He watched Jack commune with the ship. Something about that reminded him awfully of how he'd once clung to sound of the TARDIS's hum, when the drums were at their worst. She'd sung to him, sometimes, her voice just a soft descant over the noise in his head.
"We needed something simple to differentiate ourselves on sight, and didn't feel like changing out usual clothes entirely. A flower seemed simple enough. Why a white rose? Amusing irony, mostly. In the flower language of 19th century England, it stands for purity and innocence. A red rose alone stands for passionate love. A white and red rose together, however, stands for unity."