Pollination, Fertilization, and Reproduction in Plants and Humans

Pollination and Fertilization in Plants

Pollination is the mechanism to achieve the objective of transferring pollen grains, shed from the anther, to the stigma of a pistil. This transfer is termed pollination.

Kinds of Pollination

Depending on the source of pollen, pollination can be divided into three types:

• Autogamy
• Geitonogamy
• Xenogamy

Double Fertilization

After entering one of the synergids, the pollen tube releases two male gametes into the cytoplasm of the synergid. One of the male gametes moves towards the egg cell and fuses with its nucleus, thus completing the syngamy. This results in the formation of a diploid cell, the zygote. The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN). As this involves the fusion of three haploid nuclei, it is termed triple fusion.

Since two types of fusions, syngamy and triple fusion, take place in an embryo sac, the phenomenon is termed double fertilization, an event unique to flowering plants. The central cell, after triple fusion, becomes the primary endosperm cell (PEC) and develops into the endosperm, while the zygote develops into an embryo.

The embryo develops at the micropylar end of the embryo sac where the zygote is situated. Most zygotes divide only after a certain amount of endosperm is formed. This is an adaptation to provide assured nutrition to the developing embryo.

Human Reproductive Systems

The Male Reproductive System

The male reproductive system is located in the pelvis region. It includes a pair of testes along with accessory ducts, glands, and the external genitalia. The testes are situated outside the abdominal cavity within a pouch called the scrotum. The scrotum helps in maintaining the low temperature of the testes (2–2.5°C lower than the normal internal body temperature) necessary for spermatogenesis.

The Female Reproductive System

The female reproductive system consists of a pair of ovaries along with a pair of oviducts, uterus, cervix, vagina, and the external genitalia located in the pelvic region. These parts of the system, along with a pair of mammary glands, are integrated structurally and functionally to support the processes of ovulation, fertilization, pregnancy, birth, and child care.

Ovaries are the primary female sex organs that produce the female gamete (ovum) and several steroid hormones (ovarian hormones). The ovaries are located one on each side of the lower abdomen. Each ovary is about 2 to 4 cm in length and is connected to the pelvic wall and uterus by ligaments. Each ovary is covered by a thin epithelium, which encloses the ovarian stroma. The stroma is divided into two zones – a peripheral cortex and an inner medulla.

Gametogenesis

The primary sex organs – the testis in males and the ovaries in females – produce gametes, i.e., sperms and ovum, respectively, by the process called gametogenesis.

Fertilization

The process of fusion of a sperm with an ovum is called fertilization.