Difference Between Pollen Grain and Ovule

Main Difference – Pollen Grain vs Ovule

Pollen grain and ovule are the two structures which give rise to reproductive cells of angiosperms. The flower is the reproductive structure of angiosperms. It contains a sepal, petals, stamens and pistil. Stamens are the male reproductive organ of the flower whereas the pistil is the female reproductive organ. Stamens consist of anthers, where the pollen grain is produced, and filaments, which support the anther. Pistil consists of stigma – where the pollen grains land, style, and ovary – where the ovule is developed. Pollen grain is considered as the microgametophyte while ovule contains the megagametophyte. The main difference between pollen grain and ovule is that pollen grain is a male reproductive structure which gives rise to sperms whereas ovule is the female reproductive structure which gives rise to the egg cell. 

This article explores,

1. What is a Pollen Grain
      – Definition, Structure, Characteristics
2. What is an Ovule
      – Definition, Structure, Characteristics
3. What is the difference between Pollen Grain and Ovule

What is a Pollen Grain

Pollen grain is a microscopic particle produced by the flower of angiosperms. Generally, it contains a single cell along with two nuclei: tube nucleus and generative cell nucleus. The exine is the tough protective wall found surrounding the pollen grain. Intine is the other protective layer found inside the exine. Both exine and intine allow the pollen grain to survive under harsh conditions. Pollen grains are developed in the pollen sac of the anther. Pollen sacs are the chambers found inside the anther. They are enclosed by a protective epidermis and a fibrous layer. The inside of the fibrous layer is called tapetum; this stores food in order to energize future cell divisions. Four chambers of pollen sacs are found inside the anther, containing cells with large nuclei. During the development of the anther, cells inside the pollen sac undergo two mitotic divisions, resulting in a tetrad. The cells are now called microspores, which eventually become young pollen grains. Young pollen grains contain a single haploid nucleus, which is divided by mitosis in order to produce tube nucleus and the generative nucleus. Tube nucleus is responsible for the formation of the pollen tube, burrowing into ovary through the stigma and style. Generative nucleus produces two sperm cells in angiosperms, involved in double fertilization. Once pollen grains become mature, the wall between pollen sacs disappear, and mature pollen grains are dispersed. This process is called ripening of the anther.

Figure 1: Pollen grains of different plant

What is an Ovule

Ovule is a part of the ovary which contains female germ cells. It develops into a seed after fertilization. Ovary may contain one or more ovules. Ovule consists of integuments, nucellus, and the embryo sac. Integuments are the two walls which surround the ovule. Microphyl is the small opening found in the integuments, allowing the entering of the pollen tube. Nucellus contains the required nutrients for the developing embryo sac. One cell within a large number of diploid cells inside the ovule become the megaspore mother cell, dividing by mitosis in order to form four haploid cells. Only one haploid cell remains while the others degenerate. The remaining haploid cell becomes the megaspore. The enlarged megaspore undergoes three mitosis, forming eight haploid nuclei. Six haploid cells along with two polar nuclei are present inside the megaspore. This is known as the embryo sac. The embryo sac is also identified as the megagametophyte.  One of the haploid cells near themicrophyl become the egg cell.

Figure 2: Ovules of Helleborus foetidus flower

Difference Between Pollen Grain and Ovule

Definition

Pollen Grain: Pollen grain is the male reproductive structure.

Ovule: Ovule is the female reproductive structure.

Production of Cells

Pollen Grain: Pollen grain produces sperm cells.

Ovule: Ovule produces egg cells.

Alternative Names

Pollen Grain: Pollen grain is identified as the microgametophyte of the plant.

Ovule: Ovule is identified as the megagametophyte of the plant.

Location

Pollen Grain: Pollen grain is found in the anther of stamens.

Ovule: Ovule is found inside the ovary of the pistil of a flower.

Content

Pollen Grain: Pollen grains consist of a generative cell along with two nuclei and a tube cell.

Ovule: Ovules consist of integuments, nucellus and the embryo sac.

During Pollination

Pollen Grain: Pollen grain is dispersed by external pollination agents in order to be deposited on the stigma of another flower.

Ovule: Ovule is not dispersed during pollination.

After Fertilization

Pollen Grain: After fertilization, pollen grain is destroyed.

Ovule: After fertilization, ovule develops into the seed.

Conclusion

Pollen grain and ovule are the male and female reproductive structures, which are produced by the flower. Both mature structures contain haploid gamete cells. Pollen grain is produced in the pollen sacs of the anther. It is protected by tough coats in order to survive under harsh conditions. Pollen grains are microscopic structures, dispersed by external pollinating agents in order to deposit on the stigma of a flower for fertilization. Pollen grain of angiosperms contains two sperm cells, facilitating double fertilization. Ovule is found inside the ovary. Single egg cells are found in the ovule. Multi-seed plants contain several ovules inside the ovary. The egg cell is protected inside the ovary until fertilization occurs. The main difference between pollen grain and ovule is the type of gametes they bear.

Reference:
1.”Chapter 28: Sexual Reproduction in the Flowering Plant.” Leavingcertbiology.net. N.p., n.d. Web. 04 May 2017. [http://www.leavingcertbiology.net/chapter-28-sexual-reproduction-in-the-flowering-plant.html]2.”Formation of Sex Cells.” The Structure and Functions of Flowers. N.p., n.d. Web. 04 May 2017. [http://leavingbio.net/thestructureandfunctionsofflowers%5B1%5D.htm]

Image Courtesy:
1.”Misc pollen colorized” By Dartmouth Electron Microscope Facility, Dartmouth College – Source and public domain notice at Dartmouth Electron Microscope Facility (Public Domain) via Commons Wikimedia
2. “Ovules in flower”By Tameeria at English Wikipedia – Transferred from en.wikipedia to Commons. (CC BY-SA 3.0) via Commons Wikimedia