What is Pollen?

(Part II)

In part I of this discussion about pollen, I talked about wind-pollinated plants versus animal-pollinated plants with reference to the number of pollen grains produced by the plants and indicated that wind-pollinated plants produce greater numbers of pollen grains.  In addition to the number of grains produced by different plants there is often a difference in the size and external appearance of the pollen grains as well.  The size and appearance of pollen grains is extremely varied, with a range in size from very small (5-7 microns*) to quite large (over 200 microns), and wide variation in shape and ornamentation also.  Most pollen grains probably fall in the size range of 30-40 microns.  Most wind-borne pollens are small to medium in size with a common size range of approximately 15-35 microns.  There are some larger types of pollen that are transported by wind but these grains are typically modified to have air bladders which make them more buoyant in the air. (See figures below of some of these typical air-borne pollen grains.)

Pollen grains are composed of two layers, an inner layer called the intine and an outer layer called the exine.  The intine is composed of “normal” cell-wall material; substances such as cellulose and pectic compounds.  Internal to the intine is the cytoplasm of the pollen grain which includes the nuclei formed from the meiotic and mitotic divisions that were required to produce the mature pollen grain.

The outer layer of the cell wall of pollen grains is called the exine.  The exine is composed principally of a material called sporopollenin.  The exact composition of sporopollenin isn’t known but it is extremely resistant to decay, being generally insensitive to enzymatic activity and chemical degradation.  The exine layer is initiated while the grain is still present in the group of four cells called the tetrad which is formed following meiosis in the anthers, but most of its thickness is added after the pollen grains separate from one another.  The exine is often sculptured in various ways to produce intricate and beautiful patterns.  (See figures below.)  Most pollen grains have openings in the exine through which the pollen tube grows following germination of the pollen.  The pollen tube then grows down the style and makes its way to the ovule present in the ovary.  Openings in the pollen grains may be pores or furrows (called colpi) or pores within furrows.  Pollen grains with pores are said to be porate, those with furrows are described as colpate, and those having pores in the furrows are referred to as colporate.  Some pollen grains have no openings in the exine and are labeled as inaperturate pollen.

Sweetgum red cedar

Sweetgum – many pores in exine                         Red cedar – no pores or furrows

Ragweed Long leaf pine

Ragweed – small grain and tricolporate             Long-leaf pine – “Mickey Mouse” ears are air bladders – large                                                                                           pollen grain

Pollen grains vary in shape.  Some grains when viewed with a light microscope have contours that appear triangular, or circular, or oval, while others have modifications of these basic shapes.  The surfaces of pollen grains vary greatly in their appearance as well.  Some appear almost smooth while others are fine to coarsely granulate.  Some have distinct striations and others appear to have wrinkles on their surfaces.  Pollen grains produced by plants in the sunflower family have pollen grains that possess spines.

All of these variations in pollen grain shape, size, and ornamentation permit the identification of the species or family of plants which produces the pollen.  Knowing which plants are producing pollen at a particular time is important for allergy sufferers.  It means that if these individuals stay aware of when particular plants are releasing their pollen, they can take added precautions and reduce the likelihood of a major bout with allergic rhinitis or asthma.

* A micron is one millionth of a meter or a thousandth of a millimeter.  For comparison, the thickness of a piece of typical printing paper is approximately 0.1 millimeter which is 100 microns.

H. Wayne Shew, Ph.D.

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