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When you see a pine tree,
or 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
pine trees, the males cones are smaller and
located at the tree tops, while 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, while 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.
Pollen from the male
cones makes contact with the sticky surface of a female cone and
adheres there. It sprouts and grows down into the female cone,
forming what is known as a
pollen tube; this may
take as long as a year-and-a-half. The pollen
tube eventually makes contact with the part of the female cone, the ovule,
that holds the egg cells, and the sperm from the pollen moves down
the tube and fertilizes the egg cell. Many pollen produce pollen tubes and fertilize many egg cells
in a female cone.
The fertilized egg cell,
the zygote, grows into an embryo which
is encased in a
seed. The seeds have
"wings"
and are light enough to be carried off in the wind, once the female
cone "opens" to release them. The seeds that settle
in hospitable places
sprout and grow into new sporophytes. The
sporophytes have true roots, and true stems, and true leaves,
although the leaves are usually needles. These
long thin leaves are more effective than flat leaves at
conserving water. They also commonly
produce an oily waterproof layer, a cuticle, to reduce
evaporation from the inside of a needle.
Much of the patterns and even structures in
gymnosperms are found again in the angiosperms,
with a few exceptions and several additions.
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