Seeds in Fruit – Class Angiospermae (mono / dicots)
PLANTS INVADE LAND
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.
What is Pollen?
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 to the female gametophytes and sprout a "tunnel" into them that a sperm could travel down.
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).
Divisions of Gymnosperms
There are 4 groups within the Gymnosperms:
Cycads : resemble short palm trees
Gingkos, mostly known because some Chinese species have been used in landscaping around the world
Gnetales : share some traits with angiosperms
Conifers : cone-bearing shrubs and trees.
The conifers are the best-known gymnosperms, and our discussion of gymnosperm life-cycles will center on conifers.
The gymnosperms were probably the first really widely-distributed plant group, and with their rise came the rise of a major animal group: the dinosaurs.
Ginkgoes are often planted in cities not only because they're pretty trees but also because they thrive where air pollution is bad.
It's not surprising that ginkgoes are air-pollution tolerant, because they are very primitive plants; they may have evolved when the earth's atmosphere was even more sulphurous and grimy than today, because of erupting volcanoes.
Ginkgoes are living fossils
Seed coat turns fleshy not ovary wall
Gymnosperm Life cycle
When you see a pine tree, a spruce or a cone-bearing shrub, the "main plant" is a sporophyte
The gametophyte form is confined to the cones which commonly have male and female versions.
In many trees (though not all)
the males cones are smaller and located at the tree tops
the female cones are larger and found farther down.
The sizes relate to the needs of the two genders:
the female cones will generate seeds, and will need to be bigger
the male cones just produce tiny wind-carried pollen (and even though a lot of pollen is made, not much room is needed for it).
The location on the trees reflects how the pollen is wind-spread, with the male cones as high as can be for best wind access, and the female cones lower since much of the pollen will settle eventually.
The transfer of the entire microgametophyte (pollen grain) to the vicinity of the megagametophyte (cone)
Wind or animals usually do this
Roles and Uses of Gymnosperms
Roots of conifers supply amino acids to some fungi (eg.mycorrhiza)
Stems of cycads/seeds ofginkgo sources of starch
Needles of eastern white pine are rich in Vitamin C