What are Pollen grains?
- Pollen grains are known as Micro-spores.
- They are the male reproductive bodies of a flower.
- Pollen grains are microscopic structures that carry the male reproductive cell of plants.
- The pollen grain contains cytoplasm along with the tube cell (which becomes the pollen tube) and the generative cell (which releases the sperm nuclei).
- They are very minute in size and are like particles of dust.
Importance or Functions of Pollen grains:
- Pollen is essential for sexual reproduction of flowering plants and plants that produce cones.
- Each pollen grain contains male gametes necessary for fertilization.
- The scientific study of living and fossilized pollen grains is known as Palynology.
Site of the pollen grains formation:
- A stamen is the male reproductive organ of a flower (Fig 1a).
- The stamen is made up of 2 parts: the anther that is held up by a filament.
- The anther contains the pollen sac or Microsporangium in which the pollen grains are produced (Fig 1b). These pollen grains are used during pollination.
How the pollen grains are produced?
Microsporogenesis:
- The process of formation of haploid microspores from a Microspore Mother Cell (MMC) or Pollen Mother Cell is called Microsporogenesis
- An anther is a tetragonal structure consisting of four microsporangia located at the corners.
- Two microsporangia are located in each lobe of the anther.
- The microsporangia undergo development and become pollen sacs.
- Microspores are arranged in a cluster in a cluster of four cells- the microspore tetrad
- As the Anthers mature and dehydrate, the Microspores dissociate from each other and develop into pollen grains
- The pollen sacs are packed with pollen grains
Structure of Pollen grains:
- The Pollen grains represent the Male gametophytes
- Each pollen grains has a two-layered wall. The outer hard layer called the exine is made up of sporopollenin which is one of the most resistant organic material that enables them to resist high temperature and strong acid and alkali. No enzyme is yet known to degrade sporopollenin
- The region on exine where sporopollenin is absent are called the germ pores. It helps in the formation of pollen tube The inner layer is thin called as intine. It is composed of cellulose and pectin
- Chemical nature of intine and exine:
- The intine is pecto-cellulosic in nature, and generally destroyed during acetolysis, while the exine consists of cutin, and it is acetolysis-resistant layer. It is also resistant to biological and physical degradation
- A mature pollen grain contains two cells, the Vegetative and the Generative cell •The Vegetative cell is bigger, has abundant food reserve and a large irregular shaped nucleus
- The Generative cell is small and floats in the cytoplasm of the vegetative cell
- In about 60% of Angiosperms, Pollen grains are shed at this 2-celled stage
- In the remaining species, the Generative cell divides mitotically to give rise to two male gametes before Pollen grains are shed (3 celled stage)
- At certain places the exine is thin or absent and the areas may have thickened intine or deposition of callose
- It’s called germ pores (if rounded) or Germinal furrows (if elongated)
- In insect pollinated pollen grains the exine is spiny as well as covered by a yellowish, viscous sticky and oily layer called Pollenkit. Pollenkit is made up of lipid, carotenoids and some proteins. It helps in insect attraction and protect pollens against radiation
Size of Pollen grains:
- Pollen grains are very minute in size and are like particles of dust
- It is commonly Globular in outline, through several other shapes are also found.
- The diameter is mostly 25-50 µm
- The size of pollens varies from nearly less than 10µm (eg. Myosoti salpestris-Figure 2a,b,c) to more than 200µm (eg. Mirabilis jalapa and some Cucurbitaceae)
Sculpturing of pollen grains:
the tectum is an outermost layer of a pollen grain. The Tectum provides a characteristic sculpturing or designs over the surface of pollen grains. Examples: ridges, Tubercles, Spines, Reticulations
Erdtman characterised the external surface of pollen by various markings which has been mentioned below. it is also known as the exine of the pollen constitute the Sculpturing. The main types are as follows:
- Psilate (smooth)
- Foveolate (pitted)
- Fossulate (grooved)
- Scabrate (with fine projections)
- Verrucate (warty)
- Baculate (rod like elements)
- Pilate (rod like projections with swollen beads)
- Gemmate (sessile pillar like)
- Echinate (spiny)
- Regulate (with elongate irregularly distributed elements)
- Striate (with elongate parallel elements)
- Punctate (with minute pores) Reticulate (with net like elements)
Apertures of Pollen grains:
Pollen grains are generally
(a) Tricolpate (with three germ pores) Example: Dicot (Figure a)
(b) Monocolpate (with single germinal furrow) Example: Monocot (Figure b)
Shape of Pollen grains:
Erdtman (1952) demonstrated nine types of Pollen grains. The shape were classified (Figure 4) on the ratio between the length of Polar axis (P) and Equatorial diameter (E). P and E are measured from the equatorial view of a pollen grain
Spheroidal Shape of pollen grains are found in – Cucarbitaceae;
Oblate- Apocynaceae, Euphorbiaceae and Prolate- euphorbiaceae
- Peroblate (ratio of P to E < 0.50)
- Oblate (ratio of P to E between 0.50 and 0.75)
- Suboblate (ratio of P to E between 0.75 and 0.88)
- Oblate spheroidal (ratio of P to E between 0.88 and 1)
- Subspheroidal (ratio of P to E between 0.75 and 1.33)
- Spheroidal (ratio of P to E between 0.60 and 0.70)
- Prolate spheroidal (ratio of P to E between 1 and 1.14)
- Sub Prolate (ratio of P to E between 1.14 and 1.33)
- Prolate (ratio of P to E between 1.33 and 2)
- PerProlate (ratio of P to E between > 2)
References:
- A text book of Botany Angiosperms (1981) B.P. Pandey. first edition, Publishers: S.Chand & Company LTD. Ramnagar, New Delhi-110055
- Class Notes: Flower, Fruits and Seeds: https://classnotes.org.in/class-6/science/getting-to-know-plants/flowers-fruits-and-seeds/
- Socratic Q&A: https://socratic.org/questions/what-is-a-stamen
- https://www.indiabiologyneet.com/2020/08/sexual-reproduction-in-flowering-plans.html
- https://images.app.goo.gl/Ji9jmT6fzoUAabTz7
- http://balikavidyapith.com/Upload/Event/12th,%20BIOLOGY,%2002-05-2020.pdf
- https://rstomar3114.blogspot.com/p/sexual-reproduction-in-flowering-plants_56.html
- Wikipedia: the freeEncylcopedia https://en.wikipedia.org/wiki/Myosotis_alpestris
- PalDat – Palynological Database:an online publication on recent pollen- •https://www.paldat.org/search/genus/Myosotis;jsessionid=15F5DE1E0CB1D51D184DDA04F04DBCB6
- PalDat – Palynological Database:an online publication on recent pollen https://www.paldat.org/pub/Mirabilis_jalapa/302432
- Researchgate- https://www.researchgate.net/figure/Psilate-pollen-typical-for-Schismatoglottis-A-C-Schismatoglottis-celebica-A-hydrated_fig2_282077320
- Researchgate- https://www.researchgate.net/figure/Equatorial-view-polar-view-and-sculpture-details-of-pollen-grains-of-Parthenocissus_fig2_232815339
- Xinci Cui, Yunyun Zhao, Chunhai Zhao and Jiaxi Liu : PALYNOLOGY 2020, Pollen morphology of tribes Alsineae and Sperguleae (Caryophyllaceae) and its systematic significance VOL. 44, NO. 4, 597–620 •
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