35.2:
The Angiosperm Life Cycle
Angiosperms—the most diverse group of plants on Earth—include flowering and fruiting trees, grasses, vegetables, and other flowering plants.
The life cycle of angiosperms is dominated by the spore-generating sporophyte stage, rather than the sexual gametophyte stage.
Like conifers, angiosperms produce two types of spores.
These male and female gametes are produced in distinct reproductive flower organs.
Megaspores are produced in carpels. A carpel includes an ovary and its ovules. Each ovule contains a megasporangium, where megaspores are produced.
Within each megasporangium is a megasporocyte—a megaspore mother cell. A megasporocyte produces four megaspores through meiosis; one survives and develops into an embryo sac, which consists of an egg and a small number of other cells.
Microspores originate in the flower anthers at the tips of stamens. Within the anthers are microsporangia containing microsporocytes. Microsporocytes produce microspores through meiosis.
A microspore develops into a pollen grain, which contains a tube cell and a generative cell. After the pollen reaches a stigma – the upper part of the carpel – the tube cell becomes a pollen tube. The pollen tube extends down the carpel to the ovule, which contains the embryo sac. The generative cell then divides to form two sperm.
These sperm are released together into one ovule, in an act of double-fertilization. One fertilizes the egg, and the other fertilizes the central cell of the embryo sac. The fertilized egg then forms a zygote, which will develop into an embryo. The fertilized central cell forms the endosperm – a nutrient storage structure. The embryo and endosperm are packed into a seed.
In the case of many angiosperms, the ovary then develops into a fruit, which usually contains multiple seeds, or fertilized ovules. The fruit takes many possible forms, typically depending on the species, sometimes looking very similar to the original ovary and other times recruiting additional tissues or joining multiple flowering structures together to create collective fruits.
Finally, a germinated seed develops into a mature sporophyte, which can produce flowers and begin another life cycle.
35.2:
The Angiosperm Life Cycle
Plants have a life cycle split between two multicellular stages: a haploid stage—with cells containing one set of chromosomes—and a diploid stage—with cells containing two sets of chromosomes. The haploid stage is the gamete-producing gametophyte, and the diploid stage is the spore-producing sporophyte.
Today, most plants grow from seeds and produce flowers and fruit; such plants are called angiosperms. Angiosperms begin as seeds—structures consisting of a protective seed coat, a nutrient supply, and an embryo. The seed develops into a sporophyte—the familiar, flower-producing plant form.
The reproductive life cycle of angiosperms begins with flowering. Stamens and carpels contain sporangia, structures with spore-producing cells called sporocytes. Sporophytes produce spores as either eggs or sperm, depending on their origin.
For example, male spores—called microspores—are produced within anthers at the tips of stamens. A microspore develops into a pollen grain—the male gametophyte. A pollen grain contains a tube cell and a generative cell, which develops into sperm.
A carpel consists of an ovary and its ovules. Female spores, called megaspores, are produced within ovules. A megaspore develops into an embryo sac—the female gametophyte—which contains the egg.
Pollination allows the sperm-producing pollen grain to reach the egg-containing embryo sac. While the embryo sac is stationary, pollen grains can be carried by wind, water, or animals.
For sperm to fertilize an egg, pollen released from the anthers must reach the sticky stigma at the tip of a carpel. Then, the tube cell of the pollen grain becomes a pollen tube, extending down the carpel to the ovule.
Angiosperms undergo a type of double fertilization that produces an embryo and an endosperm, a nutrient store. The embryo and endosperm are packed into a seed coat, forming a seed. As the ovules become seeds, the ovary typically develops into fruit that helps protect and distribute the seeds.
Suggested Reading
Bleckmann, Andrea, Svenja Alter, and Thomas Dresselhaus. 2014. “The Beginning of a Seed: Regulatory Mechanisms of Double Fertilization.” Frontiers in Plant Science 5 (November). [Source]
Endress, P. K. 2011. “Angiosperm Ovules: Diversity, Development, Evolution.” Annals of Botany 107 (9): 1465–89. [Source]