The author's excellent 'powers of observation' [=ability to notice and pay close attention to things] are evident in the book's detailed descriptions.
The plants seem to have recovered from the disease, but they are still 'under observation'. [=someone is watching them carefully]
They're keeping him at the hospital 'for observation' [=so that doctors can watch him carefully] for another few days.
(..) Private Cowboy. Sir, he was that guy who shot all those people from that tower in Austin, TeXas, sir. That's affirmative. Charles Whitman killed people... ...from a -story observation tower at the University of TeXas. From distances of up to yards. Anybody know who Lee Harvey Oswald was?. (..)1987 Full Metal Jacket