I've been looking for this for several years, and bugs me that I can't find it...
It was a collection of about 7 - 8 sci-fi short stories geared towards younger readers that I read in the early 80's or so, and I can't remember the name of the book, but do recall some of the details of the stories it contained (not titles though)
One story was about a lady that lived alone tending her plants and her home got broken into and she was robbed (killed too?) and one of the plants attacked and captured\killed the robbers.
Another was about a new kid to school that everyone thought was strange and it turned out he was an alien that would go "home" to a UFO. (Discovered when kids followed him one night)
The story I remember the most was about a police officer of a town in a desert saw huge ships "float" towards the town as he was outside the town in the desert. He heard a loud hum and saw beam of light from the ship hit the town and when he got back in town everyone was gone, and I believe he saw Neanderthal like creatures in their place . The story ended with a future scientist being puzzled uncovering a gun from the site of where the town was in what was assumed to be millions of years later. (I believe the title of this story was something like "the final experiment" or "the last experiment")
The cover of the book had a picture of the Earth in the middle with a target over it like it was in the scope of a gun. I believe some text was on there such as "is anyone up there". It could have been a book from the scholastic book club for elementary\middle schools.