Many of these tales come from Ovid's Metamorphoses. They are creations of the Ancient Greeks as a way to explain their world.
The Cypress: An Asian Prince named Cyparissus accidently killed his favourite Stag and almost died of heartbreak. Apollo had taken a bit of a liking to the youth and in order to prevent his death, turned him into a Cypress tree.
The Heliotrope and the Frankincense: In Greek, this is a generic name for plants that turn towards the sun. The Original heliotrope was Clytia, a daughter of Oceanus and Thethys. She was one Apollo's numerous lovers. Apollo was happy enought with Clytia until he fell for an Assyrian Princess, Leucothoe. He disguised himself as an Assyrian Queen in order to be closer to the Princess and abandoned Clytia. Clytia exposes Apollo to Leucothoe's father, who in turn buries his daughter alive. Apollo could not save her but instead turned her into a tree which would become known as the Frankincense. Apollo despised Clytia for revealing him and she sat for nine days and nine nights, eating nothing, watching her lover move across the sky. She took root and turned into the heliotropion, the flower that turns its head to the sun.
The Hyacinth: Hyacinthus was an Ancient Spartan Prince who caught the fancy of both Apollo and Zephyrus (the personification of the West Wind). Only Apollo's love was requited though, and one day Zephyrus blew on a discus that Apollo had thrown so that it hit Hyacinthus in the head, resulting in his death. The sun-god raised a flower out of his lover's blood and placed his body among the constellations.
The Iris: Iris is the Greek name for the rainbow and its' personification, the Goddess Iris - the wife of Zephyrus. When Ajax committed suicide after the fall of Troy, and from the bloody Earth the Goddess Iris created the flower.
The Laurel: Apollo fell in love with Daphne, a devotee of Apollo's twin sister Artmeis. A Pisan Prince name Leucippus also fancied Daphne, and she preferred him. However, no man was allowed amongst Artemis' followers, so Leucippus disguised himself as a woman. The jealous Apollo exposed the ruse and Artemis had him kiled. Daphne fled from Apollo in fear and loathing but could not out run him. So she prayed for deliverance from Zeus. Zeus turned her into a Lauerel tree (called Daphne in Greek). In Greece, the laurel became associated with prophetic and poetic inspiration. Hence why a nation's greatest poets are its "poets laueate".
The Lotus: A nymph called Lotis prayed to be changed into a flower to escape the advances of the God Priapus, one of Dionysus' son's. The word Priapic, now means "phallic". Interestingly enough, the God Priapus was the personification of an erect penis. He was originally a God of fertility and later adopted as a Guardian or Gardens.
The Narcissus: Upon falling in love with his own reflection, Narcissus died of heartbreak when his love could not be returned. Upon his deathbed a small flower grew.