Sexual Reproduction in Flowering Plants · Lesson 2 of 9
Pollen Grains and Male Gametophyte Development
“The architecture, cell stages and lifespan of pollen grains, and how mature male gametes become available for fertilisation.”
• Differentiate pollen exine and intine and explain the role of germ pores. • Describe the vegetative and generative cells of a developing pollen grain. • Compare two-celled and three-celled pollen at shedding. • Explain pollen viability, dispersal, storage and relevant biological effects.
A pollen grain is a protected, reduced male gametophyte
The microspore produced by meiosis does not immediately function as a male gamete. It grows and undergoes mitotic development inside a protective pollen wall, forming a reduced male gametophyte. Most pollen grains are microscopic, and their shapes, apertures and outer-wall sculpturing differ greatly among flowering plants. These differences are useful to botanists who identify pollen in modern plant samples or ancient sediments. The grain's ability to withstand transport outside the anther depends on its strong wall and on retained cellular viability.
The exine, intine and germination aperture
A pollen grain has a tough outer exine, largely formed from sporopollenin, and a thinner continuous inner intine rich in cellulose and pectin. Sporopollenin is chemically and mechanically resistant, helping pollen withstand drying and environmental exposure. The exine often bears elaborate ridges or spines, which can aid species-level identification. At certain apertures or germ pores, the exine is thin or absent. During compatible pollen germination the inner contents protrude through such an aperture and the growing pollen tube emerges.
The wall is simultaneously a protective barrier and an interface with the pistil. An exine that resists environmental damage cannot remain completely rigid everywhere, so germination apertures provide regions through which the wall can expand and permit tube emergence. The intine then contributes to the wall of the pollen tube. The grain's vegetative protoplast contains stored nutrients and enzymes supporting germination; the generative lineage provides the eventual male gametes. These functions must not be confused: wall layers are protective structures, not additional reproductive cells.
How one haploid microspore becomes two or three cells
The haploid microspore undergoes an unequal mitotic division. The larger vegetative cell contains abundant cytoplasm, nutrients and a nucleus that directs pollen-tube growth. The smaller generative cell, initially enclosed within the vegetative cytoplasm, is denser and often spindle-shaped. At this stage the grain is described as two-celled. In many species pollen is released in this condition; the generative cell divides by mitosis after pollination, often while the pollen tube grows, producing two haploid male gametes.
In other species the generative cell divides before pollen is shed, so the grain is released in a three-celled condition: one vegetative cell plus two male gametes. The two gametes are genetically related products of mitosis and remain haploid. The male gametes do not swim through soil or surface water toward the egg; they are transported by the pollen tube. Note the critical distinction between the earlier meiosis of the diploid PMC, which establishes haploidy, and the later mitoses, which increase cell number without restoring diploidy.
Pollen viability, dispersal and pollen banks
Pollen viability is the capacity of a pollen grain to germinate and participate in fertilisation when it reaches a suitable stigma. It varies with species, water status, temperature and humidity. In some cereals, exposed pollen can remain viable only briefly, whereas pollen from other flowering plant families may retain viability far longer. This matters because a pollen grain reaching a stigma after viability is lost will not fertilise an ovule even if wind or an animal carried it successfully. Pollination is therefore only the first logistical step; viable pollen and compatibility are both required.
Low-temperature storage of carefully prepared pollen offers one way to conserve plant genetic resources and to cross parent plants that flower at different times. Pollen from suitable species may be stored in liquid nitrogen near −196°C after appropriate preparation. Such pollen banks complement seed banks. The practice does not mean every species' pollen survives identical storage protocols. Pollen grains may also trigger allergies in susceptible individuals; airborne grains are a biological exposure, not evidence of disease in every person. Nutritional marketing claims about pollen preparations should not be treated as established benefits without independent evidence.
Under appropriate supervision, place fresh pollen on a slide with a suitable dilute sugar solution and compare grains immediately and after an interval. Look for emergent pollen tubes. A tube indicates germination; the absence of tubes alone may reflect species, medium, timing or temperature rather than proving the pollen was dead.
Quick Quiz
Quiz
What is the major protective component of the pollen exine?
Through which region does a pollen tube typically emerge?
Which mitotic event produces the two male gametes?
A pollen grain contains a vegetative cell and two male gametes. At what stage was it shed?
Why can compatible-looking pollen on a stigma still fail to fertilise an ovule?
Practice Problems
- Differentiate a haploid microspore, a two-celled pollen grain and a three-celled pollen grain.
- Explain how the exine and germ pore cooperate during reproduction.
- A pollen mother cell has 2n = 18 chromosomes. Determine the chromosomes in the generative cell and each male gamete.
- Explain why pollen banks need controlled handling rather than simple room-temperature storage.
A microspore is the single haploid product of a pollen mother-cell meiosis. After an unequal mitosis, it forms a larger vegetative cell and smaller generative cell: the two-celled pollen grain. A further mitosis of the generative cell yields two male gametes, giving a three-celled grain. All three stages remain haploid in their gametic nuclear lineage.
Key Takeaways
• A pollen grain is a reduced male gametophyte developed from a haploid microspore. • The exine protects pollen; the intine contributes to the germinating tube. • Germ pores or apertures are regions where pollen-tube emergence can occur. • A vegetative cell supports tube growth, while a generative cell produces two male gametes. • Pollen may be released in two-celled or three-celled condition. • Meiosis produces haploid spores; later mitosis produces the gametes. • Pollen viability and compatible pistil interaction determine success after transport.