Old, old science - to the extent it could be called that at all. Eating enough calories of plant food provides enough protein, as long as it isn't all from the same, low protein plant. Why? Because aside from the variety of amino acids found in plants, you have a built-in 'amino acid buffer': your intestines. The intestinal linings, which are high in protein, slough off regularly (almost daily, IIRC), and as they do they 'fill in' any 'gaps' in amino acids in the diet. Sure you can get a protein deficiency if you try hard enough, but it's easier to just eat mostly animal products, if you want to get sick with a lot less effort.And as for the protein, sure, lots of plants contain proteins. As far as my knowledge goes, almost all of it is incomplete protein. So unless you know which protein parts are in each plant and make sure to eat the right groups(rice and beans compliment each other and your body can use them to build complete proteins, for example) then it won't do you much good.
On a related note: vegans. yes, they do tend to proselytize, just as the guys wolfing down burgers like to tell us how great ruminants are. Well, they are, of course - in numbers suited to their ecosystem. Between humans overrunning the planet, and us trying to stuff enough cattle, pigs and chickens into the environment to let us all eat like cavemen, we've created a situation where we need fewer ruminants (and humans) - not more. So question everyone, and if someone eschews meat - or loves it - question them twice.