Gametophyte

A gametophyte is one of the two main stages in the life cycles of plants. It is a special type of multicellular organism that has only one set of chromosomes, called haploid. This stage develops from a tiny spore and is very important because it is the sexual phase in a plant's life. During this phase, the gametophyte creates special structures that produce gametes, which are haploid cells that join together in a process called fertilization.

When these gametes join, they form a new cell called a zygote, which has two sets of chromosomes, called diploid. This zygote then grows into another stage in the plant's life called the sporophyte. The sporophyte can make new spores through a process called meiosis. These spores can grow into new gametophytes, continuing the cycle of life for plants. This amazing process helps plants reproduce and grow in many different places around the world.

Algae

In some types of green, red, and brown algae, the plant parts that make spores and the parts that make gametes look very similar. For example, in a type of green algae called Ulva lactuca, the gametes are all the same size and shape.

Land plants

In land plants, there is always a difference between the cells that become eggs and those that become sperm. In these plants, one part called the sporophyte and another called the gametophyte can look very different. Ancient fossil plants had gametophytes with special features, but today's plants show simpler forms.

Bryophytes

For bryophytes like mosses, liverworts, and hornworts, the gametophyte is the part we usually see. It lives longer and can make its own food, while the sporophyte grows attached to it and depends on it. When a moss spore starts to grow, it forms a chain of cells called a protonema. The mature gametophyte of mosses grows into leafy shapes and creates special structures that produce eggs and sperm.

Several gametophytes growing in a terrarium

Vascular plants

All vascular plants, like ferns and clubmosses, are more dominated by the sporophyte stage. As plants evolved to reproduce with seeds, the female gametophytes became smaller and more dependent on the sporophyte.

Ferns

In most ferns, the gametophyte is a small, green, independent organism called a prothallus. It makes gametes and supports the early growth of the sporophyte. In some fern groups, these gametophytes live underground and partner with fungi for food.

Multiple examples of the variation of cell number in mature seed plant female gametophytes prior to fertilization. Each cell contains one nucleus unless depicted otherwise. A: Typical 7 celled, 8 nucleate angiosperm female gametophyte (ex. Tilia americana). B: Typical gymnosperm female gametophyte with many haploid somatic cells illustrated with a honeycomb grid and two haploid germ cells (ex. Ginkgo biloba). C: Abnormally large 10 celled, 16 nucleate angiosperm female gametophyte (ex. Peperomia dolabriformis). D: Abnormally small 4 celled, 4 nucleate angiosperm female gametophyte (ex. Amborella trichopoda). E: Unusual gymnosperm female gametophyte that is singled celled with many free nuclei surrounding a pictured central vacuole (ex. Gnetum gnemon). Blue: egg cell. Dark orange: synergid cell. Yellow: accessory cell. Green: antipodal cell. Peach: central cell. Purple: individual nuclei.

Lycophytes

Lycophytes, such as clubmosses, have two kinds of gametophytes. In some, spores grow into bisexual underground gametophytes that work with fungi. In others, like Isoetes and Selaginella, spores produce male and female gametophytes inside the spore wall.

Seed plants

In seed plants, the gametophyte stage is even smaller and depends completely on the sporophyte. The male gametophyte in gymnosperms develops inside male cones, producing pollen grains. The female gametophyte grows inside the ovule of the female cone.

In flowering plants, the male gametophyte forms inside the anther and becomes the pollen grain. The female gametophyte develops inside the ovule of the flower. After fertilization, the egg becomes part of the new sporophyte.

Heterospory

Main articles: Dioicous and Heterospory

In some plants, there are two different types of gametophytes. These two types look and work differently, so we call them heteromorphic. The gametophyte that makes eggs is called a megagametophyte, because it is usually bigger. The one that makes sperm is called a microgametophyte.

In certain plants, there are two kinds of structures that each make one type of spore. These spores grow into one type of gametophyte. Some plants have different egg-making and sperm-making gametophytes, but they come from the same kind of spore.

In plants that make seeds, the microgametophyte is called pollen. It has a few cells when it leaves the place where it grew. The megagametophyte grows inside a special part of the plant.

In gymnosperms, the megagametophyte has many cells and makes one or more special structures, each with an egg cell. The gametophyte stores food in the seed.

In angiosperms, the megagametophyte is very small, with just a few cells, sometimes called the embryo sac. It usually has seven cells and eight nuclei, including one egg cell. Two nuclei join with a sperm nucleus to form a special structure that becomes the food storage in the seed.