Gametophyte

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

When these gametes join, they form a new cell called a zygote. This zygote has two sets of chromosomes, called diploid. The 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 process helps plants reproduce and grow in many places around the world.

Algae

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

Land plants

In land plants, there are always different cells that become eggs and sperm. In these plants, one part called the sporophyte and another called the gametophyte can look very different. Ancient plants had special gametophytes, but today's plants show simpler ones.

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 makes special structures that produce eggs and sperm.

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.

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 one egg cell.