Taxonomy and Classification ❮ 159early land plants was the need to find a way to distribute water and nutrients throughout
the plant—not as much of an issue when the plant was submerged in water. The solution
to this issue was the development of the xylem and phloem, which you will read about in
Chapter 14, Plants. The xylemis the water superhighway for the plant, transporting water
throughout the plant. The phloemis the sugar food highway for the plant, transporting
sugar and nutrients to the various plant structures.
The first vascular plants (also referred to as tracheophytes) to evolve did not have seeds.
Two major evolutionary changes occurred that allowed the transition from bryophytes to
seedless vascular plants:- The switch from the gametophyte to the sporophyte as the dominant generation of the
life cycle. - The development of branched sporophytes, increasing the number of spores produced.
The major seedless vascular plants you should know are ferns,which are homosporous
plantsthat produce a single spore type that gives rise to bisexual gametophytes. The spores
tend to exist on the underside of the fern leaves. A heterosporous plantproduces two types
of spores, some of which yield male gametophytes (microspores), and others produce
female gametophytes (megaspores). The dominant generation for ferns is the sporophyte.
Seed Plants
Gymnosperm
The third major plant category to branch off the phylogenetic tree is the seed plant. Three
major evolutionary changes occurred between the seedless vascular plants and the birth of
seed plants:- Further decline in the prominence of the gametophyte generation of the life cycle.
- The birth of pollination.
- The evolution of the seed.
A seed is a package containing an embryo and the food to feed the developing embryo that
is surrounded by a nice protective shell. The first major seed plants to surface were the
gymnosperms.These plants are heterosporous and usually transport their sperm through
the use of pollen—the sperm-bearing male gametophyte. Not all gymnosperms have pollen;
some have motile sperm. The major gymnosperms you should remember are the conifers,
plants whose reproductive structure is a cone. Members of this division include pine trees,
firs, cedars, and redwoods. These plants survive well in dry conditions and keep their leaves
year-round. They are evergreens and usually have needles for leaves.
Angiosperm
The final major plant evolutionary category to branch off the phylogenetic tree is the flow-
ering plant. Today there are more angiospermsaround than any other kind of plant. There
are two major classes of angiosperms to know: monocots (monocotyledons) and dicots
(dicotyledons). A cotyledonis a structure that provides nourishment for a developing plant.
One distinction between monocots and dicots is that monocots have a single cotyledon,
while dicots have two.
One interesting evolutionary change from the gymnosperm to the angiosperm is the
adaptation of the xylem. In gymnosperms, the xylem cells in charge of water transport
are the tracheid cells,whereas in angiosperms, the xylem cells are the more efficient
vessel elements.Don’t worry too much about this distinction, but store away in the back
of your mind that vessel elements are seen in angiosperms, while tracheid cells are seen
in gymnosperms.KEY IDEA
KEY IDEA
KEY IDEA
http://www.ebook3000.com