The bryophytes and ferns did a relatively good job of invading the wetter parts of the land environment, but one critical weakness kept them from spreading farther: the need for water to get sperm from male to female plants (or plant parts). Both had developed wind-blown spores to spread asexually, but as we've seen, asexual reproduction limits the speed that organisms can adapt to new environment, and the land was full of new environments. Still, it's quite likely that ancient forms of ferns and their relatives spread fairly widely - its not easy to tell because such organisms don't fossilize easily. We do know that, for a time, the landscape was very foresty, but the "trees" were really huge tree ferns. How much the rest of the environment resembled modern versions is harder to guess at, but animals were spreading across the land very successfully, and that suggests lots of food choices, lots of plants to occupy the first level of the food chain. Also, as often happens, there was apparently a major climate shift away from generally wet to widely drier, providing even more advantages to those best suited to the new conditions.
The adaptation that really led to an evolutionary leap was
pollen, a tiny male gametophyte that, in its first versions, could be carried by the wind like spores to the female gametophytes. Once there, the pollen sprouts a vascular-tube-like "tunnel," a pollen tube, that a sperm could travel down to get toe the egg cells. No longer would the two genders have to be close to one another and need some sort of open water between them for the sperm to swim through. Plants would still need water for their chemistry and specifically for photosynthesis, but they wouldn't be limited to environments where open water was periodically available. These new types of plants could carry gametophytes high above the ground, where sporophyte embryos could be encased with some "starting off" food and also be set off in the wind. The casings with little sporophytes in them were seeds, and all of the groups to evolve from pollen-bearing plants would also be seed plants.
Seed plants are all vascular plants, with an internal tube system that brings water and nutrients up from the roots through thick-walled xylem tubes, and phloem, that carry fuel back to the roots for use and for storage. There are two major types of seed plants: the gymnosperms (Latin for "naked seeds") and the angiosperms (Latin for "covered seeds," sort of). This chapter will deal with the gymnosperms.