Symbolism, Imagery, Allegory
Titus and Violet share their first kiss in the Garden on the Moon, plus it's one of the few times where Titus speaks in metaphor—so, it's obviously important. Check out how the garden is described:
Outside the window, there had been a garden, like, I guess you could call it a courtyard or a terrarium? But a long time ago the glass ceiling over the terrarium had cracked, and so everything was dead, and there was moon dust all over everything out there. Everything was grey.
Also, something was leaking air and heat out in the garden, lots of waste air, and the air was rocking off into space through the hole, so all of the dead vines in the garden were standing straight up, slapping back and forth, pulled toward the crack in the ceiling where we could see the stars. (15.5-6)
Notice how the atmosphere is cracked, so the air is leaking out, and everything is dead, but the vines are wriggling around, pulled toward the negative pressure of that crack? The dead moon garden is basically a microcosm of what's happening back home.
On Earth, the environment is dead, and people are partying it up, consuming what they can get, just like the vines wriggling toward the ceiling. As Titus describes it, the vines are like "a squid in love with the sky" (15.9): sea animals reaching for something that's going to kill them. In other words, it's lights-out for the human race.