Natural History
The
earliest seed plants were gymnosperms (naked seed plants) and appeared
about 360 MYA. A number of groups of plants independently developed
seeds, making gymnosperms a polyphyletic (not one ancestral species)
group of plants. However, only one line of gymnosperms would survive to
the present day. This line has four classes. Only one, the Coniferae, is
represented in Yukon. The first conifers appeared about 310 MYA. They
are more advanced than the earliest gymnosperms in that the male sperm
are delivered to the female egg by a pollen tube rather than by swimming.
Today conifers cover 25% of the earth's surface, and much more in Yukon.
Though conifers are an advanced gymnosperm, they are not the line from
which the angiosperms (flowering plants) evolved. A sister class
to the conifers called the Gnetales were the ancestral line to
the angiosperms (flowering plants).
The Naked Seed
The reason seeds of the Gymnosperms are said to be naked is because the ovule is exposed to the environment for fertilization.
Also, unlike flowering plants, the gymnosperm ovules are not protected in an ovary.
While the ovule becomes the seed coat, the ovaries of angiosperms become the fruit.
Thus with no ovary, gymnosperms produce no fruit to protect or nourish the seeds, so they are naked in this way as well.
Life Cycle
- The mature sporophyte (full-grown tree or shrub) produces both male and female cones.
- The male cones have microsporangia, which produce copious amounts of microspores in the form of pollen.
- The female cones produce female gametophytes in ovules.
- When the male cones release their pollen, the female cones open up, exposing the ovule to the air.
- When
the male pollen falls into the female cone, it will germinate, growing a
pollen tube (in coniferae class) to deliver the male gametes right into
the ovule.
- Upon fertilization, the female cone closes, protecting the zygote which grows into an embryo while a seed develops around it.
- Upon seed maturity, which can take a couple years after pollination, the seed is released.
- The seed will lie dormant until conditions are right for germination.
- Upon germination the sporophyte embryo grows into a new sporophyte (tree or shrub).
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