2.6.3.2: Angiosperm Life Cycle

Learning Objectives

• Identify structures and phases in the angiosperm life cycle; know their ploidy.
• Explain how fertilization occurs within a flower.
• Label a developing ovary cross section.

Angiosperms have a complex life cycle. The gametophytes have been further reduced: antheridia were lost in the gymnosperms and archegonia were lost in the angiosperms. Both gametophytes are now housed within the flower, a structure composed of highly modified leaves specialized for pollination. From flowers, fruits are produced, a protective structure that (usually) develops from the ovary wall and is specialized for seed dispersal.

The Microgametophyte (AKA the Pollen Grain)

The microgametophyte develops and reaches maturity within the microsporangia (Figure \(\PageIndex{1}\)). The microsporangia, which are usually bi-lobed, are also called pollen sacs. These pollen sacs are found in the anther of the stamen, which is at the end of the filament.

Figure \(\PageIndex{1}\): 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 will 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 and the pollen grains (male gametophytes) are released. (b) In these scanning electron micrographs, pollen sacs are ready to burst, releasing their grains. (credit b: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Within the microsporangium (pollen sac), many microspore mother cells divide by meiosis to each give rise to four haploid microspores, each of which will ultimately form a pollen grain (Figure \(\PageIndex{2}\)). An inner layer of cells in the microsporangium, known as the tapetum, provides nutrition to the developing microspores and contributes key components to the pollen wall. Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.

Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine (Figure \(\PageIndex{2}\)). The exine contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. Sporopollenin allows the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without undergoing damage.

Figure \(\PageIndex{2}\): Pollen develops from the microspore mother cells. The mature pollen grain is composed of two cells: the pollen tube cell and the generative cell, which is inside the tube 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. Descriptive text: The mother cell undergoes meiosis to form a tetrad of cells, which separate to form the pollen grains. The pollen grains undergo mitosis without cytokinesis, resulting in four mature pollen grains with two nuclei each. One is called the generative nucleus, and the other is called the pollen tube nucleus. Two projective layers form around the mature pollen grain, the inner intine and the outer exine. Micrograph shows a pollen grain, which looks like puffed wheat. (credit “pollen micrograph”: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Mature pollen grains contain two cells: a generative cell and a pollen tube cell. The generative cell is contained within the larger pollen tube cell. When a pollen grain reaches the stigma, it germinates into a pollen tube. The generative cell migrates with the pollen tube to enter the ovary. During its transit inside the pollen tube, the generative cell divides to form two gametes (spermatia). These, along with the tube nucleus (also known as the vegetative nucleus), migrate down the pollen tube as it grows through the style, the micropyle, and into the ovule chamber.

In Arabidopsis, the pollen tube follows a gradient of increasing concentration of a small defensin-like protein secreted by the synergids (see The Megagametophyte).

The Megagametophyte

Within the ovary of the gynoecium, ovules are produced. Ovules consist of a double-layered integument with a small opening called the micropyle. The integument surrounds the megasporangium. Both the micropyle and megasporangium are diploid tissue of the sporophyte and are connected to the ovary wall by a region of tissue called the funiculus. The funiculus connects to a region of the ovary called the placenta, where nutritive support is provided the ovary wall and supplied to the developing ovule.

Within the megasporangium a single diploid megaspore mother cell divides by meiosis to produce four haploid megaspores. One of these will survive and three will disintegrate. The nucleus of the surviving megaspore undergoes 3 successive mitotic divisions. The 8 nuclei that result are distributed and partitioned off by cell walls to form the embryo sac (Figure \(\PageIndex{3}\)). The embryo sac is composed of 7 cells. The egg cell, located near the micropylar end, is flanked by 2 synergid cells. The large central cell contains 2 polar nuclei (a dikaryotic cell). The final three cells are the antipodal cells, located on the opposite side as the egg and synergids (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{3}\): Photomicrograph of a Lilium ovary at the eight nucleate stage. The integument surrounds the megasporangium, with a gap on one side (the micropyle). Inside the megasporangium, the megagametophyte is developing. It is composed of synergid cells (on the micropylar end) flanking an egg. There is a vacuole (space) in the center of the megagametophyte. The polar nuclei and antipodal cells are on the opposite side. A-Synergid cells, B-Vacuole, C-Micropyle, D-Polar nucleus, E-Antipodal cell, F-Integument. Scale=0.1mm. Jon Houseman, CC BY-NC, via Wikimedia Commons.

Figure \(\PageIndex{4}\): As shown in this diagram of the embryo sac in angiosperms, the ovule is covered by integuments and has an opening called a micropyle. Inside the embryo sac are three antipodal cells, two synergids, a central cell, and the egg cell. Descriptive text: Illustration shows the embryo sac, which is egg-shaped. The narrow end, called the micropylar end, has an opening that allows pollen to enter. The other end is called the chalazal end. Three cells called antipodals are at the chalazal end. The egg cell and two other cells called synergids are at the micropylar end. Two polar nuclei are inside the central cell in the middle of the embryo sac.

The synergids help guide the pollen tube for successful fertilization, after which they disintegrate. One of the spermatia produced by the pollen's generative cell fuses with the egg to form a diploid zygote. This zygote will grow into the sporophyte. The second spermatium fuses with the polar nuclei to produce a triploid endosperm (Figure \(\PageIndex{5}\)). This event is called double fertilization. The endosperm will provide additional nutritive tissue for the growing embryo.

After fertilization, the integument will close the micropyle and develop into the seed coat, protecting the seed. The ovary wall will develop into the pericarp of the fruit.

Figure \(\PageIndex{5}\): In angiosperms, one sperm fertilizes the egg to form the 2n zygote, and the other sperm fertilizes the central cell to form the 3n endosperm. This is called a double fertilization. Descriptive text: Illustration shows a vase-like gynoecium composed of a swollen ovary at the base and a long style (forming the neck of the vase) ending in a stigma. A pollen grain adheres to the stigma. The pollen contains two cells: a generative cell and a tube cell. The pollen tube cell grows through the stigma and into the style. The generative cell travels inside the pollen tube. It divides to form two sperm. The pollen tube penetrates an opening in the ovule called a micropyle. One of the sperm fertilizes the egg (between the two synergids) to form the zygote. The other sperm fertilizes two polar nuclei to form a triploid endosperm, which becomes a food source for the growing embryo. Three antipodal cells remain on the other side of the embryo sac.

Fruits

An unfertilized ovary, as shown in Figure \(\PageIndex{6}\), contains one or more developing ovules produced in compartments called locules. Each ovule is attached to a nutritional region of the ovary called the placenta by a strand of tissue called the funiculus. The sporophyte supports the developing ovule through this tissue pathway. Prior to fertilization, there is a small gap in the integument called the micropyle.

Figure \(\PageIndex{6}\): Cross section of Lilium ovary, showing A=Female gametophyte, B=Ovule, C=Locule, D=Placenta, E=Dissepiment. The ovary has three locules, each with two developing ovules. Each ovule attached to the ovary by a region of tissue called the placenta. The label in red added to original contribution of File:Lilium ovary L.jpg provided by Jon Houseman and Matthew Ford. JonRichfield, CC BY-NC, via Wikimedia Commons.

After fertilization, the ovary wall develops into the fruit, surrounding the seeds. In fleshy fruits that use animals for dispersal, like the pears shown in Figure \(\PageIndex{7}\), this might include a swelling of the cells, increased sugar production, and a change in pigmentation. These are the type of fruits we are familiar with. However, all flowers turn into fruits. The fruits might be dry, spiky, or in other ways completely unappetizing, and this is become many fruits do not use animal ingestion as their method of dispersal (more on this in Chapter 8.3 Fruits and Dispersal).

Figure \(\PageIndex{7}\): These two images show clusters of pear blossoms from the same tree. The cluster on the left is in an earlier stage of development. The corolla is still attached and the stamens are radiating outward to disperse pollen. In the image on the right, the corolla has fallen off and the anthers have shriveled. The calyx is still visible and, just below it, the semi-inferior ovary and hypanthium are beginning to swell. Eventually, these will swell to many times their current size and form the fruits we know as pears. Photos by Maria Morrow, CC BY-NC.

The Full Life Cycle

The angiosperm life cycle is shown in Figure \(\PageIndex{12}\) and Video \(\PageIndex{1}\).

Figure \(\PageIndex{12}\): A diagram of the angiosperm life cycle. Pollination and fertilization take place within the ovary. The formation of the ovule takes place on the left hand side, moving from top to bottom. A macrospore ovule within the ovary undergoes meiosis, forming 4 nuclei, then mitosis, forming 8 nuclei but only 7 cells: 3 antipodals, two polar nuclei in the central cell, two synergids and an egg. This is the mature megagametophyte (embryo sac). The formation of the pollen grain takes place on the right hand side, again moving from top to bottom. Within one of the microsporangia, a microspore "mother cell" undergoes meiosis to form a pollen tetrad. Mitosis occurs within each pollen grain to produce a generative cell and a tube cell. This is the mature microgametophyte (pollen). At the center bottom of the image, pollination and double fertilization occurs, where the sperm cells fuse with the egg (forming a zygote) and polar nuclei (forming the triploid endosperm). The seed (composed of an external seed coat, small embryo, and large endosperm) develops through the center of the image, transitioning into a mature sporophyte at the top. Diagram by LadyofHats, Public domain, via Wikimedia Commons.

Watch Video \(\PageIndex{1}\) to help untangle this complex life cycle.

Video \(\PageIndex{1}\): A digital, narrated rendition of the angiosperm life cycle. Sourced from YouTube.

Attribution

Curated and authored by Maria Morrow, CC BY-NC, using the following sources:

• 32.1 Reproductive Development and Structure and 32.2 Pollination and Fertilization from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
• 8.1 Spermatophyta 2.0 from Introduction to Botany by Alexey Shipunov (public domain)

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