Because plants, of the leafy green variety, have a complex evolutionary history with lots of side trips (like lichen and mold and algae). Further earth based plants are based on the same basic DNA structure as earth based animals.
So while it is not "impossible" that an equivalent to plants evolved into exactly one species or micro-eco-system which exists in this one place on Mars, the chance that this occurred and didn't cause the whole planet to have evidence of their existence is really really small.
But don't let that discourage you from trying various plant hypotheses. The way to pursue that is to then try to figure out what things would be true if they were plants, and see if any of the data we have from Mars would allow you to get closer to proving or disproving your hypothesis. The last time I checked you could ask for and get the a copy of the raw data from JPL if you asked for it (well not some 'give me all your data' kind of ask, but "I'd like to get HiRise imagery from this part of the planet ...")
Yes, they could. But can you include some reasoning with how they would have done that? Some sound more likely than others.
The panspermia theory [1] is based on the idea that perhaps some forms of life could exist in space as it transits from place to place. One of the things that has always bothered me about that hypothesis is the orbital mechanics. Which is to say if you posit a supernova or other energetic event that accelerated a planet (or fragments of that planet) into space, and somehow the life on those fragments survived both the radiation and the effects of vacuum on volatiles, and then it arrived in our solar system, what would the relative velocities be of that material with respect to our planets? And then when that material impacted a local rocky planet how much energy would be released and how would it survive that?
The 'primordial soup' theories have thus always held more interest for me as being more probable. With papers like this one: http://arstechnica.com/science/2012/10/simple-reaction-makes... lending a bit of narrative around how it might have occurred. But the primordial soup theories also need evolution to get from a happenstance chemical mixture into something like multi-cellular organisms.
So follow your chain of reasoning and see what questions it leads you to:
"down from an external source" ...
From where?
How did the plant get there to come down?
What is needed to survive a fall from space? At what velocities?
So while it is not "impossible" that an equivalent to plants evolved into exactly one species or micro-eco-system which exists in this one place on Mars, the chance that this occurred and didn't cause the whole planet to have evidence of their existence is really really small.
But don't let that discourage you from trying various plant hypotheses. The way to pursue that is to then try to figure out what things would be true if they were plants, and see if any of the data we have from Mars would allow you to get closer to proving or disproving your hypothesis. The last time I checked you could ask for and get the a copy of the raw data from JPL if you asked for it (well not some 'give me all your data' kind of ask, but "I'd like to get HiRise imagery from this part of the planet ...")