Found: YA SF novel lunar underground-dwelling polymorphic creature found by two teenage boys

After the success in your help here finding one juvenile SF novel, I have another one I wonder if someone could help me identify. I read this in the early 1970s, so was published before this.

Two boys, living in a lunar domed city do the ‘teenage rebellion’ thing by stealing a moon buggy and going exploring. They reach another colony dome, previously abandoned. By chance, they find either some written notes or text scratched into the underside of a steel bunk bed (can’t remember which) by a former occupant who describes seeing a large flower temporarily rising from a particular crack/crater/lava tube and giving its location. Knowing he will be ridiculed if he reports this on the biologically dead Moon, the writer merely commits it to these personal notes and tells no one.

The two boys set off to investigate, find a (stranded?) single giant alien polymorphic telepathic creature living underground in tunnels and caverns of the crack/lava tube that has survived for millennia alone, is lonely and welcomes the human company. The ‘flower’ is a solar energy accumulator to provide energy for the creature to live. The boys communicate, share knowledge and befriend the alien for a short time. Eventually, knowing they will have to return home and ‘face the music’ from parents and the authorities, they resolve to keep their discovery secret for fear of how mankind would treat this creature if it were to be known about.

Sounds like The Lotus Caves by John Christopher, except the plant creature isn't quite as benevolent.

Yes, thank you. That's it. Amazing, you're all so quick on here. I have now looked up various précises and reviews of this book and they agree with the novel I read. Interesting that I had forgotten that the alien plant-thing was using some method of mind control to persuade the boys to stay and that their lunar dome city home was so boring. Probably says something about the way I thought then, imagining living in a technological future lunar city as exciting and also separately feeling being cosseted in a idyllic environment to a state of euphoria is hardly something to be avoided. Knowing now that the author was John Christopher, I can see certain recurrent ideas in some of his other novels such as the Tripods. Being YA, this is nowhere near as bleak as The World In Winter or No Blade of Grass, though.

Another one solved!

>3 WilliamBavington: Excellent!

Christopher's Empty World was a YA novel that chilled me as a kid. I have The World in Winter on my TBR pile at the moment.

Yes, I had forgotten that Empty World was also by John Christopher. Another good, rather sombre read.