Matt Russell)

The prototype below puts each of these steps in context with each other:

Prototype credit: LadyofHats Mariana Ruiz – based on Judd, Walter S. , Campbell, Christopher S. , Kellog, Elizabeth A. andStevens, Peter F. 1999. Plant Systematics: A PhylogeneticApproach.Sinauer Associates Inc.ISBN 0-878934049., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1671506

This video gives a simplified (but very engaging) overview of double fertilization, as well as reviewing flower structure:

Avoiding self-pollination

In angiosperms, pollination is the transfer of pollen from an anther to a stigma. Many plants can both cocky-pollinate and cross-pollinate. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or some other blossom on the same plant. Cross-pollination is the transfer of pollen from the anther of ane flower to the stigma of another blossom on a unlike individual.

Individuals who are well-adapted to electric current conditions may non be well adapted if and when conditions change; therefore, genetic diversity is beneficial in changing environmental or stress conditions (this is the main advantage of sexual reproduction, after all!). Although self-pollination less energetically demanding since information technology does not crave production of nectar or extra pollen as food for pollinators, cocky-pollination leads to less genetic diverseness in the population since genetic fabric from the same constitute is used to form gametes, and eventually, the zygote. In dissimilarity, cantankerous-pollination (or out-crossing) leads to greater genetic diversity considering the microgametophyte and megagametophyte are derived from different plants.

Because cross-pollination allows for more genetic diversity, development has selected for many ways to avert self-pollination in different species:

The pollen and the ovary mature at unlike times
The flowers have physical features that prevent self-pollination, such differences in anther and stigma length
Male and female flowers located on unlike parts of the institute
The male and female person flowers are located on dissimilar plants, such that each plant makes only male or only female person gametophytes
"Incompatibility genes" preclude pollen from germinating into the stigma (illustrated below)

Cocky-incompatibility genes determine whether pollen can germinate, preventing fertilization by pollen with the same genotype.

Incompatibility genes are one of the more complex ways that plants foreclose self-pollination. Self-incompatibility is controlled by a gene chosen the South (sterility) locus. If the pollen and the stigma have the same version (allele) of the factor, then and then stigma sends signals that prevent the pollen from germinating.

Pollination Syndromes

It may sound like a disease, but pollination "syndrome" merely ways the way a item plant species is pollinated. The majority of pollinators are animals, including insects (like bees, flies, and collywobbles), bats, or birds. Some plant species are pollinated by abiotic agents, such as wind and h2o. Plants that are pollinated by animals must either produce nectar to concenter and feed the animals, or extra pollen that is eaten by the animals. Plants that are pollinated by wind or h2o must produce massive quantities of pollen since the probability of the pollen landing on a stigma of the correct species is depression (air current and water pollination is analogous to broadcast spawning). The mechanism of pollination and the features of the blossom are tightly linked:

Colored, highly scented flowers tend to be pollinated bybees,collywobbles, wasps, orflies. These insects are active during the day, and are able to observe vivid colors and accept a strong sense of smell. Different smells attract different pollinators, with sweet smells alluring bees and collywobbles, and rotting smells alluring flies. Many insect-pollinated flowers have additional colour patterns in the UV range, which insects are capable of seeing while humans cannot.
White or stake-colored, highly scented flowers tend to be pollinated bymothsand bats which are active at night. The light coloring makes them easier to see at nighttime, and they tend to smell musky or fruity. Flowers pollinated past bats are larger than those pollinated by moths.
Brightly colored, odorless flowers tend to exist pollinated bybirds, which practise not have a strong sense of odor. The flowers tend to take a curved, tubular shape to accommodate the bird's beak.
Small-scale green, petal-less flowers tend to be pollinated bywind. Wind-pollinated flowers do not produce nectar, but must produce excessive quantities of pollen. Gymnosperms such equally pines, which do not have flowers, are as well pollinated past wind.
Some aquatic plants are pollinated past water; the pollen floats and the water carries information technology to another flower.

Some examples of different pollination syndromes are shown below:

Left: Insects, such as bees, are of import agents of pollination. Eye: Hummingbirds have adaptations that allow them to reach the nectar of certain tubular flowers. Correct: A person knocks pollen from a pine tree. (credit: OpenStax Biology, left: modification of work by Jon Sullivan, Lori Branham)

And this video briefly describes the different pollination syndromes listed above:

Seed Dormancy and Formation

Every bit described above, a seed enters a period of temporary development after fertilization; in nearly species, the seed then enters a menses of stasis (inactivity), called dormancy. Dormancy is triggered by loss of up to 95% of the seed's h2o content, which dehydrates the seed, causes extremely low metabolic activity, and "concentrates" the seed's sugars to protect the cells from freezing during winter months. Dormancy tin concluding months, years, or even centuries in some cases.

Once weather are appropriate for bulb growth, the seed volition thengerminate or re-initiate development. The signal to initiate seed formation is indicator that conditions are favorable for growth and, depending on the species, tin include:

h2o, indicating the start of the rainy season and rehyrdating the seed
specific wavelengths of low-cal, indicating favorable sunlight atmospheric condition necessary for photosynthesis and the seed is not buried too far under the soil
a sustained catamenia of cold, indicating that the seed does not germinate until the common cold flavor is over
fire, typical of forest trees, indicating reduced contest from existing alpine tree
scarification, or chemical treatment with acids, indicating that the seed has passed through the digestive tract of an creature

Fruit and Seed Dispersal

Later fertilization, the ovary of the flower develops into thefruit. While we tend to recall of fruits every bit being sugariness, biologically a fruit is any construction that develops from an ovary after fertilization. The biological purpose of the fruit is due south eed dispersal, allowing the seed to be spread far from the female parent constitute, so they may detect favorable and less competitive weather condition in which to germinate and grow.

Some fruit have built-in mechanisms and so they can disperse by themselves, whereas others require the help of agents similar wind, water, and animals. As with pollination syndromes and bloom structure, y'all can often predict a fruit'south dispersal mechanism based on structure, composition, and size:

Propulsion-dispersed fruits, such as violets, really "explode" out of the plant.
Wind-dispersed fruits, such equally dandelions, are lightweight and may have fly or parachute-like appendages that let them to exist carried by the wind.
Water-dispersed fruits, such as coconuts, are light or buoyant, giving them the ability to float.
Animal-dispersedfruits may exist either accept tiny "hooks" all over them so that they attach to passing animals and later autumn off in a new location (like sandburs), or very sweet or fatty and so that they will be eaten and deposited in a new location in the carrion (like blackberries). Fruits dispersed past birds tend to exist brightly colored as birds have a highly developed sense of sight; fruits dispersed by mammals tend to exist highly scented equally mammals have a highly developed sense of odour.

Fruits and seeds are dispersed past diverse means. (a) Dandelion seeds are dispersed past wind, the (b) coconut seed is dispersed by water, and the (c) acorn is dispersed by animals that enshroud and then forget information technology. (credit OpenStax Biology a: modification of work by "Rosendahl"/Flickr; credit b: modification of work by Shine Oa; credit c: modification of piece of work by Paolo Neo)