Seeds in plant tycoon are fairly simple. There are four different seeds for each plant, based on the rarity of the flower. As the flowers get rarer, seeds tend to become more complex and larger.
Common seeds come from the bluestar, citrus, fourpetal, jalapa, and spotted varieties of plants.
The next level of seeds (uncommon, if you will) come from daisy, fragrant, rosaceae, and venemous flowers.
Rare seeds are abundant: aureus, blazing, fabled, mela, mystic, nox, painted, and viola all produce this third level.
Finally, Megarare seeds are produced by plants bearing the arthurium, baccatus, lilia, tahitian, tilia, and venus flowers.
Pictures of the seeds (and their corresponding plants) can be found here. Common seeds are in the left column, uncommon in the second, rare in the third, and megarare in the far right column.