I'm trying to find a book that had three illustrated science fiction stories in it.
The first was about explorers on an alien planet. They found a forest of trees with 'horns' or 'wires', and creatures like delta-winged (triangular) aeroplanes, in different colours, with hollow sockets that the explorers could ride in. These all flew to a volcano. Inside the volcano, strange creatures sat in concentric circles around a central lava pool and passed objects around. Eventually the volcano erupted and spread giant glowing seeds across the landscape. (I think the planet was called Rokell, but I might have that confused with the name of an alien from a different book.)
The second story was about an underwater city--I think the people who lived there wore artificial gills. I don't remember much about this one, but there may have been two characters trying to escape the city.
The third story was about a spaceship that landed on a barren, dead planet and brought back rock samples to a space station. The station then started to malfunction as if someone were sabotaging it. Eventually the crew realised that the rocks from the 'dead' planet were alive and intelligent, and were attacking the station from inside. I distinctly remember a scene in which a rock came flying through an open hatch, as if it was trying to get aboard, or to hit someone in the head...
The book had illustrations (the underwater ones were mostly blue and green, the intelligent-rock ones were brown and yellow, etc). I read it in the late 80s / early 90s, in Australia, but it was a secondhand or library book, so it was probably older. I think it was written for children but I'm not sure.