The folks over at io9.com have gotten into one of those great comic book character debates. If Groot is a plant, what flower or plant did he evolve from? If not, what is he: an animal, fungus, or a virus maybe?
If Groot had terrestrial origins he would more than likely have evolved from either an African Daisy, a Sycamore Maple, a Honey Mushroom or Kudzu.
The case for the African Daisy:
Humanity has been cross-breeding Daisies for thousands of years into every imaginable color, size and shape. They can take root anywhere and in any climate. If Daisies ever became sentient their super friendly properties and adaptability could replace dogs as man's best friend. Besides, nothing is cuter than a Daisy.
The Daisy crossbreeds easily with other wildflowers and readily takes to genetic manipulating. Volunteers (plants that spring up from seeds or cuttings made by cultivated plants, but which haven't been cultivated themselves) that inherited the genes of their semi-sentient but still rooted to the earth ancestors could easily cross pollinate with a more "free-walking" extraterrestrial variety to produce a feral population of galumphing "Daisy men" over several generations.
The Sycamore Maple
In a key scene Groot produces samaras (winged seed cases) that glow. The Sycamore can produce a duo pairing of bat wing like samaras or a quad pairing of seed cases that drift like helicopters in the air.
Sycamores are urban tough. They can grow through and over concrete. They can withstand high levels of air pollution, cold, and concentrations of salt. Like Groot, Sycamores can take huge parts of their tree-osity being hacked from their being.
Their high resistance to human industrial pollution and catastrophe allow them to live in the meanest of man-made extremes, even too the point of over-powering native species. In a newly cleared lot, Sycamores would be the first tree to sprout up.
Sentient, genetically modified Sycamores can easily uproot themselves and quickly make themselves "King" over less mobile flora. They would be a tough foe if they decide to invade and propagate urban colonies. Their samaras are small enough to go undetected in spaceships, making intergalactic cross fertilization extremely likely.
The Honey Mushroom
One of Groots more mysterious qualities is his light producing capability.
Bio-luminescence in animals is rare, in flora, it is even rarer, and always man-made. The only known example, the glowing Starlight Avatar is produced by splicing genes from a bioluminescent bacteria with a common decorative plant called Nicotiana alata. Bioglow Technologies holds the exclusive patent. That would make Groot a GMO.
On the other hand, bio-luminesence is quite common among fungi. Luciferin reactions (a common fungi compound) cause Mushrooms to emit a soft green glow. Honey Mushroom roots can easily penetrate rotting wood and produce numerous glowing Luciferin reactions.
The only problem with fungi is that they are notoriously finicky growers requiring precise sprouting conditions.
Groot has probably developed a symbiotic relationship with a certain mushroom species. He could cultivate these fungi under his outer bark and deploy their light producing qualities as needed. Also Groot's natural photosynthesis will allow photon interaction-- allowing him to store-up vast amounts of energy.
The most likely plant to evolve into Groot would be Kudzu. These vines are fast growing and produce the same leaf pattern that Groot displays in several scenes. Kudzu also adapts well to any environment and most importantly, is virtually unkillable. Unlike most vines, like Poison Ivy, Kudzu is not poisonous to the touch or the stomach. Kudzu is often used as a cheap farm animal feed. Groot may be snacking on his own shoulder because he's the most nutritious thing in the room.
The last possibility that explains a Groot-like organism is a virus. A sentient plant virus will survive as long as there is the smallest bit of infested vegetation. It will rapidly grow and absorb additional mass from the other vegetation around it.