Natural History:
It is said that the evolution of seeds was the single most important development in the history of plants.
A number of lineages evolved to reproduce by seeds however only one group of seed plants became the ancestors of all todays seed plants.
A prerequisite development to the evolution of seeds, was the development of heterospory.
Heterosporus plants produce two types of spores.
One is the small male spore called the microspore the other is the larger female spore called the megaspore.
Each gives rise to a gametophyte of one gender and so can either make male or female gamites but not both.
Unlike homosporus gametophytes which produce both male and female gamites.
The next step in the evolution of seeds was the development of the ovule.
It is the ovule that becomes the seed coat.
The female megaspore becomes retained in the megasporangium on the sporophyte.
Thus the female gametophyte never becomes independent of the sporophyte.
Over time plants develop a protective covering for the female gametophyte by the fussion of nearby leaves.
This covering or ovule would remain after firtilization to protect the embrio and so become the seed coat.
At the same time the male microspores were developing into pollen.
First microspores would have become wind born germinating when they land on or near the megaspore.
At first they would still release motile gametes (sperm) as the most primitive Gymnosperms still do today.
These early seed plants were gymnosperms (naked seed plants) and appeared about 360 MYA.
Some male gametophytes would grow a tube into the female plant tissue to absorb nutrients to assist in the gametophytes growth.
Eventually this tube would be used to deliver the male gamites (sperm) directly to the egg in the ovule.
This is how firtilization occurs in the more advanced gymnosperms such as conifers.
Conifers which all Yukon gymnosperms are, appeared 310 MYA.
Today conifers cover 25% of the earth surface.
Over time the seed plants became more sophisticated.
They started to protect thier seeds in an ovary and they started to produce flowers to attract insects for firtilization.
The first angiosperms (flowering seed plants) appeared only 130 MYA but now account for 96% of all vascular plant species.
The Seed
Seeds have been called the "time capsules of life".
They have the ability to suspend the growth of the plant embrio untill conditions are right for germination.
This has proven to be a huge evolutionary advantage.
Today 96% of all plants are seed plants.
Life Cylce
With the development of the seed, the gametophyte generation became much reduced and dependent on the sporophyte.
The growth of the female gamite takes place in the ovule.
The male gamete now in the form of pollen, germinates once it has reached the appropriate part of the female reproductive structure.
There it usually deposites its microspores directly on or in the female gamitophyte.
Upon firtilization a sporophyte zygote is formed.
This zygote grows to an embrio while the rest of the female gametophyte and ovule developes into the seed.
The plant embrio will stop growing and stay in suspended animation in the seed.
The plant embrio will only germinate (commence growth) when the proper environmental conditions are met.
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