Chapter 12. Pteridology ~ The Ferns[edit | edit source]
Pteridology is the study of ferns—plants classified in the Division Pterophyta (or Filicophyta). Ferns do not have seeds the way trees and flowering plants do. Rather, they have spores the way mosses do. The haploid spores grow small haploid organisms, which then undergo fertilization and grow the diploid fern plant directly out of the haploid gametophyte, similar to the sporophyte stalk growing out of the moss. The larger part, what we think of as the fern, is the sporophyte. The gametophyte is a small green prothallus that the sporophyte grows out of. Ferns are still tied to an aquatic environment, in that once a spore grows into a prothallus, there must be moisture enough for the egg in the prothallus to be fertilized by swimming, flagellated fern sperm.
Having a large sporophyte allows ferns to produce many more spores than a moss could- recall that each sporophyte on a moss only carried one sporangia. Producing many more propagules increased fern presence and dominance. Besides having a larger sporophyte generation, ferns have many important adaptations that increase their capabilities above the mosses. Ferns have roots, which, unlike moss rhizoids, not only anchor, but take up nutrients. Ferns are vascular plants, with lignified vascular tissues. These allow active water transport. That water transport along with the strength of the ligified cells allow ferns to be much larger than their moss ancestors. At one point, ferns and fern trees were the most advanced plant life, and grew even larger than ferns today do, with great size and variety of ferns. There were no flowering plants in the early cretaceous- the first forests of the dinosaurs were composed of fern trees.
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Sporophyte Structure[edit | edit source]
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Evolution and ferns[edit | edit source]
Ferns have a big advantage over the mosses in their vascular tissue. They can grow taller, and can exist in more diverse environments. This is a trend that will continue in evolution, eventually leading to the rise of such large sporophyte generations as the great sequoia trees. But if ferns are so much more fit for survival, why are there still mosses? And if a larger sporophyte generation is more fit, why haven't sequoias become dominant enough to eliminate the ferns? While there are clear benefits to a larger sporophyte generation, in some recurring natural situations, natural selection favors mosses over ferns or ferns over trees. Spores are better at spreading by wind than many seeds are, for instance. So while in the long term, the protection of a seed allows seed plants to be dominant on the planet, in many situations the lightness and transport of a spore is still efficient in spreading ferns.
There are two gena that have showed immense diversity in the division Pteridophyta.Selaginella and Equisetum have been identified to be the only gena that are heterosporous.
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