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Angiosperm Life Cycle

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Biology

Plants, like animals, can reproduce sexually (as well as asexually). In animals, the organism’s body is always diploid and produces the haploid gametes through meiosis. Plants, unlike animals, have two different generations during their life cycle. A diploid generation produces haploid spores through meiosis, and a haploid generation produces the gametes through mitosis. Flowers are the structures that produce and bear this haploid generation in flowering plants (angiosperms). We will describe the general structure of angiosperms, how the two generations alternate in their life cycle, how this is related to their sexual reproduction, and the differences with gymnosperm reproduction.

Angiosperm structure

Angiosperms belong to the vascular plants, a group of plants that present tissue differentiation including vascular tissue for nutrient transport. The other groups of vascular plants are gymnosperms (pines, cycads, cypresses) and seedless vascular plants (ferns, clubmosses, and horsetails). All vascular plants share a basic body plan and structure of organ systems and tissues.

Organ systems

An angiosperm plant has two organ systems (Figure 1):

Angiosperm Life Cycle Angiosperm body structure StudySmarter

Figure 1. The angiosperm plant body is organized in an underground root system and an aerial shoot system. Source: Kelvinsong, CC BY-SA 3.0 , via Wikimedia Commons.

  • Root system: the underground system of a plant composed of roots. Its main functions are to anchor the plant to the soil, absorb water and minerals, and sometimes store nutrients.
  • Shoot system: the aerial part of the plant composed of stems, leaves, and flowers. The vegetative or non-reproductive organs include the leaves where photosynthesis occurs (providing the food for the plant), and the stems bear the leaves and flowers, and give structural support to keep them upright. On the other hand, the flowers are the reproductive structures that bear the gametes (sexual reproductive cells) and where sexual reproduction takes place in angiosperms.

Plant tissues

The plant organs are made of several tissues, which are divided into three main types:

  • Dermal tissue: like our skin, it is the outer cover of the plant organs that gives protection from the environment and allows gas interchange with the environment.
  • Vascular tissue: it forms the transport system of the plant, connecting the root and the shoot systems. Water and minerals absorbed by the roots and organic molecules synthesized through photosynthesis are transported throughout the plant.
  • Ground tissue: all other plant tissues that are not dermal nor vascular are ground tissues (also called fundamental tissue). They are below the dermal tissue and surrounding and inside the vascular tissue. Ground tissues have diverse functions including support, storage, and photosynthesis.

Besides these differentiated or specialized tissues, there are regions of undifferentiated cells in the plant that continuously divide. These are regions of plant growth called meristems.

You can learn more about vascular plants and their organization and structure here.

Angiosperm flowering plant life cycle

All plants and some algae have a similar life cycle known as alternation of generations. In this cycle, a sporophyte (diploid non-sexual phase or generation) alternates with a gametophyte (haploid sexual generation). The main stages of a plant life cycle, starting with the zygote formation, can be summarized as follows:

  • The male gamete fertilizes the female gamete resulting in a diploid zygote. In angiosperms, this occurs in the female part of the flower (specifically inside the ovary).
  • The zygote grows through mitosis becoming the multicellular diploid sporophyte (division through mitosis does not change the number of chromosomes).
  • At maturity, a tissue called sporangium is formed in the sporophyte flower. It contains a sporocyte cell (spore mother cell) that produces haploid spores through meiosis (division through meiosis reduces the number of chromosomes from 2n to n).
  • These haploid spores undergo mitosis, forming the multicellular haploid gametophyte.
  • The gametophyte produces the haploid gametes (sexual reproductive cells) through mitosis.

These stages and the names of tissues or structures are common for all plants and algae life cycles (except for the flower part that is specific to angiosperms). Each group of plants has variations or modifications to this basic cycle, and some specific structures differ (like cones for gymnosperms and flowers for angiosperms).

The meaning of structures’ names can help you remember their function. For example, the gametophyte produces gametes, the sporophyte produces spores.

Angiosperm life cycle diagram

The diagram below (Figure 2) shows the general life cycle of a plant, you can identify in this cycle the main stages described above. Remember that there are male and female gametes (sperm and eggs respectively) and that they come from separated male and female gametophytes. Thus, gametophyte development has variations for each sex.

The general names for structures and tissues change slightly for each gametophyte. Note that for male structures the prefix micro- is added, while for females it is mega- (for example sporocyte becomes microsporocyte and megasporocyte respectively). This reflects the fact that male spores and gametes are usually smaller (micro) than female ones (mega).

Angiosperm Life Cycle Life cycle diagram StudySmarter

Figure 2. Diagram of the life cycle of angiosperms. Source: LadyofHats, Public domain, via Wikimedia Commons.

The angiosperm life cycle includes two main parts: since the sporophyte is the non-sexual generation, it corresponds to the growth phase of a plant (some plants can reproduce asexually though). The gametophyte phase corresponds to the sexual reproduction of a plant which we describe below.

Angiosperm reproduction cycle

The reproduction cycle in plants corresponds to the sexual generation, the haploid gametophyte. In angiosperms, sexual reproduction occurs in the flower since this structure produces the gametophytes. Below we describe the process for the development of male and female gametophytes separately. The general process is the same, but with some variation for each sex.

Development of female gametophyte and gametes

The female gametophyte is produced inside the ovary (the swollen base of a carpel) (Figure 3). An ovary can have one or more ovules. An ovule is composed of a tissue called the megasporangium (plural microsporangia) enclosed by the integuments (two layers of protective tissue that have a small opening called micropyle). One cell in the megasporangium differentiates into a megasporocyte (megaspore’s mother cell). Each megasporocyte enlarges and undergoes meiosis, producing 4 haploid daughter cells that are the megaspores (the first female haploid cells in the plant life cycle). Three of these megaspores usually degenerate and only one survives.

Angiosperm Life Cycle Female gametophyte and gamete StudySmarter

Figure 3. Female gametophyte in angiosperms. Left: diagram of gametophyte development (longitudinal view); right: micrograph of a Lilly ovary (cross section), the dashed lines delineate one of the three fused carpels with two ovules showing in this section, one with a visible megasporocyte. Source: left, modified from LadyofHats, Public domain; right, modified from Ilse Anahi Carrasco, CC BY 4.0 ; both images via Wikimedia Commons.

The surviving megaspore then goes through mitosis three times, but without cytokinesis (the splitting of the cytoplasm), resulting in one large cell with eight haploid nuclei. Membranes form around six of the nuclei, resulting in six small cells and the original large central cell, with the two remaining nuclei. These seven cells and eight nuclei form the embryo sac that is the female gametophyte. The eight nuclei have specific locations inside the embryo sac:

  • Three cells stay near the micropyle, one being the egg cell (female gamete). This makes sense as the pollen tube that delivers the sperm enters through the micropyle. The other two, called synergid cells, seats one on each side of the egg and are thought to attract the pollen tube.
  • Other three cells, called the antipodal cells, remain on the opposite side of the micropyle. Their function is unknown.
  • The remaining two nuclei, called polar nuclei, stay at the center of the large cell. These also participate in the fertilization process.

Development of male gametophyte and gametes

The male gametophyte is produced inside the anther (Figure 4). The anther contains four pollen sacs called microsporangia (singular microsporangium) that enclose many microsporocytes (microspore’s mother cells). Each microsporocyte undergoes meiosis and produces 4 haploid daughter cells that are the microspores (Figure 4, right image, shows numerous microsporocytes and some are undergoing, or have undergone, meiosis resulting in two or four cells). These spores are the first male haploid cells in the plant life cycle and are immature pollen grains.

Angiosperm Life Cycle Male gametophyte and gamete StudySmarter

Figure 4: Male gametophyte in angiosperms. Left: diagram of gametophyte development (longitudinal view of anther); middle: mature anthers with pollen grains; right: micrograph of the four microsporangia (cross-section of anther) containing numerous microsporocytes. Source: left, modified from LadyofHats, Public domain, via Wikimedia Commons; middle: Ali Shah Lakhani, unsplash.com; right, Bruce Kirchoff, CC BY 2.0, flickr.com

Each microspore then goes through mitosis once, resulting in a mature pollen grain with two cells. The mature pollen grain is the male gametophyte. The cells are the tube cell and the generative cell, they are not the gametes yet, the process finishes when the pollen reaches the female reproductive structure.

At this point, the anther matures and bursts to release the pollen grains. The pollen is transported from the anther to the pistil of another flower through pollination. once on the female reproductive structure, the pollen grain germinates and the generative cell divides once by mitosis producing two cells, the male gametes (two sperms).

Pollination is the transfer of pollen from the male reproductive structure to the female reproductive structure in seed plants.

Difference between angiosperm and gymnosperm life cycle

In vascular plants (ferns and allies, gymnosperms, and angiosperms), the sporophyte is the dominant phase. Gymnosperms and angiosperms (both seed-producing plants) bear the gametophytes inside the organism all the time (the gametophyte is completely dependent on the sporophyte). This means that when you see a fern, pine, cypress, or any flowering plant you see the sporophyte generation. In fact, to see the gametophyte of gymnosperms and angiosperms you would have to look for them in the plant reproductive structures (cones in gymnosperms and flowers in angiosperms) under a microscope because they are so small.

There are some differences between the life cycles of gymnosperms and angiosperms, specifically with reproduction. Table 1 below summarizes these differences:

Feature

Gymnosperms

Angiosperms

Structure that bears the reproductive parts

Female cones, male cones

Flowers that can be bisexual or unisexual

Male reproductive structure

The microsporangia are located on the surface of a scale

The microsporangia are inside the anther

Female reproductive structure

The megasporangium is inside an ovule located on the surface of a scale. The ovule has one layer of integument but is not enclosed in an ovary. The female gametophyte is composed of numerous cells and contains two or three archegonia (the structures that enclose an egg cell)

The megasporangium is inside an ovule enclosed in an ovary. the ovule has two layers of integument. The female gametophyte is the embryo sac composed of seven cells and eight nuclei (only one is an egg cell).

Fertilization

Simple fertilization, one sperm fertilizes one egg

Double fertilization, one sperm fertilizes the egg cell, and another sperm fertilizes the two polar nuclei

Seed

The seed develops on the surface of the scale. It contains the embryo and a nutritious tissue that is haploid (derived from the female gametophyte), protected by a seed coat

The seed develops inside the ovary, the ovary becomes the fruit. It contains the embryo and a nutritious tissue that is triploid (originated from the double fertilization), protected by the fruit.

Table 1: The differences between the life cycles of gymnosperms and angiosperms.

Angiosperm Life Cycle - Key takeaways

  • Angiosperms are organized into a root (underground) and a shoot (aerial stems, leaves, and flowers) systems.
  • All plants’ life cycle consists of an alternation of generations that includes a non-sexual sporophyte phase and a sexual gametophyte phase.
  • The dominant generation in vascular plants (ferns and allies, gymnosperms, and angiosperms) is the sporophyte.
  • The gametophyte in angiosperms is completely dependent on the sporophyte (pollen grains only move out for fertilization but do not grow outside of the sporophyte).
  • Gamete production and fertilization in angiosperms, which are processes for sexual reproduction, occur in the flower.
  • In angiosperms, the male gametophyte (pollen grain) is produced inside the anther while the female gametophyte (embryo sac) is produced inside the ovule.

References

Mary Ann Clark et al., Biology 2e, Openstax web version 2022

Images links

Figure 1: https://commons.wikimedia.org/wiki/File:Plant.svg

Figure 2: https://commons.wikimedia.org/wiki/File:Angiosperm_life_cycle_diagram-en.svg

Figure 3: left, https://commons.wikimedia.org/wiki/File:Angiosperm_life_cycle_diagram-en.svg; right, https://commons.wikimedia.org/wiki/File:Lilium_Ovary_10x-.25.png

Figure 4: middle, https://unsplash.com/photos/mNLGCmE35VA; right, https://www.flickr.com/photos/brucekirchoff/21386937054/in/photostream/

Angiosperm Life Cycle

Reproduction in angiosperms takes place in the flower, specifically inside the ovary.

We can describe five stages of growth in angiosperms as follows: embryo growth, seed germination, sporophyte growth, flowering, and fruit production. Note that flowers and fruits are also part of reproduction in angiosperms, but we refer here to the growth of the sporophytic tissue (diploid) that forms them.

Flowers play critical roles in an angiosperm life cycle because they are responsible for their sexual reproduction. Flowers produce and bear the sexual reproductive cells (gametes), attract pollinators, and are the place where sexual reproduction takes place.

The life cycle of an angiosperm in simple steps: fertilization: male gamete (n) and female gamete (n) fusion results in a zygote (2n); zygote growth: through mitosis, becoming the multicellular sporophyte (2n); spore formation:  the sporangium (2n) in a flower contains a spore mother cell (2n) that produces spores (n) through meiosis; gametophyte formation: the spores (n) undergo mitosis, forming the multicellular gametophyte (n); gamete production: the gametophyte produces the gametes (n) through mitosis.

Final Angiosperm Life Cycle Quiz

Question

Which of the following statements are true for angiosperms’ life cycle?

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The first products of meiosis are the gametes

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Angiosperms lack the following in their life cycle:

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an ovule surrounded by integument

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The main functions of the root system in angiosperms are:

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water transport

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Some of the main functions of the shoot system in angiosperms are:

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water transport

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The functions of dermal tissue in angiosperms are:

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protection from the environment

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The functions of the vascular tissue in angiosperms are:

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protection from the environment

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The functions of ground tissue in angiosperms are:

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protection from the environment

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In angiosperms, the ___ is the female gametophyte, which, with the surrounding ___ compose the __ that is enclosed by the ___.

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embryo sac, integument, ovule, ovary

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In angiosperms, the ___ is the male gametophyte, which is produced by a ___ that is enclosed in a __ located in the ___.

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pollen grain, microspore, microsporangium, anther

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In angiosperms, the female gametophyte contains:

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synergid cells

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In a pollen grain, the generative cell produces the ___ through ____ while the tube cell produces the ___.

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gametes, mitosis, pollen tube

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The reproductive structures are found in ____ in angiosperms and in ____ in gymnosperms.

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Flowers in angiosperms, cones in gymnosperm

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Gymnosperms present the following:

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naked ovules on the surface of a scale

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Angiosperms present the following:

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naked ovules on the surface of a scale

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What are the differences between the reproduction of angiosperms and gymnosperms?

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Angiosperms: ovules enclosed by an ovary that become seeds enclosed by a fruit, seed’s nutritious tissue is triploid, flowers, microsporangia are enclosed by the anther, gametophyte composed of 8 nuclei and 7 cells.

Gymnosperms: naked ovules on the surface of scales, seed’s nutritious tissue is haploid, cones, microsporangia on the surface of scales, gametophyte composed of one or more egg cells.

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The dominant generation in angiosperms is ___, and the alternate generation is ___ of the dominant.

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sporophyte, dependent

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The role of double fertilization in angiosperms, in contrast to single fertilization, is to produce:

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an endosperm

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The products of double fertilization in angiosperms are:

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A diploid zygote and a triploid endosperm. 

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Gymnosperms do not present double fertilization; therefore, they do not produce: 


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a zygote

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Both gymnosperms and angiosperms seeds contain:


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an embryo

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The nutritious tissue originates from the ___ in gymnosperms, and from the ___ in angiosperms:


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female gametophyte, fusion of sperm cell and two female nuclei

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The endosperm in flowering plants is formed by the fusion of a sperm cell with:


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two polar nuclei

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Which ones of the following eventually disintegrate?


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antipodal cells

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The ___ secrete chemicals signals to guide the pollen tube:


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antipodal cells

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The zygote becomes the ___, the endosperm is the ___, and the integument becomes the ___. 


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embryo, nutrient storage, seed coat

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The ovule becomes the __ and the ovary becomes the ___. 


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seed, fruit

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Which of the following are advantages of double fertilization in angiosperms?


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the formation of a zygote

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The ___ is usually the direct nutrient source for embryos of monocotyledon species. 


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triploid endosperm

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The ___ is usually the direct nutrient source for embryos of dicotyledon species.


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triploid endosperm

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In angiosperms, unlike gymnosperms, the endosperm is formed: 


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before fertilization

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Which of the following is necessary for a pollen grain to germinate? 


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the absorption of water from the stigma

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Pollination is related to: 

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plant sexual reproduction

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Which of the following are abiotic agents of pollination? 

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non-living agents

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Which of the following are flower traits related to wind-pollination? 


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bright coloration

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Which of the following are flower traits related to biotic pollination?


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feathery stigma

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.Which of the following options shows the correct order from the most common pollination type to the less common one, in angiosperms?

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wind, water, biotic

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Because of the coevolution of traits between a flower and its pollinator, what do you think would happen if the pollinator goes extinct?


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The plant would probably go extinct too if it is a highly specialized relationship. The plant can have other pollinators, but their visits are probably less frequent or they do not successfully pollinate the plant, thus the reproduction success of the plant population could decrease enough to endanger the population. 

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Why do many plants avoid self-pollination? 


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because it decreases genetic diversity 

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In what ways can a plant avoid self-pollination? 


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By not synchronizing pollen release and stigma receptivity, positioning anther and stigma at different lengths, self-incompatibility, production of unisexual flowers, or separation of female and male flowers in different plants. 

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Bee pollinated flowers usually present these traits: 


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pale or white color

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If a flower has a pale or white coloration, is aromatic, and is open at night, it is most likely pollinated by: 


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bees

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Which of the following are traits of flowers pollinated mostly by birds?


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yellow and red colors

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This is a trait shared by flowers pollinated by birds and bats: 


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bright colors

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Gymnosperms are mostly pollinated by____ while angiosperms are mostly pollinated by____. 


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wind, animals

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Pollination is critical for plant reproduction because it enables: 


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fertilization

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Describe one example of “deceptive pollination”, when a flower attracts a pollinator but gives no rewards. 


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Answer

Carrion flowers that attract flies through an odor of rotten meat and with a reddish or brownish coloration. 

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