From Destinypedia, the Destiny wiki
In the darkest days, we didn't have many Guardians. And those who Ghosts had revived were scattered across the solar system, far from the people who needed protection as they made their way to the City in the Traveler's shadow.
But Rangers only had one life to give. When they died, they were gone forever. So the Takanome Rangers took to planting blue flowers along the roads they watched. That way, when they fell, a piece of them would live on to guide weary travelers home.
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