Pollen Germination on Stigma Through a Permanent Slide

Pollen germination refers to the growth of pollen tubes from pollen as it lands on stigma. As soon as it lands on the stigma, the stigma releases a sticky organic sugary substance that allows the germination of the pollen grain. The transfer of pollen grains from male part of the flower, i.e, anther to the female part of the plant, i.e, stigma is referred to pollination. The germination is also assisted by the absorption of nutrients and water by pollen. 

Key Terms: Germination, Pollen Grains, Pollen Tube, Stigma, Flower, Plant, Vegetative Cell, Generative Cell, Gametes, Pollination, Anther

Pollen Grains

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Pollen grains are tiny powdery substances, usually spherical in shape with size ranging from 25-50 μm in diameter. Its outer layer is called exine and the inner layer is called intine. The exine consists of a complex substance called sporopollenin known as the most resistant biological material known. It can withstand high temperatures, alkalis, and strong acids, and show resistance against degradation enzymes. The intine is thin and uniform and is made up of pecto-cellulose. It comes out of the germ pore and gives rise to the pollen tube during germination of pollen.

Pollen Germination

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The process of pollen germination is described below:

• Upon the arrival of pollen grain on the stigma of the plant, a tube emerges from the germ pore of the pollen grain and moves through stigma, style, and finally reaches the ovary. 
• The pollen tube is nourished by the sugary substance and grows until it reaches the ovule in the ovary. The length of the pollen tube varies between a few millimetres to 45 cm (in maize) in different plants and depends on the length of the style. 
• During the growth of the pollen tube, the vegetative cell or the tube cell enlarges and comes out of the pollen grain through the germ pore and forms the pollen tube. 
• The generative cell within the pollen grain tube cell divides to form two sperm cells (male gametes).
• As the pollen tube grows, the two male gametes (present inside each pollen grain) go into the tube. 
• The pollen tube finally releases two male gametes into the embryo sac to carry out the process of double fertilisation.

Experiment

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Aim: To observe pollen germination on stigma through permanent slide.

Material Required:

• China Rose
• Boric Acid
• Forcep
• Microscope
• Sucrose (10 g)
• Coverslips
• Slide
• Dropper

Procedure:

1. The first step is to prepare the nutrient rich solution by mixing sucrose and boric acid together in water.
2. Pluck stigma from China rose (bisexual flower) using forcep.
3. Put the stigma on the slide.
4. Now with the help of a dropper, add a dropwise solution on the slide and hold for 20 seconds.
5. Cover with cover slide.
6. Observe under a microscope.

Observation:

The germination of pollen grains in nutrient rich medium can be seen as the vegetative cells enlarge. The nucleus grows and gives rise to a pollen tube which emerges from one of the germ pores. Followed by the growth of the pollen tube, two male gametes (spherical or lenticular in shape) are formed. The pollen grains that germinate are known as viable pollen grains whereas the pollen grains which do not germinate are known as non-viable pollen grains. 

Precautions:

Some precautions as mentioned below should be taken while conducting the experiment:

• Use freshly plucked flowers only.
• Use fresh and dry slides.
• Use only 1-2 drops of sucrose solution, otherwise it may hinder the process of germination of pollen.

Things to Remember

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• Pollens are powdery substances of different shape and size present in anther, consisting of a bulk of microspores. These microspores undergo two mitotic divisions to form male gametophyte. 
• First mitotic division results in vegetative cells (non-reproductive) whereas second mitotic division results in formation of generative cells that give rise to two male gametes.
• The generative cell is small and enclosed within the cytoplasm of the vegetative cell.
• When pollen lands on stigma, they first identify whether they are of their own species or different. If they are of different species, it does not allow them to germinate.
• During pollen germination, vegetative cells enlarge and come out of the pollen grain through the germ pore and form the pollen tube. 
• Pollen tube goes through stigma, style and finally reaches the ovary. 
• The generative cell within the pollen grain tube cell divides to form two sperm cells (male gametes). It releases gametes into ovules for double fertilisation.
• To study the germination of pollen on stigma, the pollen must be present on stigma. China rose was preferred as it has both male and female reproductive parts in one flower.