Pollen Structure and formation

Pollen is a powdery substance produced by seed plants.It consists of pollen grains (highly reduced microgametophytes) that produce male gametes (sperm cells). Pollen grains have a hard shell made of sporopollenin that protects the gametophytes during their movement from stamens to pistils in flowering plants, or from male cones to female cones in gymnosperms.If pollen lands on a compatible pistil or female cone, it will germinate, producing pollen tubes that transfer sperm to the ovule containing the female gametophyte.Individual pollen grains are small enough that zooming in is required to see details.The study of pollen is called palynology and is very useful in paleoecology, paleontology, archaeology and forensics.Pollen in plants is used to transfer haploid male genetic material from the anthers of one flower to the stigma of another flower in cross-pollination.In the case of self-pollination, the process occurs from the anthers of one flower to the stigma of the same flower.Pollen is rarely used as a food and food supplement. It is often contaminated with pesticides due to agricultural practices.

Structure and formation 

Formation

Pollen is produced in microsporangia in the male cones of conifers or other gymnosperms or in the anthers of angiosperm flowers. Pollen grains come in a wide variety of shapes, sizes, and species-specific surface markings (see electron micrograph, right).The pollen grains of pines, firs, and spruces are winged.Smallest pollen grain, Forget-me-not (Myosotis spp.) are 2.5-5 µm (0.005 mm) in diameter.Corn pollen grains are large, about 90–100 µm.Most grass pollen is about 20-25 µm. In angiosperms, during flower development, the anthers consist of a large number of undifferentiated cells, except for a partially differentiated dermis.As the flower develops, four groups of sporogenous cells form within the anther.The fertile sporogenous cells are surrounded by a sterile cell layer that grows in the anther wall. Some cells grow into vegetative cells that provide nutrition for the microspores formed by the sporogenous cells through meiosis.

In a process called microsporogenesis, after meiosis, each diploid sporogenous cell (microspore mother cell, pollen mother cell, or mother cell) produces four haploid microspores.After the formation of the four microspores contained in the callose wall, the pollen grain wall begins to develop.The callus wall is broken down by an enzyme called callus, and the released pollen grains become larger and take on their characteristic shape, forming a resistant outer wall called the exine and an inner wall called the inner wall.The outer wall is what is preserved in the fossil record.Two basic types of microsporogenesis are recognized, simultaneous and sequential.In simultaneous microsporogenesis, meiotic steps I and II are completed before cytokinesis, whereas in sequential microsporogenesis, cytokinesis follows.Although a continuum of intermediate forms may exist, the types of microsporogenesis have systematic significance.The main form in monocots is continuous, but there are important exceptions.During microgametogenesis, unicellular microspores undergo mitosis and develop into mature microgametophytes containing gametes.Among some flowering plants,The germination of the pollen grain may even begin before it leaves the microsporangium, the germ cell that forms the two sperm cells.

Structure

With the exception of some submerged plants, mature pollen grains have double walls.The vegetative and germ cells are surrounded by an unaltered thin, fragile cellulose wall called the endospore or inner wall, and a tough outer cutinous wall composed mainly of sporopollenin, called the exospore or exine.The outer walls are often thorny or warty, or have various carvings, marking features that are often of value in identifying genus, species, or even cultivars or individuals.Spines may be shorter than 1 micron in length (spinulus, plural spinuli), called spinulose (scabrate), or longer than 1 micron (echina, echinae), called echinate.Various terms also describe sculpture, such as reticulation, a net-like appearance consisting of elements (murus, muri) separated from each other by cavities (plural lumina).These mesh structures may also be called brochi.The pollen wall protects the sperm as the pollen grain moves from the anther to the stigma; it protects vital genetic material from desiccation and solar radiation.The surface of the pollen grain is covered with waxes and proteins that are held in place by structures called sculptural elements on the surface of the grain. The outer pollen wall, which prevents the pollen grain from shrinking and crushing genetic material during drying, consists of two layers.These two layers are the tectum and the foot layer, which is just above the viscera. The canopy and foot layers are separated by areas called colums, which consist of reinforcing rods.The outer wall is made of a resistant biopolymer called sporopollenin.Pollen pores are regions of the pollen wall that may involve thinning of the exine or a marked reduction in exine thickness.

They allow the grain to shrink and expand due to changes in moisture content.The process of shrinking the grain is called harmony.The elongated pores or grooves in the pollen grain are called colpi (singular: colpus) or sulci (singular: sulcus).More circular pores are called pores. Colpi, sulci, and pores are the main characters to identify pollen classes.Pollen can be called aporous (without pores) or aporous (with pores).The pores may have covers (opercles) and are thus described as covered.However, the term non-aperture covers a wide range of morphological types, such as functionally non-aperture (cryptic pore) and fully open pore.Unsaturated pollen grains usually have thin walls, which facilitate germination of pollen tubes at any position.Terms such as monoporous and triporous refer to the number of pores present (one and three, respectively).Spiraperturate means that one or more pores are spiral-shaped.The orientation of the furrow (relative to the original tetrad of microspores) classifies the pollen as furrowed or furrowed.When the pollen grain is in a tetrad, there is a groove in the middle of the outer surface of the trough pollen.If the pollen has only one furrow, it is described as single furrow, with two furrows, as double furrow, or more, as multifurrow.Colpate pollen can also be described as polycolpate if there are more than two wrinkles in the middle of the outer surface.Syncolpate pollen grains have two or more colpi fused at the end.Pollens of eudicots have three colpi (tricolpate) or have a shape evolved from tricolpate pollen.The evolutionary trend of plants is from single-slot to multi-slot or porous pollen.In addition, gymnosperm pollen grains often have air sacs or vesicles called vesicles.The sacci aren't actually balloons, but spongy, which increases the buoyancy of the pollen grain and helps it float in the wind, since most gymnosperms are wind plants.Pollen can be unicastic (contains one sac) or dicystic (contains two sacs). Modern pine, spruce, and yellowwood trees all produce sac pollen.