How Does Pollination Work in Non Flowering Seed Plants?
Plants can be divided into four types--mosses, ferns, non-flowering seed plants and flowering plants--each of which represents evolutionary progress within the plant kingdom. Non-flowering seed plants first achieved seeds, an important evolutionary development, but did not perfect methods of pollination the way that flowering plants would. Instead, the seeds of non-flowering plants develop slowly and rely on the wind for pollination. Does this Spark an idea?
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Evolution
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Early plants, such as mosses and ferns, did not produce seeds. Instead, they relied on water to transport their reproductive cells or spores. The development of seeds is regarded as a major advancement in plant evolution because seeds could disperse with greater ease and had added protection that allowed them to withstand extreme conditions and delay germination until growing conditions were ideal. Non-flowering seed plants dominated during the age of the dinosaurs, and the eventual evolution of flowering plants would represent another leap forward for plant evolution that would result in the diminishment of non-flowering seed plants.
Seed Structure
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Non-flowering seed plants are called gymnosperms, which means "naked seed," so named because seeds do not grow inside of an ovary but form on the outside of a cone. Cones are modified leaves, and the most familiar gymnosperms are conifers. Both male and female cones exist, with the male cones producing pollen and the female cones producing female sex cells that, when fertilized, result in seeds that can germinate into new plants.
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Pollen and Egg Cell Formation
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Pollen contains sperm cells and forms on the scales on male cones. Like their non-seed-bearing predecessors, most gymnosperms rely on mechanical means, usually the wind, to transport pollen to female cones. In female cones, development occurs more slowly, and cells divide to form egg cells that await the delivery of pollen and fertilization.
Seed Fertilization
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When pollen lands on a female cone, part of the pollen cell develops into a pollen tube, which punctures the covering of the female cells and extends until it reaches the egg. Within the pollen, the generative cell splits to form two sperm cells, one of which fertilizes the egg and, eventually, produces an embryo. Development of gymnosperm seeds is a slow process, and fertilized seeds often mature for two years before being shed. Ohio State University's Department of Horticulture and Crop Science says this explains why evergreen trees often have cones of different sizes and at different phases of development on the same tree, as it may take up to three years for seeds to develop.
Comparison with Flowering Plants
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Flowering plants, or angiosperms, represent further enhancement of the mechanisms used for pollination. While gymnosperms rely to a degree on happenstance to fertilize their seeds--they must wait for the wind to carry the pollen to a female cone of the correct species--the bright, sweet-scented flowers of angiosperms attract animals, such as bees and hummingbirds, that transport pollen from plant to plant. The relative efficiency of angiosperm pollination compared to gymnosperm pollination explains why flowering plants are now the dominant plants found on Earth.
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References
- Photo Credit winter pine cone image by Mary Lane from Fotolia.com
