The sexual process consists in merging of sex cells - male and female, that are called gametes. The result of fusion of two gametes is a cell called

Sexual reproduction in plants (isogamy, anisogamy, oogamy) [Types, Forms, morphology]

The sexual process consists in merging of sex cells — male and female, that are called gametes.

The result of fusion of two gametes is a cell called a zygote. With further development, it gives rise to a new organism. In the vegetable world there are three types of sexual reproduction, representing three stages of its evolution.

Isogamy

The simplest type is isogamous sexual process (isogamy). In this case, both gametes are mobile and are absolutely identical in size and appearance. During the fusion they retain mobility by flagella at the front end of rounded or pear-shaped body. Isogamy occurs in lower plants — algae, fungi (diagram 132, 1).

Anisogamy (heterogamy)

The next step is heterogamous sexual process (heterogamy or anisogamy). In anisogamy both gametes are mobile, they have flagella, but differ in size. One of them, the smaller, has more mobility and is considered to be a male gamete. Another one, a little larger, is less mobile. In its plasma, there is a certain amount of reserve nutrients. It is considered to be a female gamete.

Anisogamy occurs only in lower plants, such as in some green algae (Chlamydomonas Braunii) and brown algae (e.g. Ectocarpus) (diagram 132, 2).

The gametes are usually produced in special cells that sometimes do no differ from conventional plants’ vegetative cells, but more often have different distinctive form in comparison with the latter. The cells that produce gametes are called gametangia. Taken from http://worldofschool.org

Diagram 132. Various types of sexual reproduction in green algae: 1 — isogamy in Ulothrix; 2 — anisogamy in Gonium; 3 — oogamy in Sphaeroplea: a — sperm; b — egg

Oogamy

The next stage is oogamous sexual process (oogamy). In this case, gametes sharply differ in shape, size and character. One of them — male — is very shallow, its protoplast consists mainly of the nucleus and a thin layer of the cytoplasm; in front of it there is a small amount of plasma, from which depart flagella. Male gamete is actively mobile and has a special name — spermatozoid. Female gamete is deprived of motility. It's called an egg or ovum. Its major protoplast has a large nucleus. The cytoplasm contains a large amount of nutrients (diagram 132, 3).

Oogamy occurs in all higher plants and in vast majority of lower plants. Male organs, in which develop spermatozoids, are called antheridia; Female, in which form the egg cells, in lowest plants are called oogonia, in higher plants — archegonium. Oogonium is always a unicellular form; archegonium is already a multicellular body having a complex structure.

In flowering plants, as well as in some of the gymnosperms (Gnetum and Welwitschia), archegonium, as well as female prothallia, is greatly reduced.

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