Some of the earliest plants which evolved to bear seeds Gymnosperms ( like cycads) did not have their ovules covered in the ovary. They called it naked ovules. They were even borne on the leaves in some species which are now represented in fossil records. Covering the ovules in a cover of ovary which is modified leaf in broadest sense was sign of evolution in plants.
The primitive angiospersms had big flowers. The ovules once covered in the ovary were hardly visible to male pollens to fertilze them. So male pollens have to be told where lies the egg by huge petals attracting them. Nature made flowers to attract male pollens or insect which will bring pollens. Why the insect should do this job . Nature loaded their plants with honey nectar so that insects visit them for collecting nectar. This lead to pollen transmission to female flowers.
Just like developing societies had elaborate marriage ceremonies lasting for weeks or months ( even in India 20 years ago marriage lasted minimum of 3 to 5 days)
So were the plants in the beginning. Gymnosperms needed almost 3 seasons or years from pollination to seed setting ( in Pines it can be seen even today).
Gradually with evolution as the time between proposing and fertilization or consumation of marriage got reduced from years to some days to some hrs ( or marriage instituion was being replaced by live ins ) the evolution in plants was reflected in disappearance in organs of attracting insects like beutiful petals , nectar etc. The highly evolved plants like highly evolved societies did not need elaborate and wasteful arrangements of marriage to produce offsprings which was only ment to produce offsprings. The monocot plants (like wheat, rice, maize etc) have highly reduced flowers and invest minimum energy in flowering parts and make maximum gain of grains. The long affairs of marriage and attracing males were replaced with simple things.
However once again ovules are getting uncovered to attract pollens or males . Are we moving forward or backwards in ladder of evolution only fossil records will show millions of years henceafter.
Signs of evolution in plants
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