Malvaceae: Pollen Grain
Pollen grain is a fine coarse powder containing the micro gametophytes of seed plants, which produce the male gametes (sperm cells). Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the female cone of coniferous plants.
When pollen land on a compatible pistil or female cone (ie, when pollination has occurred), it germinates and produces a pollen tube that transfer the sperm to the ovule (for female gametophyte).
The individual pollen grains are small enough to require magnification to see details. The study of pollen is called palynology and is highly used in paleoecology, paleontology, archeology and forensics. Pollen itself is not the male gamete.
Each pollen grain contains vegetative (non-reproductive) cells (only a single cell in most flowering plants but several in other seed plants) and a generative (reproductive) cell containing two nuclei: a tube nucleus (that produces the pollen tube) and a generative nucleus (that divides to form the two sperm cells). The group of cells is surrounded by a cellulose rich cell wall called the intine, and a resistant outer wall composed largely of sporopollenin called the exine.
Pollen is produced in the “microsporangium” (contained in the anther of an angiosperm flower, male cone of a coniferous plant, or male cone of other seed plants pollen grains come in a wide variety of shapes (most often spherical), sizes and surface markings characteristics of the species (see electron micro graph, right). Pollen grains of pines, firs, and spruces are winged.
The smallest pollen grain, that of the forget-me-not (Myosotis Spp), it around 6µm (0.006mm) in diameter. Wind-borne pollen grains can be as large as about 90-100µm.
In angiosperms, during flower development, the anther is composed of a mass of cells that appear undifferentiated, except for a partially differentiated dermis. As the flower develops, four groups of sporogenous cells from
within the anthers. The fertile sporogenous cells are surrounded by layers of sterile cells that grow into the wall of the pollen sac. Some of the cells grow into nutritive cells that supply nutrition for the microspores that form by meiotic division from the sporongenous cells. In a process called microsporogenesis, four haploid microspores are produced from each diploid sporogenous cell (microsporocyte), after meiotic division.
After the formation of the four microspores, which are contained by cellose walls, the development of the pollen grain walls begins.
The cellose wall is broken down by an enzyme called cellase and the freed pollen grains grow in size and develop their characteristic shape and form a resistant outer wall called the exine and an inner wall called the intine. The exine is what is preserved in the fossil record.
The pollen wall protects the sperm while the pollen grain is moving from the anther to the stigma; it protects the vital genetic material from drying out and solar radiation. The pollen grain surface is covered with waxes and proteins, which are held in place by structures called sculpture elements on the surface of the grain.
The outer pollen wall, which prevents the pollen grain from shrinking and crushing the genetic material during desiccation, is composed of two layers. These two layers are the tectum and the fo0t layer, which is just above the intine. The tectum and foot layer are separated by a region called the columella, which is composed of strengthening rods.
The outer wall is constructed with a resistant biopolymer called sporopollenin. The pollen tube passes through the wall by way of structures called APERTURES.
Pollen apertures are various modifications of the wall of the pollen grain that may involve thinning, ridges and pores. They serve as an exit for the pollen contents and allow shrinking and swelling of the grain caused by changes in moisture content. The elongated apertures or furrows in the pollen grain are called colpi (singular: colpus), which along with pores, are a major criterion for the identification of classes of pollen.
The orientation of furrows (related to the original tetrad of microspores) classify the pollen as colpate or sulcate. Eudicots have three colpi (tricolpate) or pollen shapes that are evolutionarily derived from tricolpate pollen other groups having one sulcus (monosulcate).
Except in the case of some submerged aquatic plants, the mature pollen grains has a double wall, a thin delicate wall unaltered cellulose (the endospore of intine) and a tough outer cuticularized exospore or exine.
The exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual. In some flowering plants, germination of the pollen grain often begins before it leaves the microsporangium, with the generative cell forming the two sperm cells.