Which aspect of the angiosperm life cycle and morphology is not unique to angiosperms?

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.

Angiosperm organ systems

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

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.

Angiosperm 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.

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).

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.

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.

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:

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/