Virginia Tech
Virginia Polytechnic Institute: Greenhouse Dinosaur Extinction Theory
This comprehensive page discusses the Asteroid Impact vs. Volcano-Greenhouse dinosaur extinction debate. It was written by paleontologist, professor, and scientist Dr. Dewey McLean.
Other
Hamilton College: Plant Kingdom Spore Producers
A solid overview of the few plants that reproduce by spore production. Includes examples and an explanation of the early evolution of these plants.
University of Hawai'i
University of Hawaii: Bryophyte Reproductive Cycle
An explanation of the plants that reproduce by the production of spores. Includes extensive graphics.