PLANT DEVELOPMENT & REPRODUCTION - CBSE International

The inner wall of pollen grain is called intine. It is a thin and continuous layer made upof cellulose and pectin. The cytoplasm of pollen grain is surrounded by plasma membrane.When pollen grain gets matured, it contains two cells, the vegetative cell and generative cell.Vegetative cell is bigger, contains abundant food reserve and a large irregularly shaped nucleus.The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle shapedwith dense cytoplasm and a nucleus. In 60 percent of angiosperms, pollen grains are shed atthis 2-celled stage. In the remaining species, the generative cell divides mitotically to give riseto the two male gametes before pollen grains are shed (3-celled stage). When once pollen grainsare shed, they have to land on the stigma before they lose viability if they have to bring aboutfertilization. The period for which pollen grains remain viable is highly variable and to someextent, depends on the prevailing temperature and humidity. In some cereals such as rice andwheat, pollen grains lose viability within 30 minutes of their release, and in some members ofrosaceae, leguminosae and solanaceae, they remain viable for months.You may have heard of storing semen/sperms of many animals including humans for artificialinsemination. It is also possible to store pollen grains of a large number of species for years inliquid nitrogen (at -196 ºC temperature). It is called cryopreservation. Such pollen banks storepollen and can be used as pollen reserves or banks (similar to seed banks), in crop breedingprogrammes.Pollen Grains and Human HealthWhat are the advantages anddisadvantages of pollen shedding atdifferent stage? How long, do youthink, the pollen grains retain viability?Could you explain what may be thepossible reason for short life span ofpollen grain in wheat, rice?Unit 1 Plant Development & Reproduction 30