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Originally Posted by merkur 
Are plants all essentially clones of earlier plants since plants don't (as far as I know) reproduce sexually? I mean if an orange tree produces oranges (which are kind of like plant eggs right?) which fall to the ground and the seeds in the oranges germinate, won't a genetically identical orange tree (to its parent) sprout up? If not, then where does the genetic variation in plants come from? Do plants undergo meiosis or mitosis? 
I'll leave the dynamics of plant reproduction to the experts, but plants all have unique genotypes just like animals. In face, forensic plant DNA evidence has even been used in court!
http://www.utexas.edu/courses/stross.../paloverde.htm
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For the first time a murderer has been convicted on DNA evidence obtained from a plant. The case was described in the PBS TV series, Scientific American Frontiers, amiably hosted by a continually bemused Alan Alda, the perfect foil for the scientists he interviews. The murder of a young woman occurred in Phoenix, Arizona, and the finding of a pager at the scene of the crime led the police to a prime suspect in the case. He admitted picking up the victim, but claimed she had robbed him of his wallet and pager. The forensic squad examined the suspect's pickup truck and among the bits of evidence collected were pods later identified as the fruits of palo verde (Cercidium spp.). One detective went back to the murder scene and found several palo verde trees, one of which showed some damage that could have been caused by a vehicle. The detective's superior officer innocently suggested the possibility of linking the fruits and the tree by using DNA comparison, not realizing that this had never been done before. Several researchers were contacted before a geneticist at the University of Arizona in Tucson agreed to take on the case. Of course, the first crucial study was to establish evidence that would stand up in court on whether individual plants (especially the palo verde trees) have unique patterns of DNA. A preliminary study on samples from different trees from the murder scene and elsewhere quickly established that each palo verde tree was unique in its DNA pattern. It was then a simple matter to link the pods from the suspect's truck to the damaged tree at the murder scene and obtain a conviction.
