a small grain that consists of a few cells. To the naked eye it appears as
a yellowish (pale yellow) dust-like substance that is either dispersed by wind
Pollen is formed within the sacs (or microsporangia) in the anthers that are located in plant flowers. The development of plant anthers also involves the growth and differentiation of tissue that develop to produce the pollen sacs. This is then followed by a process of cell division (meiosis) that results in the formation of clusters referred to as quartets within the pollen sac chambers.
The quartet then undergoes further divisions to produce pollen, which serves as the male gametophyte of plants (seed plants) for reproduction. Depending on the type of plant, pollen is either dispersed by wind or insect to the receptive stigma.
* A plant that is pollinated through wind is referred to as anemophilous while those by insects are known as entimophilous.
* Once a pollen grain falls on the receptive stigma, it starts absorbing water and becomes activated. This causes the gametophyte in the grain to develop a tube reaching the ovule of the flower, which in turn allows male gamete cells to be transported to the ovule for fertilization.
* If the pollen is not to be used immediately, it should dried and stored frozen.
When viewing pollen grains under stereo microscope, it is advisable to view treated pollen (washed using a little alcohol) and untreated grains separately in order to see the difference.
The procedure involves the following simple steps:
Students may also place a healthy plant anther with pollen (without treating it with alcohol or adding glycerine) on the stage of the stereo microscope and try observing how it appears
* Pollen grains have an oily surface that can be easily removed using alcohol. Therefore, alcohol may be used to remove the oily layer and thus get a better look at the grain surface. Fifty percent glycerine is used for the purposes of hydrating the grains and making them swell for good viewing.
When viewed under the stereo microscope, pollen grains will appear as grossly shaped, irregular structures/particles. However, the shape and appearance of the grains will vary depending on the type of pollen under investigation. For untreated grains, there is poor contrast compared to treated pollen grains.
Glycerol Jelly Method
The glycerol jelly technique is one of the most common mounting techniques used for pollen. It involves using glycerol jelly, which consists of 10 grams of gelatin, 35ml of distilled water as well as 30 ml of glycerol.
For best possible results, students are advised to use freshly collected pollen.
When viewed under the microscope, the stained slide is clearer because of better contrast. The grains will appear as tiny ovoid particles with what seems like a scaly surface or ornamentations.
The unstained slide appears more transparent /translucent and does not give a clear view of the grain surface. However, the appearance of the grains is also largely dependent on the type of plant from which the pollen was obtained.
Wet Mount Technique
* For this technique, toluidine blue or acetocarmine may be used for staining the sample for clarity.
Dry Mounting Technique
This is the simplest method that involves the following simple steps:
Pollen tubes are important structures that grow down the style allowing for gametes to the transported to the ovary for fertilization. To view pollen tubes under the microscope, Farmer's solution and sodium hydroxide can be used.
When viewed under the microscope, the tubes will be seen to look like fragile tubes (like flexible straws).
For electron microscopy, acetolysis is one of the most popular methods used for preparing pollen. However, it has been shown to cause distortions, which influenced the development of a new and better method. One of the best and most recent methods involves the use of Aerosol-OT and amyl acetate.
* acetolysis involves the use of acetic acid for the purposes of breaking down compounds (organic compounds)
When viewed under the SEM, the grains can clearly be seen as inflated or deflated structures (of varying shapes) with rough surfaces or cleavages depending on the type of pollen being viewed.
When viewed under the microscope (compound microscopes or electron microscopes) the grains may have different types of ornamentations with the ornamentations being irregularly distributed on the surface of the grains.
For instance, whereas some of these ornamentation may be visible all across the surface of the grain, others may only be present on polar ends of the grain or across given sections on the surface of the grain.
Cameron Thompson (2016) Techniques for Viewing Pollen Tubes in Angiosperm Flowers.
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