What are Pollen Grains?
Pollen grains are tiny particles that carry the male reproductive cells (sperm cells) in flowering plants. They contain special cells called microgametophytes, which develop into sperm cells during fertilisation. The study of pollen and its various characteristics is known as Palynology.
Pollen grains are essential for Pollination, the process of transferring the male part of the flower to the female part for successful reproduction. In many flowering plants, pollen grains in flower structures help ensure genetic diversity and the continuation of plant species.
Structure of Pollen Grain
Each pollen grain has three main components:
Living Cytoplasm: This is the inner part that contains the cell’s vital components. It typically breaks down quickly if the pollen grain is fossilised.
Intine: This inner layer of the cell wall is made of cellulose and pectin. Just like the cytoplasm, the intine often degrades rapidly once fossilisation takes place.
Exine: This is the tough outer layer composed of sporopollenin (a resistant, terpene-based compound). Sporopollenin is highly durable, allowing many pollen grains to remain intact for long periods, even when buried underground.
Size, Shape, and Colour
Size: Although pollen grains are microscopic, their size varies widely among species, typically ranging from about 3 micrometres to 200 micrometres.
Shape: Pollen grains can appear round, oval, triangular, disc-like, or even bean-shaped. Some have smooth surfaces, while others have a spiky texture.
Colour: Most pollen grains are naturally white, but certain species produce pollen in shades of yellow, orange, or cream.
Types of Pollen Grains
Different types of pollen grains can be classified based on factors like their shape, surface texture, and the number of pores or furrows on their exine. For instance, some grains have multiple furrows (colpi) on their surface, while others may have just one or two. These variations help botanists identify and differentiate pollen grains belonging to different plant species.
Importance of Pollen Grains in Flower Reproduction
Fertilisation: Pollen grains are produced in the anther, which is part of the stamen (the male reproductive organ of a flower). Once mature, pollen grains transfer to the stigma (female reproductive organ) during pollination.
Pollen Tube Formation: After landing on the stigma, a tube cell within the pollen grain forms the pollen tube, allowing the generative cell to travel and release sperm cells for fertilisation.
Biodiversity and Survival: Successful transfer of pollen ensures genetic variation and the survival of pollen grains plants, contributing to a healthy ecosystem.
Additional Facts and Unique Insights
Fossil Record: Because of the durable exine layer, pollen grains are well-preserved in sediments. Scientists use these fossilised grains to learn about ancient plant life and climate changes.
Allergies: Pollen grains are also known to cause allergies in some people, especially when they are airborne. However, not all pollen is allergenic; it depends on the plant species.
Quick Quiz (With Answers)
Where are pollen grains produced in a flower?
a) Ovary
b) Anther
c) Stigma
d) Sepal
Answer: b) Anther
Which pollen grain layer mainly consists of sporopollenin?
a) Intine
b) Exine
c) Cytoplasm
d) Nucleus
Answer: b) Exine
Which term describes the study of pollen grains and their characteristics?
a) Botany
b) Ecology
c) Palynology
d) Zoology
Answer: c) Palynology
Related Topics
FAQs on Pollen Grains: Definition, Structure, and More
1. Why is sporopollenin important?
Sporopollenin is a very tough substance found in the outer layer (exine) of pollen grains. It makes the grain highly resistant to physical and chemical damage, helping pollen survive harsh conditions.
2. How do pollen grains help in plant reproduction?
Pollen grains carry the male reproductive cells. When they land on the female part of the flower (stigma), they germinate and form a pollen tube that transports the sperm cells to the ovule for fertilisation.
3. Why are pollen grains of different shapes and sizes?
Different species of plants produce uniquely shaped pollen grains. The variations in size, shape, and surface patterns help botanists identify plant species and also play a role in how the grains are dispersed (e.g., by wind or insects).
4. What happens to pollen grains during fossilisation?
During fossilisation, the living cytoplasm and intine typically degrade quickly, while the exine layer remains intact due to the presence of sporopollenin. This makes fossilised pollen crucial for studying ancient plant life and climates.
5. What is the typical range in size for pollen grains?
Pollen grains can range from about 3 micrometres to 200 micrometres in diameter, depending on the plant species.
