Strangler Fig
In his Strangler Fig article Martin Fowler uses a botanic metaphor to talk about projects of legacy modernization. The stranger fig (the new system) grows on existing tree (the legacy system) and once mature it can become autonomous. It's a way to replace a legacy system with parts that start by taking support on the existing system, then, once mature enough they become autonomous and progressively replace the existing system.
As it grows, it draws nutrients from the host tree until it reaches the ground to grow roots and the canopy to get sunlight. It can then become self-sustaining, and its original host tree may die leaving the fig as an echo of its shape. This gradual process of replacing the host tree struck me as a striking analogy to the way I saw colleagues doing modernization of legacy software systems.