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.
adaptation that really led to an evolutionary leap was
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 sperm 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 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.