Pollen grains are the biological basis of plant reproduction. Grains produced from a mature flower are the consequence of what is known as pollination. Pollination is the fusion between male and female parts of the flower, during which pollen from a male part is transferred to the female part with the help of external agents. Factors that help in pollination may include insects such as honey bees, wind, water, etc. The scientific study of pollen and spores is known as palynology. This article is designed to understand the structure of typical pollen grain.
Structure and characteristics of pollen
Ovules produces ovum-the female gametophytes whereas, pollen is produced by the anthers of the stamens. In gymnosperms, pollen is produced in the microsporophylls of the microstrobili (male pollen cones). It is a yellowish, smooth substance , nevertheless, the colour may vary between yellow, dark yellow, white, red, purple, orange, etc. When you closely watch the sticky pollen under a microscope, you can observe range of characteristic variations such as ; sizes, shapes, designs and other morphological features. Pollen is spherical, measuring approximately 25-50 micrometres in diameter. A typical pollen has a central nucleus that assist in the fertilization. Nucleus has prominent two-layered walls called exine and intine. The outer layer is called exine and the inner layer is the intine. Exine is, made up of sporopollenin -a tough and resistant organic material. Sporopollenin can resist harsh climatic conditions such as high heat from sun, corrosive effect of strong acids and alkali. Pollen grains are able to sustain their actual structure and nature for a prolonged time period so that, they can be preserved as fossils. Intine is structurally made up of cellulose and pectin. Following the exine and intine layers, the fertilization zone begins with cytoplasm surrounded by a plasma membrane. In a mature pollen grain(Papaver dubium L), there are two cells namely the vegetative cell that has a rich supply of food and nutrients and a generative cell that floats in the cytoplasm of the vegetative cell. The generative cell is responsible for fusion with the ovules.
Viability of pollen
In about 60 % of cases, pollen grains are shed at this 2-celled stage (vegetative, generative) but the rest of them undergo cell division mitotically to reach the 3-celled stage. Pollen shedding requires a flower to be completely matured and the temperature of the environment to be generally hot. When the pollen has shed out of the male parts, they have a viability period (period of survival) until then the pollen is active and energetic to fertilize. Fortunately, the period of viability of pollen grains is very high hence it can survive in the high temperature and humidity. Viability varies between species, for example, in some cereals such as rice and wheat, pollen grains lose viability within 30 minutes after their release, but in some of the plants like Rosacea, Leguminosae, and Solanaceae, the viability stays for months.
Uses of pollen (commercial and domestic)
Pollen is sold as packed products. Honey bee farmers use pollen to refill the nutritional requirement of honey bees that are domestically bred. Since pollen grains are rich in nutrients, pollen tablets are used as supplements. Some experimental studies have proved that the pollen consumption boosts the performance of athletes, however,they can be a cause for many allergic diseases . People with skin allergy, asthma, bronchial inflammatory disorders are at high risk for pollen allergy.