This is the 1971 (Collier edition) reprinting of the 1969 edition, the one I have. The dedication is: "To Julia for the spark that broke the logjam".
The basic premise is that two bored Lunar colonists (teenage boys born on the Moon while their parents were working on a long-term contract) go on what amounts to a joyride. In the process, they encounter evidence (a fragment of a journal) which suggests that a colonist at First Station (since abandoned, because the colony moved to the larger (but still small) Bubble) did not in fact just go off his rocker and suicide, but was pursuing a quixotic quest to try to find what he described as a giant flower (on the airless Moon).
Following the leads in the journal, the boys end up falling through a poorly reinforced ceiling in the titular Lotus Caves.
The Plant (which refers to itself in the plural, and seems to have many surface manifestations) has created a habitat for itself, which is not quite fully insulated yet (the Plant says it arrived in the Solar System before Earth was cool enough to be habitable (? wouldn't the Moon have been too hot then, as well?)). But it's not in any hurry (it doesn't seem to have much temporal sense), and it's still not finished fully closing itself off.
The boys meet the missing colonist (his name is Thurgood, and he's been missing for 70 years, but though he'd be over 100 years old, he doesn't seem to've aged significantly). Questioning him and the Plant, they discover that the Plant's solution to intruders is to bribe them into complacency (there's a suggestion that the food the Plant supplies may be drugged). The comparison that's repeatedly made is that the Plant acts like a mollusk, and coats irritants until they're smooth and non-threatening 'pearls'.
The history of Earth in this book is sketchy at best. The Lunar colonists are taught what is a very sanitized version of history, apparently--and the children don't have any basis to make experiential corrections. They're even prevented from experiencing the smells of the planet they've never seen, apparently lest the emotional stimuli might activate unrequitable desires. So the reader has to try to fill in the missing parts.
It seems evident that the Plant's fears of mass invasions of its private elysium are well founded. The boys find that they have to face ethical concerns as well as their growing sluggishness while planning their escape.
There are questions, however, about their proposed solution. For example, the description of how the Plant came to the Solar System leaves the question of how it (they?) reproduce(s). Are there other Plants out there, releasing seeds? What happens when (if?) this one decides it's time to reproduce? PROBABLY its spores will leave the Solar System, but...
Another problem is that while the Plant is about at climax stage, and will finish sealing up its habitat soon (by its standards), humanity is evidently not near climax. It's been possible to conceal the Plant's existence for the more than 70 years since humans first settled on the Moon--but what happens when more Bubbles are established? What if there's a general effort to terraform the Moon?
In general, Christopher seems fairly skilled at selenography (especially given that he was writing in 1969)--but he makes a few mistakes. One is the assumption that there're no colors in the maria and other lunar features. Another is that at one point he describes the Lunar rocks as 'weathered'. Obviously, lunar rocks are NOT weathered. They may, however, have been altered by extralunar impactors--which are almost entirely ignored in this book. What WOULD happen to the Plant if a major impactor struck nearby? Examination of the craters indicates that rock splashes like mud in such situations--and though there's little atmosphere on the Moon (not 'none' by the way. Lunar gases are at very low pressure (it's described in 'torricellis': One torricelli = approximately 1/760th of an 'atmosphere'--which is almost 15 psi)), vibrations travel through the Lunar surface--which would definitely have an impact on the Plant.
All in all, I'd say there was plenty of scope for a sequel.